Download Hybrid Sterility and Segregation Distortion in Drosophila

Hybrid Sterility and Segregation Distortion in Drosophila pseudoobscura and Drosophila persimilis
by
Shannon Rose McDermott
University Program in Genetics and Genomics
Duke University
Date:_______________________
Approved:
___________________________
Mohamed Noor, Supervisor
___________________________
Beth Sullivan
___________________________
John Willis
___________________________
Amy Bejsovec
Dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor of Philosophy in the University Program of Genetics
and Genomics in the Graduate School
of Duke University
2012
ABSTRACT
Hybrid Sterility and Segregation Distortion in Drosophila Pseudoobscura and Drosophila Persimilis
by
Shannon Rose McDermott
University Program in Genetics and Genomics
Duke University
Date:_______________________
Approved:
___________________________
Mohamed Noor, Supervisor
___________________________
Beth Sullivan
___________________________
John Willis
___________________________
Amy Bejsovec
An abstract of a dissertation submitted in partial
fulfillment of the requirements for the degree
of Doctor of Philosophy in the University Program in Genetics and Genomics in the Graduate
School
of Duke University
2012
Copyright by
Shannon McDermott
2012
Abstract
Speciation has occurred countless times throughout history, and yet the genetic
mechanisms that lead to speciation are still missing pieces. Here, we describe the genetics of two
processes that can act alone or together to cause speciation: hybrid sterility and meiotic drive.
We use the Drosophila pseudoobscura/D, persimilis species as a model system to study these
processes. We expanded on a prior study and saw little variation in strength of previously known
hybrid sterility alleles between distinct strains of D. persimilis and the Bogota subspecies of D.
pseudoobscura. Introgression of an autosomal, noninverted hybrid sterility allele from the USA
subspecies of D. pseudoobscura into D. persimilis demonstrated that the D. pseudoobscura copy of a
D. persimilis hybrid sterility factor also causes hybrid male sterility in a D. pseudoobscura bogotana
background. This allelism suggests that the introgressed allele is ancestral, but was lost in the
Bogota lineage, or that gene flow between D. pseudoobscura USA and D. persimilis moved the
sterility-conferring allele from D. persimilis into D. pseudoobscura. To further understand the
genetic basis of speciation, we asked if meiotic drive in D. persimilis is associated with hybrid
sterility seen in D. persimilis/D. pseudoobscura hybrids. QTL mapping of both traits along the right
arm of the X chromosome, where both drive and hybrid sterility loci are found, suggest that some
of the causal loci overlap and may be allelic.
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Contents
Abstract .........................................................................................................................................iv
List of Tables .............................................................................................................................. viii
List of Figures ............................................................................................................................... ix
Acknowledgements ...................................................................................................................... x
1. The role of meiotic drive in hybrid male sterility ................................................................. 1
1.1 Introduction....................................................................................................................... 1
1.2 Forms of meiotic drive and how they may (or may not) contribute to hybrid
sterility ..................................................................................................................................... 2
1.3 Recent empirical studies linking meiotic drive and speciation ................................. 7
1.4 Population genetics of drive and speciation ............................................................... 12
1.5 Concluding Remarks...................................................................................................... 15
2. Genetics of hybrid male sterility among strains and species in the Drosophila
pseudoobscura species group ....................................................................................................... 16
2.1 Introduction..................................................................................................................... 16
2.2 Methods ........................................................................................................................... 19
2.2.1 Fly Stocks .................................................................................................................... 19
2.2.2 Fly Crosses .................................................................................................................. 19
2.2.3 Fertility Assays .......................................................................................................... 20
2.2.4 Microsatellite Genotyping ........................................................................................ 21
2.2.5 Data Analysis ............................................................................................................. 22
2.3 Results .............................................................................................................................. 22
2.3.1 Variation among strains in effect of inverted regions on hybrid sterility ......... 22
v
2.3.2 Variation among strains in hybrid sterility effect of collinear regions .............. 26
2.3.3 Allelism in collinear regions of D. pseudoobscura USA and D. persimilis ........... 28
2.4 Discussion ........................................................................................................................ 30
2.4.1 Variation in hybrid incompatibilities within species ........................................... 31
2.4.2 Allelism in hybrid incompatibilities between species .......................................... 32
3. Association of within-species segregation distortion in D. persimilis with hybrid
sterility between D. persimilis and D. pseudoobscura ............................................................... 36
3.1 Introduction..................................................................................................................... 36
3.1.1 Sex chromosome segregation distortion, genetic conflict, and speciation ........ 36
3.1.2 Drosophila persimilis "sex-ratio" and hybrid sterility alleles on the X
chromosome ........................................................................................................................ 38
3.2 Materials and Methods .................................................................................................. 41
3.2.1 Fly stocks .................................................................................................................... 41
3.2.2 Fly crosses ................................................................................................................... 41
3.2.3 Fertility assays............................................................................................................ 43
3.2.4 Microsatellite genotyping......................................................................................... 44
3.2.5 Data Analysis ............................................................................................................. 44
3.3 Results .............................................................................................................................. 45
3.3.1 Characterization of D. persimilis SR ....................................................................... 45
3.3.2 Preliminary test of linkage between hybrid sterility and segregation distortion
............................................................................................................................................... 46
3.3.3 QTL mapping hybrid male sterility between SR D. persimilis SCI and D.
pseudoobscura white.............................................................................................................. 47
3.3.4 QTL Mapping segregation distortion in SR D. persimilis ..................................... 48
vi
3.4 Discussion ........................................................................................................................ 50
3.4.1 Linkage of hybrid sterility and segregation distortion factors ........................... 50
3.4.2 QTL Mapping............................................................................................................. 51
3.4.3 Characterization of segregation distortion in D. persimilis .................................. 52
4. Summary .................................................................................................................................. 54
Appendix A.................................................................................................................................. 57
Appendix B .................................................................................................................................. 58
Appendix C .................................................................................................................................. 59
References .................................................................................................................................... 60
Biography ..................................................................................................................................... 79
vii
List of Tables
Table 1: Variation in percent fertility between lines of D. pseudoobscura bogotana at each
inversion genotype. ................................................................................................................... 24
Table 2: Variation in percent fertility between lines of D. persimilis at the inversions. .....25
Table 3: Variation in percent fertility between lines of D. persimilis at the collinear QTLs.
.........................................................................................................................................................27
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List of Figures
Figure 1: Backcross Design. . .................................................................................................... 20
Figure 2: Percent fertility of individuals with a D. pseudoobscura USA copy of QTL 2. .. . 29
Figure 3: Orientation of inversions and location of markers along XR. . ............................ 40
Figure 4: Crossing scheme to map sterility and segregation distortion on the X
chromosome. ............................................................................................................................... 42
Figure 5: QTL map along XR of hybrid sterility between SR D. persimilis SCI and D.
pseudoobscura white. .................................................................................................................... 48
Figure 6: QTL map of segregation distortion on the D. persimilis SCI XR. ......................... 49
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Acknowledgements
First and foremost, I thank the current and previous members of the Noor lab,
for their extensive support and assistance. Throughout my time at Duke, the help of our
technical staff (Samantha Roth, Callie Barnwell, Sarah Bennett, Allie Graham, Jennifer
Merrill, and Brenda Manzano-Winkler) has been invaluable. I also appreciate the help of
Steve Schaeffer at Penn State for performing the salivary chromosome preparation to
confirm presence of the SR inversion.
I am grateful to the many people who have read, edited, and critiqued my
writing over the years, including Dan Barbash, Norman Johnson, Richard Kliman, and
members of UPGG and Biology.
While at Duke, I received awards from UPGG (Marcy Speer Memorial
Fellowship), Biology (Biology Department Fellowship), and the Graduate School (Dean’s
Award for Excellence in Teaching). I truly appreciate the accolades, and am grateful for
their generous financial support.
I thank members of UPGG, Biology, and my committee: John Willis, Beth
Sullivan, and Amy Bejsovec, for support, advice, and feedback throughout my
dissertation. My advisor, Mohamed Noor, is the best advisor a student could ask for; I
appreciate him more than I can say in words. Thank you for everything, Mohamed.
x
1. The role of meiotic drive in hybrid male sterility
Meiotic drive causes the distortion of allelic segregation away from Mendelian expected
ratios, often also reducing fecundity and favoring the evolution of drive suppressors. If different
species evolve distinct drive-suppressor systems, then hybrid progeny may be sterile as a result
of negative interactions of these systems' components. Although the hypothesis that meiotic
drive may contribute to hybrid sterility, and thus species formation, fell out of favor early in the
1990s, recent results showing an association between drive and sterility have resurrected this
previously controversial idea. Here, we review the different forms of meiotic drive and their
possible roles in speciation. We discuss the recent empirical evidence for a link between drive
and hybrid male sterility, also suggesting a possible mechanistic explanation for this link in the
context of chromatin remodeling. Finally, we revisit the population genetics of drive that allow it
to contribute to speciation.
1.1 Introduction
Species formation occurs when gene exchange between two populations has mostly or
completely stopped. Gene exchange may be prevented by extrinsic factors, such as geographic
isolation, but such extrinsic factors may say little or nothing about the genetic divergence that
allows species to coexist without losing their identity. Instead, most studies of "speciation" focus
on intrinsic genetic differences between populations that limit gene exchange. Typical examples
of such differences are behavioral mating discrimination and hybrid sterility.
Hybrid sterility and other hybrid incompatibilities have been a focus of genetic studies of
speciation. Part of this intensive focus comes from their association with one of the most
consistently followed patterns in evolutionary biology: Haldane's Rule. This rule states that the
1
heterogametic (XY or ZW) sex is more likely than its counterpart to exhibit F1 hybrid inviability
or sterility (Haldane 1922). Studies mapping the genetic basis of F1 hybrid male sterility have
identified a major cause being interactions between genes on the sex chromosomes and
autosomes, and a few of the interacting genes have been cloned in the last few years (see reviews
in Noor and Feder 2006; Orr et al. 2007; Presgraves 2007).
Despite this progress in mapping the genes and their interactions, researchers have not
yet determined whether particular evolutionary or genetic forces disproportionately lead to the
evolution of hybrid sterility. Some of the cloned genes associated with hybrid sterility exhibit a
strong signature indicating recent natural selection (Ting et al. 1998) while others do not exhibit
this pattern (Masly et al. 2006; Phadnis and Orr 2009). One hypothesis for a force contributing to
hybrid male sterility is meiotic drive, the subject of this review. Most studies of meiotic drive and
its association with hybrid male sterility have been performed in Drosophila, which potentially
limits the scope of the research covered in this review. Such an association has been postulated in
other systems, but in those cases, meiotic drive could not be distinguished from other genotype
frequency distortions in hybrids (e.g., inviability of particular hybrid genotypes) or was not
linked to mapped hybrid sterility QTLs (e.g., Mimulus: see Fishman and Willis (2005)). The lack
of studies of meiotic drive involved in hybrid sterility in other systems presents a large gap in
this field that future studies are encouraged to address.
1.2 Forms of meiotic drive and how they may (or may not)
contribute to hybrid sterility
The term “meiotic drive” can be used in a variety of contexts (described below), though it
is generally defined as a distortion of segregation away from Mendelian ratios of allelic
2
inheritance in heterozygotes (Presgraves 2008). Meiotic drive is characterized by the differential
formation or success of gametes, and this meiotic or gametic failure may reduce overall fertility.
Usually, homologous chromosomes segregate equally (one homologue to each gamete); however,
alleles or chromosomes containing a “drive element” will be transmitted to more than 50% of
offspring, at the expense of their homologue. Despite the reduction in fertility of the bearer, the
allelic transmission advantage can allow the driving chromosome to spread within a species.
Driving alleles also tend to be located in inversions or other regions of low recombination, and
therefore deleterious mutations linked to the driver may increase in frequency via hitchhiking.
Drivers located on the sex chromosomes can strongly distort sex ratios, which could possibly lead
to extinction. These many deleterious effects select for drive suppressors, which have evolved for
most known drive systems. We describe two types of meiotic drive that can lead to hybrid
sterility: female (chromosomal) and male (genic).
Female or “chromosomal” drive occurs via competition among oocyte chromosomes for
deposition into the resultant egg cell; however, this competition runs an added risk of causing
hybrid male sterility. Most Drosophila oocytes undergo four rounds of incomplete cytokinesis
during meiosis; only one of the resulting cells will become an egg and pass its chromosomes to
offspring. Meiotic products may compete for deposition into the egg cell via spindle fiber
attachment (Zwick et al. 1999; Pardo-Manuel de Villena and Sapienza 2001). Variation in satellite
sequences at the centromere (the site of spindle attachment) and numerous replacement
polymorphisms at the centromeric histones suggests rapid evolution, and further support the
hypothesis of an arms race to be “chosen” to be relegated to the egg cell (Malik and Henikoff
2001; Malik et al. 2002). This arms race for spindle attachment can have negative side effects;
3
centromeric and telomeric sequences in different populations may be divergent enough to cause
misalignment of the centromeres, thereby disrupting meiosis in hybrid males and causing
sterility (McKee 1998; Henikoff et al. 2001; Zwick et al. 1999). However, although a possible
contributing force, female drive may not provide the single best explanation of Haldane’s Rule
for two reasons: female drive does not explain hybrid inviability, and female drive seemingly
would not apply to haplodiploid species or species lacking an XY or ZW sex (whereas Haldane’s
Rule still applies to both species types). Further, in haplodiploid or XO species, competition
between the two chromosomes would be eliminated and no arms race would occur to increase
the chance of hybrid incompatibilities (reviewed further in Coyne and Orr 2004). The remainder
of this review will focus mainly on X chromosome male drive and its relevance to speciation.
While chromosomal drive is characterized by a competition between centromeres for
spindle fiber attachment, male drive (also called genic drive) is a competition between alleles
during sperm development. The mutations caused by this competition may increase divergence
between two species and therefore also run the risk of causing hybrid male sterility. During
normal spermatogenesis, half of the newly formed haploid sperm cells should contain X
chromosomes and the other half should contain Y chromosomes. Deviations from a 50-50 sex
ratio in offspring (typically towards females) often indicate genic drive of sex chromosomes in
males. Male drive may result from lack of production of Y-bearing sperm or producing Ybearing sperm rendered incapable of fertilization (see Recent Empirical Studies section). In many
instances, sex chromosome meiotic drive is specifically associated with a particular chromosomal
rearrangement (see Population Genetics of Drive and Speciation section) and in Drosophila, these
rearrangements are dubbed "sex-ratio (or SR) chromosomes." While sex chromosome drive
4
appears more common than autosomal drive, this perception may result from an observational
bias: sex chromosome drive is easily detected by sex-ratio disruptions ranging from 60-100
percent female offspring (reviewed in Jaenike 2001 [supplemental material]). Autosomal drive is
more difficult to observe directly unless a visible phenotypic trait difference is closely linked to
the driving locus.
Suppressors of meiotic drive are likely to evolve on the autosomes as a response to a
change in the sex ratio caused by X-linked drivers, causing them to be observed often in sex
chromosome drive systems. Autosomal suppressors will adjust the sex ratio back to normal;
however, they may not spread to fixation quickly. Y-linked suppressors can also restore the sex
ratio and will exhibit intense positive selection since nonsuppressing Y-chromosomes will not
appear (Atlan et al. 2003; Hurst and Pomiankowski 1991). However, the size, gene content, and
number of autosomes compared to the Y chromosome provide greater possibility for suppressors
to evolve (Hurst and Pomiankowski 1991). Atlan et al. (2003) further showed that, when the
strength of autosomal suppression of drive was substantially greater than Y-linked suppression,
autosomal suppressors would spread faster. This observation suggests a reason why autosomal
suppressors are observed in so many drive systems and why drive systems that contain Y-linked
suppressors nearly always contain autosomal suppressors as well (Stalker 1961; Carvalho and
Klaczko 1993; Carvalho and Klaczko 1994; Jaenike 1999).
Almost twenty years ago, two independent publications discussed the likelihood of
mutations in male drive systems explaining hybrid sterility and Haldane’s Rule in particular
(Frank 1991; Hurst and Pomiankowski 1991). These authors described a model where rapid
coevolution of drivers and suppressors increases the divergence between species. A reduction in
5
fertility is a common byproduct of meiotic drive, and thus accelerated evolution at drive loci may
disproportionately impact spermatogenesis genes, inadvertently increasing the chance of hybrid
sterility-causing interactions. The arms race between the evolution of drivers and suppressors is
thought to cycle, each triggering evolution in the other (Carvalho et al. 1997; Hall 2004). Because
distorters may be evolving repeatedly on the X chromosome, this cycle model reinforces the idea
that rapid evolution on the sex chromosomes could greatly increase divergence between species
and may be a contributor to the large-X effect observed in genetic studies of hybrid sterility
(Coyne and Orr 1989; Turelli and Orr 2000; Masly and Presgraves 2007; Tao et al. 2007b;
Charlesworth et al. 1987).
However, these meiotic drive theories for hybrid male sterility were quickly criticized for
multiple shortcomings (Coyne et al. 1991; Charlesworth et al. 1993). For example, recombination
between distorter alleles and their responders can prevent the spread of new drive systems if the
responder allele is recombined onto the same chromosome as the distorter, thereby causing the
distorter to drive against itself (Lessard 1985; Charlesworth and Hartl 1978; Prout et al. 1973;
Hartl 1975; Thomson and Feldman 1976; Liberman 1976). Because of the absence of crossing over
between the X and Y chromosomes, Frank (1991) argues that drivers should evolve faster on the
sex chromosomes than autosomes. While Coyne et al. (1991) argue that in certain cases,
autosomes are actually more likely to evolve drivers, recent empirical studies have repeatedly
identified X-linked drivers, including the four discussed in the next section.
Perhaps the strongest criticisms of the meiotic drive models came from the lack of
supporting data from empirical studies at the time. Frank’s (1991) model assumed that meiotic
drive should occur in interspecies crosses, yet early studies failed to detect evidence of meiotic
6
drive in fertile backcross hybrid males (Coyne and Orr 1993; Johnson and Wu 1992). Hurst and
Pomiankowski (1991) further predicted that the Y chromosome would play a major role in the
association of meiotic drive and hybrid male sterility, but at least some studies of hybrid sterility
detected no involvement of the Y chromosome, and Haldane's Rule is even observed in species
not bearing a Y-chromosome (Coyne et al. 1991; Charlesworth et al. 1993). Despite such
arguments against these early meiotic drive hypotheses for causing hybrid sterility, several recent
studies have demonstrated compelling links between male drive and hybrid male sterility (see
below).
1.3 Recent empirical studies linking meiotic drive and speciation
Recently, numerous empirical studies linked drive and sterility in hybrids, yet few have
proposed a precise mechanism for this link. Most drive systems are characterized as “true
drive,” where the drive actually takes place during meiosis. This class of drive causes targetcarrying sperm to never be produced, as opposed to “gamete-killing drive,” where the targetcarrying sperm are produced but are incapable of fertilization due to disruptions occurring
anytime after meiosis. True drive, such as fragmentation of the Y chromosome (Novitski et al.
1965) seems to be more common in sex chromosomal drive systems (see discussion in
Montchamp-Moreau 2006), whereas autosomal drive systems, such as SD in Drosophila
melanogaster and the t haplotype in mice, exhibit gamete-killing drive (Lyon 2003; Kusano et al.
2003).
Hauschteck-Jungen (1990) identified a link between hybrid sterility and a driving X
chromosome derived from one population of Drosophila subobscura. Some wild D. subobscura
males collected from Tunis produced 90% female offspring. These males carried A2+3+5+7, an SR
7
arrangement on the X chromosome containing four inversions relative to the standard.
Hauschteck-Jungen (1990) further observed that males possessing the Tunis A2+3+5+7 X
chromosome with autosomes derived from another strain (Küsnacht) were sterile, while males
possessing the Küsnacht X chromosome and Tunis autosomes were fertile. Hence, presence of
SR X chromosome arrangement contributed to sterility in hybrids within species when paired
with autosomes that had not coevolved with it.
A driving factor also causes sterility in hybrids between Drosophila simulans and D.
mauritiana when made homozygous (Tao et al. 2001). Tao et al. (2001) introgressed 259
homozygous segments of the D. mauritiana third chromosome into D. simulans and scored the
offspring for fertility and drive. Two segments interact to cause both drive and sterility: too much
yin (tmy) and broadie. Too much yin is presumed to be an autosomal suppressor of X chromosome
drive. The D. simulans copy of Tmy appears to be dominant to the D. mauritiana copy (tmy),
which explains why their study only observed sterility when the D. mauritiana introgression was
homozygous in the D. simulans background. When the D. simulans copy was present in the same
background, all males were fertile with no sex ratio distortion and the same results were
observed for Tmy/tmy males. About half of the males homozygous for D. mauritiana tmy in a D.
simulans background were sterile (235/452) while the others were fertile with a strong sex ratio
bias. They determined that, while tmy alone fails to suppress the sex ratio bias, the added
presence of the broadie modifier causes complete male sterility. They further fine-mapped tmy,
narrowing it down to a region spanning 80kb and demonstrated that recombination cannot
separate the two phenotypes (sterility and drive), indicating that tmy can act alone to cause both.
Multiple links between meiotic drive and hybrid sterility have been made through
8
genetic mapping studies in the Drosophila pseudoobscura species group. The first such link is
inferential. Early mapping studies by Wu and Beckenbach (1983) and Orr (1989) identified effects
of the right arm of the X chromosome of D. persimilis as contributing to sterility in a D.
pseudoobscura genetic background. These studies both used strains of D. persimilis bearing a SR
driving X chromosome. Curiously, the opposite effect was observed by Noor et al. (2001a),
where the same D. persimilis X chromosome region derived from non-driving D. persimilis X
chromosomes reduced hybrid sterility-conferring interactions elsewhere in the genome when in a
D. pseudoobscura background. This discrepancy suggests a link between the driving X
chromosome and hybrid sterility, but this link is only inferential until more precise mapping is
conducted.
More recently, drive and sterility have been directly linked in hybridizations between the
D. pseudoobscura subspecies. One direction of the subspecies cross (bogotana females crossed to
pseudoobscura males) produces very weakly fertile male progeny (Orr and Irving 2005).
Moreover, these hybrid males produce strongly female-biased offspring, a driving effect masked
in pure D. pseudoobscura bogotana due to autosomal suppressors. The X-linked factor causing
segregation distortion was localized near the sepia locus; however, this factor is necessary but not
sufficient: distortion requires additional D. pseudoobscura bogotana introgressions to exhibit drive.
The location implicated here for meiotic drive is the same location mapped in a previous study
for hybrid male sterility (Orr and Irving 2001), leading the authors to suggest that the same factor
causes both drive and hybrid male sterility (Orr and Irving 2005). This region was recently
mapped further and a gene, Overdrive, was identified that is responsible for both phenotypes
(Phadnis and Orr 2009). Overdrive encodes a protein with a Myb/SANT-like domain in an Adf-1
9
(MADF) DNA-binding domain. Adf-1 is a transcription factor known to activate genes regulated
during Drosophila development (England et al. 1992).
Some meiotic drive systems and hybrid sterility-conferring genes share common factors
that can be incorporated into a potential single mechanistic explanation for the link between these
two phenotypes: chromatin remodeling. Heterochromatin formation was recently implicated in
causing hybrid female lethality between Drosophila melanogaster males and D. simulans females
(Ferree and Barbash 2009). In hybrid females, the region of the D. melanogaster X chromosomes
containing the Zygotic hybrid rescue (Zhr) gene disrupts separation of chromatids and the D.
simulans machinery is unable to compensate. The aforementioned gene Overdrive, which
contributes to both meiotic drive and hybrid sterility, may also be involved in chromatin
remodeling. Further, Overdrive is not the only Drosophila hybrid sterility gene to contain a
MADF domain; Hybrid Male Rescue (HMR) in Drosophila melanogaster contains four MADF
domains (Barbash et al. 2003; Maheshwari et al. 2008). HMR causes hybrid male lethality and
hybrid female sterility between D. melanogaster and its sibling species, D. mauritiana, D. simulans
and D. sechellia (Hutter and Ashburner 1987; Barbash et al. 2003; Barbash and Ashburner 2003).
Recent studies suggest that some of HMR’s MADF domains may be involved in chromatin
binding (Maheshwari et al. 2008; Barbash and Lorigan 2007), while other studies have also
implicated defects in chromatin remodeling as a potential mechanism of meiotic drive (Tao et al.
2007a; Hauschteck-Jungen and Hartl 1982; Montchamp-Moreau 2006).
Silencing on the X chromosome also involves changes in chromatin states that occur
during spermatogenesis. The X chromosome is inactivated in primary spermatocytes during
spermatogenesis in Drosophila, mammals and nematodes (Lifschytz and Lindsley 1972; Kelly et
10
al. 2002; Hense et al. 2007). Lifschytz and Lindsley (1972) demonstrated that factors that prevent
proper silencing during inactivation may cause sterility. A potential change in chromatin states
could result from the different arrangement of the X chromosome in SR Drosophila; the
rearrangement then causes a failure of the X and Y chromosomes to align during metaphase in
hybrids, leading to an arrest of spermatogenesis (McKee 1998). Hence, changes in chromatin
states due to meiotic drivers may also negatively impact X chromosome silencing and therefore
fertility. Drosophila XO males also undergo X chromosome inactivation, and if such males
exhibit disrupted silencing due to the aforementioned hypothesis, this explanation may address
one of the issues raised by Coyne and Orr (2004) regarding the inability of female drive to explain
sterility in XO species. Interactions involving chromatin remodeling, and the MADF binding
domain in particular, may prove to be involved in multiple hybrid incompatibilities.
Finally, our suggestion that changes in chromatin modifications can affect hybrid sterility
is further strengthened by a recent finding by Bayes and Malik (2009). The Drosophila mauritiana
allele of OdysseusH (OdsH) contributes to hybrid male sterility when in a D. simulans genetic
background (Ting et al. 1998). The endogenous protein binds to heterochromatin, particularly loci
on the X and 4th chromosomes. Coexpression of the Drosophila mauritiana and D. simulans OdsH
alleles in sterile hybrids led to anomalous binding of the D. mauritiana copy to the Ychromosome, resulting in altered chromosome condensation. Transgenic lines into multiple
species also exhibited altered localization of OdsH, reflecting the rapid evolution seen within the
OdsH homeodomain (Ting et al. 1998). These results demonstrate rapid coevolution between a
hybrid sterility gene and its heterochromatic targets across this clade.
11
1.4 Population genetics of drive and speciation
Understanding how drive contributes to speciation requires an understanding of how
drive arises and is maintained despite associated fitness detriments. Drive systems on sex
chromosomes arise more easily than on autosomes, yet become fixed less easily. This delay
between emergence and fixation allows more time for selection to act against sex-chromosome
drive systems via recombination, introduction of suppressors and association of unpreferred
traits. However, certain effects of drive are also selected, thus causing the cyclical evolution of
drivers and suppressors mentioned previously. Again, this cycling can increase the opportunity
for hybrid incompatibilities to arise and cause speciation. We discuss each of these processes in
this section.
Drive systems are inherently more detectable to investigators when they arise on sex
chromosomes than autosomes because of the associated skew in sex ratio. However, despite the
observational bias, there are also evolutionary reasons why drive systems would favor the X
chromosome. Autosomal drivers must arise on an insensitive chromosome and then drive
against a sensitive chromosome (Charlesworth and Hartl 1978). Taking into account the fitness
cost (usually a reduction in male fertility) typically associated with drive, Hurst and
Pomiankowski (1991) showed that, for autosomal drivers to invade, the frequency of the
insensitive chromosome must be low, and the driver must be in strong linkage disequilibrium
(LD) with the target. Sex-linked drive systems require fewer starting conditions; since the X and
Y chromosomes do not recombine, it is easier to maintain LD between the driver and target (Wu
and Hammer 1991). Thus, autosomal drivers must depend on strong LD for invasion, whereas
sex-linked drivers rely only on the equilibrium that must be maintained between selection and
12
drive (Hurst and Pomiankowski 1991).
Sex-linked drive systems become fixed less easily than autosomal systems, and this
feature may predispose them to acquiring suppressors (Wu 1983a; Jaenike 1996; Jaenike 2001;
Wilkinson and Fry 2001; Taylor and Jaenike 2002; Atlan et al. 2004; James and Jaenike 1990;
Wilkinson et al. 2006; Capillon and Atlan 1999). A system with an X-linked driver will skew the
sex ratio of the population such that there is less competition for females, and thus males mate
more often (Jaenike 1996). However, driver-carrying males often exhibit reduced fertility (Hartl
et al. 1967; Beckenbach 1978; Wu 1983a; James and Jaenike 1992; Jaenike 1996; Atlan et al. 2004;
Fry and Wilkinson 2004; Wilkinson et al. 2006). Therefore the driver is held at equilibrium, where
selection against it increases as its frequency increases (Wu 1983a; Jaenike 1996; Jaenike 2001;
Wilkinson and Fry 2001; Taylor and Jaenike 2002; Atlan et al. 2004; James and Jaenike 1990;
Wilkinson et al. 2006; Capillon and Atlan 1999). This balance between selection and frequency
allows greater time for a drive suppressor to evolve and drivers that skew the sex ratio provoke
selection against themselves. Therefore, evolution of suppressors is more likely under X-linked
drive systems (Hurst and Pomiankowski 1991).
Non-Mendelian sex ratios are generally disadvantageous, and thus in sex-linked drive
systems, selective pressure is applied that pushes the ratio back to normal (Lyttle 1979). This
selective pressure may yield greater probability of inducing the arms race discussed earlier,
where rapid evolution at drivers and suppressors increases divergence and thus hybrid
incompatibilities are more likely to arise (Frank 1991). Given the fertility reduction in SR males,
offspring would be more fit if recombination broke up drivers and their linked targets; however,
many drivers are located in inversions, where recombination is prevented. Thus, recombination
13
may prevent drivers from initially fixing, but recombination will have little effect if the drivers
are already fixed or reached a stable equilibrium. Furthermore, in Drosophila, where drivers are
typically located within inversions, the X chromosome drive loci are thought to have originated
prior to the introduction of the inversions (Babcock and Anderson 1996; Wu and Beckenbach
1983; Lyttle 1991). These drive loci would not have been linked without the inversions, and thus
no drive would occur. However, as the inversions evolved, more of the X would become linked
and eventually the final inversion would have linked the drive loci together, causing meiotic
drive and increasing the frequency of the inversion arrangement in the population (Babcock and
Anderson 1996).
In addition to recombination, the evolution of suppressors also will push the sex ratio
back to Fisherian proportions (Lyttle 1979). Continued driver presence will favor the evolution of
suppressors; however, once suppressed, a different driver may arise and start the cycle over (Hall
2004). Again, this cycling hypothesis could explain how meiotic drive is linked to hybrid
sterility, where this rapid evolution at different regions of the genome can cause increased
divergence and therefore heighten the likelihood of hybrid incompatibilities arising.
Selecting for traits associated with drive can further affect driver frequency, which may
induce an arms race between drivers and suppressors. In mice and stalk-eyed flies, females can
distinguish between males carrying driving elements and non-driving males (Wilkinson et al.
1998; Lenington and Egid 1985; Carroll et al. 2004). In both species, the females actually
preferentially mate to non-driving males, and thus drive is selected against (Lenington et al. 1992;
Wilkinson et al. 1998). Females of the stalk-eyed fly species C. dalmanni and C. whitei prefer mates
with the longest eye-stalks (Burkhardt and de la Motte 1985) and eye span (Wilkinson and Reillo
14
1994; Burkhardt and de la Motte 1988). However, longer eye-stalks are associated with genetic
factors that suppress female-biased sex chromosome drive.
1.5 Concluding Remarks
While meiotic drive was not a popular explanation for hybrid sterility just over a decade
ago (Coyne and Orr 1993; Johnson and Wu 1992), many recent studies implicating an association
between meiotic drive and hybrid male sterility suggest that the community ruled out this
possibility too quickly (Wu and Beckenbach 1983; Hauschteck-Jungen 1990; Tao et al. 2001; Orr
and Irving 2005; Phadnis and Orr 2009). Theoretical studies suggest evolutionary reasons why
meiotic drive may contribute to hybrid sterility, and recent empirical studies suggest explicit
links between these two phenomena. Furthermore, exploring the genetic architecture of both
drive and sterility may provide clues as to the mechanistic basis of an association between them.
Future studies of drive and hybrid male sterility should consist of cloning the underlying genes
as well as clarifying the role of chromatin remodeling and X chromosome inactivation during
spermatogenesis. Overall, this area is ripe for further investigation to help us understand hybrid
sterility and the origin of biodiversity.
15
2. Genetics of hybrid male sterility among strains and
species in the Drosophila pseudoobscura species group
Taxa in the early stages of speciation may bear intraspecific allelic variation at loci
conferring barrier traits in hybrids such as hybrid sterility. Additionally, hybridization may
spread alleles that confer barrier traits to other taxa. Historically, few studies examine withinand between-species variation at loci conferring reproductive isolation. Here, we test for allelic
variation within Drosophila persimilis and within the Bogota subspecies of D. pseudoobscura at
regions previously shown to contribute to hybrid male sterility. We also test whether D.
persimilis and the USA subspecies of D. pseudoobscura share an allele conferring hybrid sterility in
a D. pseudoobscura bogotana genetic background. All loci conferred similar hybrid sterility effects
across all strains studied, though we detected some statistically significant quantitative effect
variation among D. persimilis alleles of some hybrid incompatibility QTLs. We also detected
allelism between D. persimilis and D. pseudoobscura USA at a second chromosome hybrid sterility
QTL. We hypothesize that either the QTL is ancestral in D. persimilis and D. pseudoobscura USA
and lost in D. pseudoobscura bogotana, or gene flow transferred the QTL from D. persimilis to D.
pseudoobscura USA. We discuss our findings in the context of population features that may
contribute to variation in hybrid incompatibilities.
2.1 Introduction
The process of speciation results when gene flow is prevented between populations,
creating reproductive isolation. A wide variety of traits can inhibit gene flow, and understanding
the genetic differences that lead to these traits informs us about the genetic changes that create
new species. More progress has been made in mapping , characterizing, and cloning genes that
16
cause incompatibilities, such as fitness reduction, in species hybrids (reviewed in Presgraves
2010) than in traits that prevent formation of hybrids (e.g., mate preference, pollinator
differences), although both have been studied extensively (Coyne and Orr 2004, Chapters 5 and
6).
However, the genetic mapping studies that lead to cloning incompatibility genes are
often unreplicated. A typical speciation genetics study will identify quantitative trait loci (QTLs)
differentiating one strain of one species from one strain of another species but does not test
between other pairs of strains. Yet, the localized QTL may be specific to the strains studied and
not actually characteristic of the entire species, and many recent studies have demonstrated
intraspecies variation in QTLs (Demuth and Wade 2007; Case and Willis 2008; Good et al. 2008;
Nolte et al. 2008; Reed et al. 2008; Sweigart et al. 2007; Teeter et al. 2010). For example, Barnwell
and Noor (2008) examined variation within and between populations of Drosophila pseudoobscura
in species mate discrimination. Although the behavioral difference between these populations
was fixed, the underlying QTLs differentiating one pair of strains failed to contribute to the
behavioral difference in any other pair of strains surveyed (see also Ortiz-Barrientos et al. 2004).
Thus, two questions must be asked of speciation genetics QTL mapping studies: 1) can the same
QTLs be detected in different pairs of strains, and if so, 2) are the effects similar in magnitude?
The Drosophila pseudoobscura/D. persimilis system has been used extensively as a model for
hybrid male sterility, pioneered by Dobzhansky (1933; 1936) and continued by others (Orr 1987;
Orr and Irving 2001; Phadnis and Orr 2009; Wu and Beckenbach 1983). Noor et al. (2001b) found
that much of the genetic basis for hybrid sterility in this system localizes to three regions bearing
fixed inversion differences between the two species. Brown et al. (2004b) and Chang and Noor
17
(2010; 2007) found that a geographically isolated subspecies of D. pseudoobscura (D. pseudoobscura
bogotana) had additional factors outside the inverted regions that conferred sterility in hybrid
males between the subspecies and D. persimilis. These findings supported a model in which
sterility factors in hybridizing species (D. pseudoobscura USA and D. persimilis) persist in fixed
inverted regions that differ between the two species. Unlike the inverted regions, most collinear
(uninverted) regions of the genome are prevented from diverging if the nascent species were
sympatric or are homogenized by interspecies gene flow after secondary contact if the nascent
species emerged in allopatry (Machado et al. 2007; Noor et al. 2007; Kulathinal et al. 2009). In
contrast, non-hybridizing species (D. pseudoobscura bogotana and D. persimilis) can maintain
additional sterility factors across the genome, both in inverted and collinear regions.
Here, we examine the system in greater detail, specifically the D. pseudoobscura bogotanaD. persimilis hybridization, to determine the extent of genetic variation contributing to hybrid
sterility within and among these taxa. We test whether and how variation within species affects
hybrid sterility by reexamining the data of Brown et al. (2004b) and collecting new data to see if
the inverted regions contributed similar effects in D. persimilis/D. pseudoobscura bogotana hybrids
when using different strains of the two taxa. We also replicate the experimental design of Chang
and Noor (2007) in multiple strains of D. persimilis to detect variation among collinear factors
contributing to hybrid sterility between D. persimilis and D. pseudoobscura bogotana. Finally, to
understand the evolutionary history of hybrid sterility in this system and study between-species
variation, we test for shared alleles in a collinear region of D. pseudoobscura USA and D. persimilis
that confers sterility in D. persimilis /D. pseudoobscura bogotana hybrids. Demonstration of allelism
(that this region has the same effect when derived from either D. persimilis or D. pseudoobscura
18
USA) could suggest that this hybrid sterility-conferring allele introgressed from D. persimilis into
D. pseudoobscura USA in collinear regions from the homogenizing gene flow described above.
2.2 Methods
2.2.1 Fly Stocks
The D. persimilis MSH1993 line was derived from females collected at Mount Saint
Helena, California, in 1993 (Noor 1995). The D. persimilis MSH3 and MSH1 lines were derived
from females collected at Mount Saint Helena, California, in 1997 (stock number 14011-0111.49).
The D. persimilis Santa Cruz Island (SCI) was obtained from the stock center (number
14011-0111.50, year: 2004). The D. pseudoobscura USA Flagstaff1993 line was derived from four
different lines, each collected in Flagstaff, Arizona, in 1993 (Noor 1995). Because the collections
vary both spatially and temporally, we assume that our sampling does not affect our tests of
variation. The D. pseudoobscura bogotana line is a subculture of the D. pseudoobscura bogotana El
Recreo line collected in 1978 (provided by H. A. Orr). Lines of D. pseudoobscura bogotana and
crosses described in the re-examination of the Brown et al. (2004b) data are described therein.
2.2.2 Fly Crosses
Drosophila pseudoobscura bogotana females carrying a white eye mutation (hereafter bogw) were
collected as virgins and maintained for at least six days. After six days, bogw females were
crossed to D. persimilis MSH3, MSH1993, MSH1, or SCI males (Figure 1). F1 females were
backcrossed to bogw males to generate backcross males (hereafter “BCbogw males”) for fertility
assays. Only those male progeny bearing the white mutation were scored because the mutation
is linked to an inversion on the left arm of the X chromosome (XL), which is known to confer
sterility between the two species (Brown et al. 2004b; Dobzhansky 1936; Noor et al. 2001a; Orr
19
1987). Because a D. persimilis XL inversion would cause sterility in a D. pseudoobscura bogotana
background, we selected white-eyed flies, which maintain a D. pseudoobscura bogotana XL
inversion, allowing sterility conferred from other loci to be detected.
For the test for allelism of a hybrid sterility locus, regions of the D. pseudoobscura USA
Flagstaff1993 second and fourth chromosomes that correspond to D. persimilis MSH1993 QTLs
associated with hybrid male sterility when in a D. pseudoobscura bogotana background (Chang and
Noor 2007), were introgressed separately into a D. persimilis MSH1993 background. These
introgressed males were crossed to bogw females, and F1 females were backcrossed to bogw
males to generate backcross males (BCbogw males) used for fertility assays. All crosses were
performed on standard sugar/yeast/agar medium at 20° ± 1° and 85% relative humidity.
2.2.3 Fertility Assays
BCbogw males were collected as virgins and maintained for seven days in food-
Figure 1: Backcross Design. Males from each line of D. persimilis were crossed to
white-eyed D. pseudoobscura bogotana (bog) females. The fertile F1 hybrid females were
backcrossed to white-eyed D. pseudoobscura bogotana males. The white-eyed backcross male
progeny (BCbogw) were then dissected and genotyped.
20
containing vials holding 1–20 males. On day 7, the fertility of each backcross male was assessed
by dissection of the testes in Ringer’s solution following the method of Coyne (1984). A male was
scored as “fertile” if at least one motile sperm was observed and “sterile” if no motile sperm were
observed. Treating fertility as a binary trait has been shown to be conservative (Campbell and
Noor 2001), although other methods of scoring fertility exist (e.g., White-Cooper 2004). All
dissected BCbogw males were labeled and stored at -20°.
2.2.4 Microsatellite Genotyping
DNA was extracted from all dissected BCbogw males following the protocol of Gloor
and Engels (1992). Microsatellite genotyping was performed in two steps. First, all BCbogw males
were genotyped for markers associated with each inversion that distinguishes D. pseudoobscura
from D. persimilis. The markers used for this initial screen were DPSX002 or DPSX046 (left arm of
X chromosome- XL), DPSX030 (XR), and DPS2022 or DPS2026 (2). These markers denoted the
species identity of the inversion arrangement on these chromosome arms. Second, only those
BCbogw males that were hemizygous or homozygous for the D. pseudoobscura bogotana allele at
the three inversion markers (hereafter “BCbogwLim males”) were further genotyped at the three
collinear QTLs identified previously (Chang and Noor 2007). Again, these markers were selected
because the D. pseudoobscura bogotana inversion alleles permit detection of sterility caused by
autosomal loci. The markers used for genotyping at these QTLs were DPS2_27.05 and
DPS2_2395c for chromosome 2, DPS3001 for chromosome 3 and two of the following for
chromosome 4: DPS4G1a, DPS4G1h, DPS4033h, DPS4611966. Primer sequences for all markers
used in this study are available in appendix A. PCR amplification followed a touchdown
21
protocol: 95° for 1 min; 3 cycles of 94° for 30 sec, 56° for 30 sec, 72° for 30 sec; 3 cycles of 94° for 30
sec, 53° for 30 sec, 72° for 30 sec; and 30 cycles of 94° for 30 sec, 50° for 30 sec, 72° for 30 sec. PCR
products were visualized on acrylamide gels on LiCor 4200 and 4300 DNA sequencer/analyzers.
2.2.5 Data Analysis
Following the protocols of Brown et al. (2004b) and Chang and Noor (2007), each
individual from each cross was parsed into categories based on genotype using Microsoft Excel.
Counts of the number of individuals in each category in each cross were summed. The number
of fertile progeny for each genotype category was divided by the total number of progeny with
that genotype to obtain the percent fertility of individuals with that genotype. Crosses of a similar
type were also analyzed together (i.e. backcrosses of different lines of D. persimilis to the same
line of D. pseudoobscura bogotana). The fertile individuals in each of those crosses of that type
were summed and divided by the total number of progeny to calculate percent fertility of a
certain genotype for all crosses together. Statistical differences in fertility between each of the
crosses of a similar type were calculated using a G-test or Chi-Square (if the G-test failed).
Analyses were performed using StatView5 (SAS; Cary, NC), although Chi-Square probabilities
were estimating using the online calculator at: http://faculty.vassar.edu/lowry/fisher2x4.html.
2.3 Results
2.3.1 Variation among strains in effect of inverted regions on hybrid sterility
Brown et al. (2004b) examined the fertility of backcross hybrids between D. persimilis and
D. pseudoobscura bogotana, specifically limiting analysis to those hybrids that were homo- or
hemizygous for all inversions from D. pseudoobscura bogotana. We reanalyzed the raw data they
obtained (Brown et al. 2004a) before they imposed this limitation. We tested if the effects of the
22
inverted regions on hybrid sterility are similar between crosses using different strains of D.
persimilis. We split the data into two classes (discussed below) for analysis because in some of the
previous research (Brown et al. 2004b; Chang and Noor 2007) and in all of our new research,
selection of progeny for phenotyping and genotyping was limited to males bearing the XL
chromosome arm from D. pseudoobscura bogotana. This limitation is based on the "white" visible
mutation located on the XL; flies bearing an XL from D. persimilis were almost invariably sterile
(see below). We focus on the fertility of backcross progeny from the cross shown in Figure 1.
Two classes of backcross progeny were analyzed: strains of D. pseudoobscura bogotana
with an XL from either species (Table 1, corresponding P-values1), and strains with only a D.
pseudoobscura bogotana-derived XL (Table 1, corresponding P-values2) while using a single strain
of D. persimilis. We found that all backcrosses bearing a D. persimilis-derived XL had greatly
reduced fertility (less than 15%), regardless of the genotypes at the other markers. Among all
backcross progeny, fertility remains comparatively high in individuals with D. pseudoobscura
bogotana genotypes at the XL (Table 1, lines 1-4). However, we observe a strong reduction in
fertility when the XR chromosome arm is derived from D. pseudoobscura bogotana, in combination
with a heterozygous second chromosome (Table 1, line 2).
Generally, we detect few statistically significant differences in fertility among the
backcross progeny classes of the same genotype derived from different strains of D. pseudoobscura
bogotana. Among the different genotypes, we observe very similar reductions in fertility when
particular D. persimilis inverted regions are introduced. Although a few genotypes exhibited
statistically significant variation in hybrid sterility effects among strains (Table 1, lines 2), the
difference is in the magnitude of associated hybrid sterility rather than the absence or presence of
23
an effect on hybrid sterility.
Although the crosses from Brown et al (2004b) used the same strain of D. persimilis, but
varied the strains of D. pseudoobscura bogotana, we performed an analogous cross using the D.
pseudoobscura bogotana ER white strain for background but varied the D. persimilis strain from
which the inverted regions were derived (strains MSH1993, MSH1, MSH3 and SCI). Only white
males were selected, so we surveyed backcross hybrid males with an XL inversion derived from
D. pseudoobscura bogotana. These results are presented in Table 2. Again, we observed a
noticeable reduction in fertility when the XR chromosome arm is derived from D. pseudoobscura
bogotana but the second chromosome is heterozygous for D. persimilis (Table 2, line 2). We
detected statistically significant differences in fertility among the backcross hybrids from
different strains when the XR chromosome arm was derived from D. persimilis (Table 2, lines 3-4),
Table 1: Variation in percent fertility between lines of D. pseudoobscura bogotana at
each inversion genotype. For each cross, D. persimilis MSH1993 was backcrossed to each of
three D. pseudoobscura bogotana strains (described in Brown et al (2004b) and the percent
fertility at each genotype is listed.
1The p-value listed in this column represents statistical significance of the difference
between the percent fertilities of only crosses (D. pseudoobscura bogotana Susa6 and
Sutatausa5.
2The P-value listed in this column represents statistical significance of the difference
between the percent fertilities of all three crosses (D. pseudoobscura bogotana Susa6,
Sutatausa5, and ER white). Both types of analyses were performed using a G-test (see
Methods). “bog”: D. pseudoobscura bogotana allele, “per”: D. persimilis allele, and “het”:
heterozygous for both a D. pseudoobscura bogotana and D. persimilis alleles.
24
25
Table 2: Variation in percent fertility between lines of D. persimilis at the inversions. For each cross, a different strain
of D. persimilis was backcrossed to D. pseudoobscura bogotana ER white and the percent fertility at each genotype is
listed. The p-value listed represents statistical significance of the difference between the percent fertilities of each
cross, and was calculated using a G-test (see Methods). "bog": D. pseudoobscura bogotana allele, "per": D. persimilis
allele, and "het": heterozygous for both D. pseudoobscura bogotana and D. persimilis alleles.
but again, the differences are in the magnitude of associated hybrid sterility rather than the
absence or presence of an effect on hybrid sterility.
In summary, we detect generally small, though statistically significant, variance among
strains of D. pseudoobscura bogotana and D. persimilis in hybrid sterility effects associated with
inverted regions.
2.3.2 Variation among strains in hybrid sterility effect of collinear regions
Chang and Noor (2007) identified three D. persimilis autosomal QTLs outside the fixed
inversions contributing to hybrid sterility when in a D. pseudoobscura bogotana background. Using
the MSH1993 line of D. persimilis, we tested the effects of one of the three collinear QTLs using
backcrosses involving three other lines of D. persimilis: MSH1, MSH3 and SCI, into the D.
pseudoobscura bogotana background. The collinear QTL tested is located on chromosomes 2 (Q2),
where Q2 is 1.42 Mb long. The QTL on chromosome 3 (Q3) was located within an inversion
polymorphic within the species, rendering fine-mapping this QTL impossible, and it is not
examined here. The third QTL is located on the fourth chromosome (Q4) and is not tested
because of its much weaker effect on hybrid sterility compared to Q2 (Chang and Noor 2007) .
Following Chang and Noor (2007), we focused on backcross hybrid individuals bearing the left
and right arm inversions of the X chromosome (XL and XR) and the second chromosome
inversion derived from D. pseudoobscura bogotana. The effects of each collinear QTL in each of the
three additional lines tested, including the original, D. persimilis MSH1993 is presented in Table 3.
Each QTL is represented by two flanking markers (listed in appendix A). As expected, having
both QTLs with D. persimilis genotypes in a D. pseudoobscura bogotana background strongly
26
27
Table 3: Variation in percent fertility between lines of D. persimilis at the collinear QTLs. Each line of D.
persimilis was backcrossed to D. pseudoobscura bogotana ER white, and the percent fertility at each genotype is listed for
each cross. The p-value listed represents statistical significance of the difference between the percent fertilities of each
cross, and was calculated using a G-test (see Methods). "bog": D. pseudoobscura bogotana allele, "per": D. persimilis allele,
and "het": heterozygous for both D. pseudoobscura bogotana and D. persimilis alleles.
reduces fertility in all lines (Table 3, line 4; appendix B, line 8). The very slight reduction in
fertility in the presence of a D. persimilis Q4 by itself (Table 3, line 2) confirms the observation that
this foreign allele can be made homozygous and maintained in a D. pseudoobscura bogotana
background (Chang and Noor 2010). The slight effect of a D. persimilis Q4 is amplified when in
combination with a D. persimilis Q2, and is consistent with the epistatic effects discussed in
Chang and Noor (2010). As with our other analysis of the inverted regions, the fertility of
backcross hybrid males bearing particular genotypes are not significantly different across the
strains surveyed.
2.3.3 Allelism in collinear regions of D. pseudoobscura USA and D.
persimilis
The results from the experiments described above demonstrate only some quantitative
within-species variation in the hybrid sterility effects of particular QTLs. Both D. pseudoobscura
USA and D. persimilis form sterile hybrid males when crossed with D. pseudoobscura bogotana, and
work reported here and elsewhere (e.g., Orr and Irving 2001) has shown that dominant, collinear,
autosomal QTLs contribute to this sterility. Many studies have shown evidence for gene
exchange between D. pseudoobscura USA and D. persimilis in such collinear regions (Machado et
al. 2007; Noor et al. 2007; Kulathinal et al. 2009), suggesting the hybrid-sterility-conferring allele
may have even spread from between the two species. We tested for allelism between D.
pseudoobscura USA and D. persimilis at one collinear QTL (Q2) that confers sterility in a D.
pseudoobscura bogotana genetic background. Q3 was not introgressed because of its linkage to an
inversion, and Q4 is demonstrated a much weaker effect on hybrid sterility between D. persimilis
and D. pseudoobscura bogotana (Chang and Noor 2007). First, we introgressed approximately 1.42
28
Mb of a 2 Mb hybrid sterility QTL on chromosome 2 (Q2) identified in Chang and Noor (2007)
from D. pseudoobscura into D. persimilis. Once made homozygous in D. persimilis, those flies were
backcrossed to D. pseudoobscura bogotana. If hybrid sterility is observed to be associated with this
QTL, D. pseudoobscura USA must contain the same hybrid-sterility-conferring allele as D.
persimilis when in a D. pseudoobscura bogotana genetic background. The presence of the D.
pseudoobscura USA copy of the D. persimilis hybrid sterility QTL on chromosome 2 was
significantly associated with hybrid sterility (Figure 2). We also examined whether the D.
pseudoobscura USA alleles have the same effect on hybrid sterility at the Q2 regions as the D.
persimilis alleles (comparing Figure 2 to Table 3/appendix B). Both the D. persimilis Q2 and D.
pseudoobscura USA Q2 show a greater fertility reduction when in combination with either a D.
persimilis Q3 or Q4, and an even stronger reduction when all three QTLs are heterozygous.
Figure 2: Percent fertility of individuals with a D. pseudoobscura USA copy of QTL 2.
Genotypes for each QTL (Q2, Q3, and Q4) are listed vertically. Sample sizes for each genotype
combination are listed below the genotypes. "bog": D. pseudoobscura bogotana allele,
"per/bog": heterozygous for D. persimilis and D. pseudoobscura bogotana alleles, "ps/bog":
heterozygous for D. pseudoobscura USA and D. pseudoobscura bogotana alleles.
29
The effects of the D. pseudoobscura USA Q2 are indeed similar to the effects of the D. persimilis Q2,
especially when in combination with other QTLs. BCbogw males with a D. pseudoobscura USA
Q2 and two other D. persimilis MSH1993 alleles at Q3 and Q4 exhibit 53.5% fertility (Figure 2). In
Table 3, BCbogw males with D. persimilis MSH1993 alleles at Q2 and Q4 exhibit 35% fertility, and
in appendix B, BCbogw males with D. persimilis MSH1993 alleles at Q2, Q3 and Q4 exhibit 30%
fertility. Although the D. pseudoobscura USA allele at Q2 may not reduce fertility as much as D.
persimilis, the effect of the Q2 marker is still highly significant (p<0.0001) and the epistatic effects
between a D. pseudoobscura USA Q2 and D. persimilis Q3/Q4 are still maintained. Therefore, the
D. pseudoobscura USA allele at Q2 is associated with hybrid sterility in a D. pseudoobscura bogotana
background.
2.4 Discussion
Historically, most genetic studies mapping reproductive isolation have been unreplicated
and make the unstated assumption that the mapped loci are representative across the population
or species. This study tests that assumption in the D. pseudoobscura- D. persimilis system.
Previous studies in this system demonstrate an association of inverted regions of the D. persimilis
genome with hybrid male sterility when crossed to D. pseudoobscura USA (Noor et al. 2001a; Noor
et al. 2001b) or to D. pseudoobscura bogotana (Brown et al. 2004b). In addition, Chang and Noor
(2007) demonstrate an association of collinear regions of D. persimilis with hybrid male sterility
when crossed to D. pseudoobscura bogotana. We test for variation in those regions responsible for
hybrid male sterility, both between lines of the same species as well as between species. We find
that, in this system, effects of both inverted and collinear regions on hybrid male sterility vary
minimally when derived from different strains within species, suggesting little intraspecific
30
variation in their effect on reproductive isolation. We also found a hybrid sterility QTL shared
between D. pseudoobscura USA and D. persimilis when introgressed into D. pseudoobscura bogotana.
These findings are discussed in turn.
2.4.1 Variation in hybrid incompatibilities within species
We found that the same QTLs across the different lines occasionally exhibited statistically
significant variation in effect on hybrid male sterility. However, all regions exhibited statistically
significant effects in the same direction; any variation detected was in magnitude of effect rather
than presence or absence, unlike the differences observed in Case and Willis (2008), Nolte et al.
(2008), Barnwell and Noor (2008), Good et al. (2008), and Sweigart et al. (2007). Further, the
effects of these QTLs from various lines were assayed over long, non-overlapping periods of
time, which may contribute environmental variance. Therefore, a lack of temporal control may
have overestimated differences in effect among lines. However, only three strains of D.
pseudoobscura bogotana and four strains of D. persimilis were used – inclusion of additional strains
might demonstrate greater variation in hybrid sterility effects. Additionally, the effects of the
inverted regions were likely averaged across many loci because of the multi-megabase length of
each inversion, which may have underestimated differences in effect by averaging across many
loci. This study further tested the effects of the collinear regions on hybrid male sterility, where
we still detected minimal variation in sterility effects among strains within species.
Testing the presence or absence of effects of localized QTLs across lines within species is
extremely important, yet rarely performed. Variation in reproductive isolation factors has been
demonstrated in Crepis (Hollingshead 1930), Mimulus (Case and Willis 2008; Sweigart et al. 2007)
Drosophila (Patterson 1952, Chapter 10; Reed and Markow 2004), Tribolium (Wade et al. 1994), and
31
mouse (Good et al. 2008). Despite these recent studies showing variation in hybrid
incompatibilities within species, many studies assume fixation of the same hybrid incompatibility
alleles within species. Part of this assumption may be tied with the observation that directional
selection and/or meiotic drive seem to be associated with the fixation of multiple cloned hybrid
incompatibility genes (Barbash et al. 2004; Ting et al. 1998; Phadnis and Orr 2009; but see Masly et
al. 2006; Presgraves et al. 2003).
Several factors may contribute to the amount of segregating variation in factors
contributing to hybrid incompatibilities. These may include divergence time, amount of
hybridization, amount of standing molecular genetic variation, effective population size, or
disadvantage as a homozygote. In the case of D. pseudoobscura bogotana and D. persimilis, the
divergence time is roughly 0.5-1.0 million years (Aquadro et al. 1991; Wang et al. 1997; Leman et
al. 2005; Hey and Nielsen 2004), which is long enough to observe limited variation in hybrid
incompatibility alleles within species. Also, D. pseudoobscura bogotana and D. persimilis do not
hybridize because they are on different continents, and D. pseudoobscura bogotana has a very small
effective population size and little standing genetic variation (Machado et al. 2002). Given the
long divergence time and low effective population size, this system may be less likely than others
to bear extensive standing genetic variation for hybrid incompatibilities.
2.4.2 Allelism in hybrid incompatibilities between species
A previous study demonstrated the significant effect of D. persimilis Q2 on hybrid male
sterility when in a D. pseudoobscura bogotana background (Chang and Noor 2007), and here we
demonstrate a significant effect of a D. pseudoobscura USA Q2 allele on hybrid male sterility in a
D. pseudoobscura bogotana background. The D. pseudoobscura USA region may contain distinct
32
hybrid-sterility-conferring loci that are not identical to those in D. persimilis. However, such loci
are not ubiquitous: Chang and Noor (2010) introgressed a large collinear region of chromosome 2
between D. pseudoobscura bogotana and D. persimilis and found no effect on hybrid sterility.
Further, our introgression of Q2 interacts precisely the same way as the D. persimilis region with a
D. pseudoobscura bogotana genetic background (although there may be additional interactions
between the D. pseudoobscura USA Q2 and D. pseudoobscura USA Q4 that we were unable to tease
out). That said, the effects of a D. pseudoobscura USA allele at Q2 is perhaps slightly reduced
compared to the effects of a D. persimilis allele at Q2 (see results in Chang and Noor 2010).
Therefore, our results are suggestive of allelism between D. pseudoobscura USA and D. persimilis at
Q2, and the effect of the D. pseudoobscura allele appears to be weaker than the D. persimilis allele,
but multiple linked factors cannot be completely ruled out.
Several scenarios could explain the possible allelism between D. pseudoobscura USA and
D. persimilis at the hybrid-sterility-conferring Q2 locus. The simplest explanation is that this allele
may represent the ancestral form, and a subsequent change within the lineage leading to D.
pseudoobscura bogotana eliminated this effect, hence allowing for the other changes in D.
pseudoobscura bogotana that led to the incompatibility. Serial changes within one lineage resulting
in hybrid lethality have been suggested to have occurred in D. mauritiana (Cattani and Presgraves
2009). If these alleles are ancestral, perhaps the observed variation in strength of Q2 was caused
by functional divergence of the incompatibility alleles in D. persimilis and D. pseudoobscura USA.
Another intriguing possibility, for which there is some limited support, is that the
sterility-conferring allele arose in D. persimilis, and gene flow between D. persimilis and D.
pseudoobscura USA in the collinear regions led to allelism at Q2 after the split of D. pseudoobscura
33
USA and D. pseudoobscura bogotana. These species are known to hybridize in the wild
(Dobzhansky 1973), molecular data suggest interspecies introgression has occurred in or near the
Q2 region (Noor et al. 2007; Machado et al. 2007; Kulathinal et al. 2009), and this introgression
appears to have affected the genetic architecture of hybrid sterility in this system (Brown et al.
2004b; Chang and Noor 2007). Machado et al. (2007) also demonstrated extensive gene flow and
higher variation in collinear regions on the second chromosome (not only near Q2), as well as in
both arms of the X chromosome, which collectively make up approximately 60% of the genome.
The D. persimilis Q2 allele could have introgressed into D. pseudoobscura USA following
hybridization. This explanation is supported by evidence from Kulathinal et al. (2009), who
demonstrated that at least some gene flow occurred in collinear regions between D. pseudoobscura
USA and D. persimilis after the divergence of the two D. pseudoobscura subspecies.
We know of no demonstrations in which introgression between two species led to hybrid
sterility with respect to a third, but a vaguely similar situation where subdivisions within one
lineage (e.g. D. pseudoobscura USA and bogotana) have experienced divergence from each other via
reproductive isolation with a different lineage (e.g. D. persimilis) was observed in green-eyed tree
frogs (Litoria genimaculata). Premating isolation between the northern and southern lineages of
Litoria genimaculata led to divergence not only from each other but also between an isolate of the
southern lineage found in the northern region (Hoskin et al. 2005). To distinguish between a loss
of ancestral Q2 in D. pseudoobscura bogotana and introgression of Q2 from D. persimilis into D.
pseudoobscura USA, the causative loci would need to be cloned and sequenced.
This study has demonstrated that some minor within-species variation in strength of
hybrid male sterility exists within D. persimilis and D. pseudoobscura bogotana; however, even the
34
minor variation we observe is likely overestimated due to environmental variation in
experimental setup. Despite the negative finding here, QTL mapping studies should continue to
test relevant QTLs in multiple lines. We have also determined that a collinear region on
chromosome 2 in D. persimilis is allelic to the same region in D. pseudoobscura USA. This study
raises the question of whether this region is involved not only in speciation between D. persimilis
and D. pseudoobscura bogotana, but also between the two D. pseudoobscura subspecies. If so,
perhaps other regions are more closely related between D. pseudoobscura USA and D. persimilis
than either are to D. pseudoobscura bogotana.
35
3. Association of within-species segregation distortion
in D. persimilis with hybrid sterility between D.
persimilis and D. pseudoobscura
In contrast to the prevailing dogma in the 1990s, recent studies have suggested that an
evolutionary history of segregation distortion within species may contribute to sterility in species
hybrids. However, these recent works identified segregation distortion exclusively in species
hybrids, and thus could not confirm a causal link between segregation distortion exhibited within
species and hybrid sterility. In this work, we expand on previous studies by more directly testing
for an association between an evolutionary history of segregation distortion and hybrid sterility.
We use a strain of Drosophila persimilis exhibiting segregation distortion within species, and we
compare our segregation distortion QTL map to a hybrid sterility QTL map generated from D.
persimilis/D. pseudoobscura backcross hybrid males, to determine co-localization of hybrid sterility
and segregation distortion factors. The maps indicate an overlapping QTL, suggestive of an
association between the two traits. Our work provides preliminary evidence for a true association
between segregation distortion within species and hybrid sterility, supporting additional a
genetic conflict model of speciation.
3.1 Introduction
3.1.1 Sex chromosome segregation distortion, genetic conflict, and
speciation
Diploid taxa typically transmit the two alleles from each locus to their offspring in equal
Mendelian proportions. In contrast, “segregation distortion” (SD) describes the phenomenon
wherein alleles are not found in equal proportions among the offspring. Meiotic drive is a form of
SD characterized by the loss of gametes carrying a certain allele, such that by the end of meiosis,
36
the favored allele is passed on more often (Werren 2011; Sandler and Novitski 1957). Many
Drosophila species exhibit SD of the entire X chromosome in males (reviewed in Jaenike 2001),
but it is sometimes unclear whether this SD occurs during meiosis, and is truly "meiotic" drive
(Policansky 1974; Pardo-Manuel de Villena and Sapienza 2001), so we conservatively refer to this
phenomenon as X chromosome segregation distortion or the “sex-ratio” (SR) trait. The SR trait
appears during either meiosis or after, during sperm maturation (reviewed in Presgraves 2008)
and biases transmission of the male’s X chromosome, causing a higher ratio of female relative to
male progeny. Since the distorter is located on the X chromosome, the resultant females produce
male offspring of which at least half exhibit SR. Continuing the cycle, those SR male offspring
then produce a disproportionately large number of female SR-carrier offspring relative to male
non-carrier offspring. Some SR factors are associated with fertility reduction, so though they are
not selected for, SR factors can rise in frequency quickly due to the transmission advantage alone.
The spread of factors causing segregation distortion, such as SR, can initiate a genetic
conflict, where selfish genetic elements are detrimental to their host or decrease transmission of
other genetic elements, while increasing transmission of themselves (Werren 2011). Due to the
spread of SR X chromosomes, selection will favor other chromosomes that bear loci suppressing
SR, hence recovering their own transmission. After SR suppression, new X chromosome
distorters (or new modifiers that restore the original distorter) that arise will again spread
through the population, reinitiating selection for new suppressors. As these elements jockey for
higher frequency, an arms race is set off – potentially accumulating modifiers that suppress
distortion, as well as modifiers that enhance distortion, and likely increasing divergence between
populations (Carvalho et al. 1997; Hall 2004). A suppressor may be specific to a particular
37
distorter, and therefore may not function in hybrids when confronted with different distorters.
Suppression malfunction in hybrids could lead to defects in chromosomal segregation, for
example, mutual distortion from two unsuppressed homologs may prevent both homologs from
entering gametes, resulting in aneuploidy, gamete failure, and hybrid sterility (Hurst and
Pomiankowski 1991; Frank 1991). Since hybrid sterility prevents gene exchange between taxa,
genetic conflict can possibly contribute to speciation via arms races and the resultant formation of
reproductive incompatibilities.
If this type of genetic conflict leads to speciation, then hybrid sterility between species
should sometimes be associated with segregation distortion (reviewed in: Presgraves 2010;
McDermott and Noor 2010; Johnson 2010). Indeed, studies in Drosophila simulans/D. mauritiana
and D. pseudoobscura USA/D. pseudoobscura Bogota have identified that genes causing hybrid
sterility also cause segregation distortion in hybrids (Tao et al. 2001; Phadnis and Orr 2009).
However, the distorter/sterility alleles from these two studies have only been detected in systems
of “cryptic distortion,” where the distortion was not identified within species – only in species
hybrids. Such cases may suggest an evolutionary history of segregation distortion within species
causing hybrid sterility, or merely a hybrid defect, acting simultaneously with hybrid sterility but
without causation. In contrast, the model system used here, Drosophila persimilis, has alleles
causing SR within species. Observing an association between alleles causing segregation
distortion within species and alleles causing hybrid sterility between species provides a new type
of support for the genetic conflict models of speciation.
3.1.2 Drosophila persimilis "sex-ratio" and hybrid sterility alleles on
the X chromosome
Here, we investigate a strain of Drosophila persimilis which exhibits SR (SR D. persimilis),
38
and carries the distorter(s) within an inversion on the right arm of the X chromosome (XR)
(Sturtevant and Dobzhansky 1936). The XR of SR D. persimilis has a different inversion
arrangement than Standard (non sex-ratio) D. persimilis, but shares the chromosomal
arrangement with Standard D. pseudoobscura (used to map hybrid sterility—see Figure 3). D.
pseudoobscura is the sister species of D. persimilis, and when they hybridize, male hybrid offspring
are sterile, while female hybrid offspring are fertile. Some strains of D. pseudoobscura also exhibit
the SR trait, but those strains carry a chromosomal arrangement that differs from both Standard
D. pseudoobscura and SR D. persimilis (Sturtevant and Dobzhansky 1936). Since the XR of SR D.
persimilis and Standard D. pseudoobscura is homosequential, recombination along the XR can
proceed in hybrid female gametes, unlike in inversion heterozygotes where recombination
products from within the inversion are not recovered in the offspring. Historically, fine-mapping
hybrid sterility loci between D. persimilis and D. pseudoobscura has been difficult because Standard
lines of both species (which are heterozygous for large inversions) are often used. Since hybrid
sterility loci in these lines are located within inversions (Noor et al. 2001a), the only surviving
progeny in Standard hybrids are non-recombinant, making mapping within the inversion
impossible.
Previous studies mapped sterility factors between D. persimilis and D. pseudoobscura
(Noor et al. 2001a; Dobzhansky 1936; Orr 1989; Wu and Beckenbach 1983), and distorters in SR D.
persimilis (Wu and Beckenbach 1983) to the XR chromosome arm. Wu and Beckenbach (1983)
were the first to describe a possible association between segregation distortion and hybrid
sterility in D. persimilis and D. pseudoobscura. They introgressed sections of the SR XR from D.
persimilis into D. pseudoobscura, and observed that certain segments (at least three loci) of the SR
39
Figure 3: Orientation of inversions and location of markers along XR. (A) Arrows
indicate the orientation of the inversion, located from band 27D to band 38 on the D.
pseudoobscura polytene chromosome (images of chromosomes adapted from Figures 8 and 9 of
Schaeffer et al. (2008), with permission from Genetics Society of America). SR D. persimilis
and Standard (nonSR) D. pseudoobscura are homosequential for the XR inversion, while
Standard D. persimilis carries a unique arrangement. (B) The XR of D. pseudoobscura was used
to map the marker locations, given its better coverage and annotation (Schaeffer et al. 2008).
Markers represented only three of the eight scaffolds (group 6, group 8a, and group8b), since
our work focused on the region carrying the inversion. The marker locations are indicated to
scale based on the physical location; the recombination distance (cM) indicated between each
marker is based on our sterility map results.
D. persimilis XR marked by visible mutations resulted in hybrid sterility in a D. pseudoobscura
background. Orr (1989) also observed that the SR XR from D. persimilis increased the proportion
of sterile hybrids. In contrast, when the Standard XR from D. persimilis is crossed into a D.
pseudoobscura background, it reduces the proportion of sterile hybrids (Noor et al. 2001a). This
combination of results provides circumstantial evidence of an association between segregation
distortion and hybrid sterility in these species, associated with factors on the XR chromosome
arm. Here, we expand on Wu and Beckenbach’s work in this system by using more markers to
map the responsible regions.
Our work describes the first direct test of an association between segregation distortion
40
within species and sterility in hybrid males-- an expansion on previous studies. We found
evidence of an association between the two phenotypes and mapped both traits to overlapping
regions on the X chromosome. We speculate that the sex-ratio trait in D. persimilis contributes to
hybrid sterility between D. persimilis and D. pseudoobscura, and therefore contributes to speciation.
3.2 Materials and Methods
3.2.1 Fly stocks
The D. persimilis MSH1993 line (Standard XR chromosome arrangement) was derived
from females collected at Mount Saint Helena, California, in 1993 (Noor 1995). D. persimilis Santa
Cruz Island (SCI) was obtained from the Drosophila Species Stock Center (stock number
14011-0111.50). This line contained both SR and Standard flies – sublines were established of
Standard D. persimilis SCI, and strongly distorting SR D. persimilis SCI. The strongly distorting
subline was created by selecting individuals each generation that produced a strong femalebiased sex ratio. This selection was continued for eleven generations. The strongly distorting
subline was used here for the crosses to map both hybrid sterility and segregation distortion. The
D. pseudoobscura white line was obtained from the Drosophila Species Stock Center (stock number
14011-0121.12).
3.2.2 Fly crosses
For the cross used to map sterility (Figure 4A): D. pseudoobscura white (hereafter psw)
females were collected as virgin adults and maintained for at least six days to reach sexual
maturity and receptivity. After six days, psw females were crossed to SR D. persimilis males from
41
Figure 4: Crossing scheme to map sterility and segregation distortion on the X
chromosome. Long boxes indicate X chromosomes, and short boxes indicate the Y chromosome
(autosomes are not shown). The crosses for sterility (A) and segregation distortion (B) both
start by crossing D. pseudoobscura white (white) to SR D. persimilis SCI (black). F1 hybrid
females are then either backcrossed to D. pseudoobscura white for the sterility cross, or crossed
to D. persimilis MSH1993 (gray) for the distortion cross. The resultant recombinant males are
then backcrossed to their respective paternal species, BCps males from panel A are scored for
fertility, and sex ratios of their progeny are counted.
the strongly distorting subline of D. persimilis SCI. F1 females were backcrossed to psw males to
generate backcross male progeny (hereafter “BCps males”) for fertility assays and preliminary
tests of linkage between sterility and distortion factors. Single pair crosses of BCps males and
psw females were set up, and after 2 days, the male was collected, assayed for fertility, and
genotyped. The female remained in the vial for another 7-8 days to lay eggs. If larvae are present
after a total of 9-10 days, the female would be discarded, and the cross would be kept to later
count the sex ratio of the progeny.
As we were only interested in mapping hybrid sterility factors that reside on the XR
chromosome arm (where distortion factor(s) are located), we sought to limit the flies used for
analysis to those without other regions known to contribute to hybrid male sterility. We limited
the dataset in two ways. First, since the red eye allele is associated with hybrid sterility factors on
42
the D. persimilis XL (Orr 1987; Noor et al. 2001a), we only assayed white-eyed BCps male
progeny, which would bear the fertile XL alleles in a D. pseudoobscura background. Therefore any
observed hybrid sterility can be attributed to factors on the XR without interference from the XL.
Second, prior to scoring markers on the XR chromosome arm, we scored microsatellite markers
on the XL and sterility-conferring 2nd chromosome, and limited the dataset to just those flies
bearing the D. pseudoobscura (non-sterility conferring) allele at both loci (see below).
For the cross used to map segregation distortion (Figure 4B): The F1 females from the
sterility cross were crossed to D. persimilis MSH1993 males to generate backcross male progeny
(hereafter “BCper males”). Red-eyed BCper males were selected to minimize hybrid sterility
effects, since the XL inversion would be of D. persimilis origin. For this cross, all white-eyed
BCper males are fully sterile, thus interfering with mapping segregation distortion, which
requires fertility to observe. Single-pair crosses of BCper males and D. persimilis MSH1993
females were set up, and if larvae were present in the vial after 9-10 days, the male was collected
for genotyping, and the female was discarded. Numbers of male and female progeny were
counted for each fertile cross to obtain sex ratios. All crosses were performed on standard
sugar/yeast/agar medium at 20° ± 1° and 80% relative humidity.
3.2.3 Fertility assays
BCps adult males were collected two days after setting up the cross to psw females, and
maintained alone in food-containing vials. One to three days later, the fertility of each backcross
male was assessed by dissection of the testes in Ringer’s solution following Coyne (1984). A male
was scored as “fertile” if at least one motile sperm was observed and “sterile” if no motile sperm
were observed. Treating fertility as a binary trait is conservative (Campbell and Noor 2001),
43
although other methods of scoring sperm motility exist (Davis and Wu 1996; Reed et al. 2008;
Moehring et al. 2006). All dissected BCps males were labeled and stored at -20°.
3.2.4 Microsatellite genotyping
DNA was isolated from all dissected BCps and BCper males following the Gloor and
Engels (1992) protocol. Microsatellite genotyping was performed in two steps. First, all BCps
males were genotyped for markers associated with each inversion that distinguishes D.
pseudoobscura from D. persimilis. The markers used for this initial screen were DPSX046 (XL),
DPSX014 (XR), and DPS2026 (2). Primer sequences for all markers are available in Appendix C.
Since we are interested in mapping traits located on the XR, two additional XR markers were
added: DPSX063, and DPSX021B2. These markers denoted the species identity of the inversion
arrangement on these chromosome arms. Second, only those BCps males that were hemizygous
or homozygous for the D. pseudoobscura allele at the XL and 2nd chromosome inversion markers
were further genotyped at additional markers on the XR. The markers used on this limited set of
flies were DPSX029, DPSX007, DPSX048, DPSX024, DPSX030, DPSX012, and DPSX062. Locations
of these and the preliminary XR markers are shown in Figure 3B. PCR amplification protocol and
touchdown cycle details are described in Noor et al. (2001a) and Palumbi (1996), respectively.
PCR products were visualized on acrylamide gels on LiCor 4200 and 4300 DNA
sequencer/analyzers.
3.2.5 Data Analysis
To analyze hybrid sterility in BCps males, all males included in the limited set were used
for mapping (a total of 1898 males). All males that produced 20 or greater progeny were also
used for a preliminary test of linkage between sterility and distorter loci. Scoring at least 20
44
progeny enhances detection of moderate segregation distortion, so crosses producing fewer than
20 progeny were not scored for this phenotype. Numbers of females and male progeny from each
BCps male were counted and tested for distortion using a Chi-square test for goodness-of-fit,
comparing the observed sex ratio to an expected 1:1 sex ratio. The number of estimated false
positives was determined using sex ratio data collected from Standard males that produced 20+
progeny, and equals the number of sex ratios (fraction of females) greater than 2 standard
deviations above the mean. To map segregation distortion in BCper males, only males that
produced 15 or greater progeny were used to ensure any segregation distortion would be
detectable.
For BCps and BCper analyses, recombination distances were obtained using OneMap
(Margarido et al. 2007) via R 2.13.1 (Ihaka and Gentleman 1996) (see Figure 3B). Both traits were
mapped using Composite Interval Mapping (CIM) in Windows QTL Cartographer 2.5_009
(Wang et al. 2011), and permuted 1000 times to establish a significance threshold. While CIM
does not account for binary data (e.g. our sterility data), Categorical Trait Multiple Interval
Mapping (CT-MIM) in Windows QTL Cartographer does. However, CT-MIM fails when using
sex-linked and hemizygous data (Shengchu Wang, personal communication), and using CIM to
analyze binary data appears comparable to CT-MIM (Moehring et al. 2004). The BCper analysis
to map segregation distortion used the fraction of the total progeny that are female, and therefore
CIM is appropriate.
3.3 Results
3.3.1 Characterization of D. persimilis SR
Unlike previous reports for homozygous SR D. pseudoobscura (Wallace 1948, Curtsinger
45
and Feldman 1980, Beckenbach 1983, Beckenbach 1996, but see Gebhardt and Anderson 1993),
our homozygous SR D. persimilis females from Santa Cruz Island (SCI) were fertile, and therefore
stocks of SR were easily made. The D. persimilis SCI line from the Drosophila Species Stock
Center was polymorphic for the SR inversion, so we created sublines homozygous for either the
SR or Standard inversion. The subline used in the crosses exhibited an average of 73% female
progeny. The presence of the D. persimilis SR inversion was confirmed by polytene squash, and
genetic markers were used later that differentiated between the inversion types.
3.3.2 Preliminary test of linkage between hybrid sterility and
segregation distortion
Using the results of the hybrid sterility cross, we were able to perform a preliminary test
of our primary question: whether loci causing hybrid sterility and segregation distortion are
linked. Results from this test would approximate the extent of linkage. If hybrid sterility and
segregation distortion are caused by completely linked or the same loci, we should expect that
backcross hybrid (BCps) males containing both loci will be sterile, and segregation distortion as
measured by the sex-ratio of the nonexistent offspring will be undetectable. BCps males will
either inherit both loci and be sterile, or inherit neither locus and be fertile with equal sex ratios.
However, if the loci causing both traits are incompletely linked, recombination should be able to
separate them. In this case, we will also observe sterile BCps males inheriting only the sterility
factor, but also BCps males inheriting only the segregation distortion factor, who will be fertile
with biased sex ratios. Therefore, the presence of biased sex ratios in the progeny of BCps males
serves as a preliminary test of linkage of the two traits. Of the 350 BCps males that produced
progeny, 9 showed sex ratios significantly different from 1:1 (Chi-Square, p<0.05). Of these 9, 1
shows a significant excess of males and 7 have no region from D. persimilis on their XR, indicating
46
that at least these 8 are false positives. Only 1 BCps male showed a significant excess of female
progeny and contained a region of the XR of D. persimilis origin. This BCps male contained D.
persimilis genotypes at markers DPSX014 and DPSX029; however, as described below, our QTL
map of segregation distortion is only marginally significant at those markers. To determine the
predicted number of false positives, we pooled sex ratio data from 216 crosses performed outside
this work, all using a Standard male parent- so no segregation distortion was acting. Sex ratios of
offspring from 9 out of 216 males were greater than 2 standard deviations above the mean (mean
fraction of females: 0.56, SD: 0.103). If 9/216 sex ratios are false positives, we predict 14/350 to be
false positives in our BCps dataset. We cannot exclude that the single female-biased, D. persimilisbearing BCps male is a recombinant between separate segregation distortion- and sterilityconferring factors, but a more parsimonious conclusion is that it is a false positive.
3.3.3 QTL mapping hybrid male sterility between SR D. persimilis SCI
and D. pseudoobscura white
To demonstrate that hybrid male sterility and segregation distortion are under the
control of the same or linked genes, QTL maps of both traits should exhibit peaks at the same
locations. To create the QTL map for hybrid sterility, 5329 BCps males were mated, tested for
fertility, genotyped at 2 initial markers: DPSX046 (chromosome XL), DPS2026 (chromosome 2),
and genotyped at 3 XR markers: DPSX014, DPSX063, and DPSX021B2. Of these 5329, 1898
exhibited D. pseudoobscura genotypes at the XL and second chromosome. This limited set was
genotyped at an additional 7 XR markers at locations shown in Figure 3B. We used CIM in
Windows QTL Cartographer to generate a QTL map, shown in Figure 5, of hybrid sterility
between Drosophila persimilis SCI and Drosophila pseudoobscura white. Our QTL map shows a large
proximal region of no significant effect, followed by one broad significant region with possibly
47
Figure 5: QTL map along XR of hybrid sterility between SR D. persimilis SCI and D.
pseudoobscura white. Map generated using Composite Interval Mapping (CIM) of BCps males
with ps genotypes at XL and second chromosome inversions. Significance threshold (LOD 1.5)
was calculated by 1000 permutations, shown as the horizontal line. Marker locations are
indicated with solid gray arrowheads.
three peaks, and a less significant minor peak near the distal end of the inversion. The broad
region starts at marker DPSX007 and ends at DPSX030, and is approximately 5.23 Mb long. The
second region is between DPSX012 and possibly continues beyond DPSX021B2, and is at least 1.9
Mb long. These regions are too large to identify candidate loci.
3.3.4 QTL Mapping segregation distortion in SR D. persimilis
To create the QTL map for segregation distortion, 10,683 BCper males were mated and
genotyped at the same 10 markers used in the sterility map. Of the 10,683 BCper males, only 109
had over 15 progeny. We used CIM in Windows QTL Cartographer to generate the QTL map for
segregation distortion shown in Figure 6. Between the markers DPSX014 and DPSX029 is a minor
48
Figure 6: QTL map of segregation distortion on the D. persimilis SCI XR. Map
generated using Composite Interval Mapping (CIM) of BCper males producing 15 or greater
progeny. Significance threshold (LOD 1.5) was calculated by 1000 permutations, shown as the
horizontal line. Marker locations are indicated with solid gray arrowheads.
peak that is 3.69 Mb long. However, given that only two markers define that peak, we cannot be
positive of its effect and position. Between the markers DPSX024 and DPSX012 (2.06 Mb) is a
peak of major effect, which overlaps with the broad hybrid sterility peak at both DPSX024 and
DPSX030. Our segregation distortion map also has a third minor peak between DPSX012 and
DPSX021B2, but unlike the peak at this same location in the hybrid sterility map, the peak for
segregation distortion appears to stop before DPSX021B2. Maps of both traits appear to overlap
at two peaks: the major peak in the center of the inversion, and the minor peak distal to the
centromere. The segregation distortion map also appears to contain a third proximal peak
distinct from the hybrid sterility map. The presence of multiple peaks suggests multiple factors
may be required for segregation distortion, as initially observed by Wu and Beckenbach (1983).
49
3.4 Discussion
In this study, we characterized and mapped SR Drosophila persimilis SCI segregation
distortion, and mapped hybrid sterility of this species with its sister species, D. pseudoobscura.
Additionally, we found that these two maps partially overlap, indicating the potential for shared
loci causing both segregation distortion within D. persimilis and hybrid sterility between these
two species.
3.4.1 Linkage of hybrid sterility and segregation distortion factors
We failed to identify clear recombinants (barring one possible case in 350 that may be a
false positive) between XR factors conferring distortion and factors conferring sterility, indicating
that the two traits are caused by the same factor or factors very close together. We cannot
estimate the actual recombination rate without including the sterile individuals, in whom
recombination is undetectable– they could have inherited both sterility and segregation
distortion factors (non-recombinant), or inherited only the sterility factor (recombinant).
Regardless, the segregation distortion and hybrid sterility factors in D. persimilis are closely
linked to each other, raising the possibility that the factors are associated or identical.
The major peaks of both hybrid sterility and segregation distortion maps colocalize,
despite the large width of the sterility map. They also share a minor peak towards the telomere of
the XR, flanking marker DPSX021B2. Combined with the data from the preliminary linkage test,
evidence suggests an association between the two traits. However, the broad peak of the hybrid
sterility map could be hiding multiple factors contributing to hybrid sterility between D.
persimilis and D. pseudoobscura. Multiple peaks in our segregation distortion map also suggest
multiple required factors (Figure 6). A similar finding suggested at least 3 segregation distortion
50
factors on the XR (Wu and Beckenbach 1983).
Using a species like D. persimilis that exhibits segregation distortion within species to
understand the possible link between segregation distortion, hybrid sterility, and speciation
provides for more direct testing than using cryptic hybrid distortion – which may merely be a
hybrid incompatibility itself. This work built on studies done by Tao et al. (2001) and Phadnis
and Orr (2009) by directly testing for an association between the two traits, and expanded on Wu
and Beckenbach (1983) by adding more individuals to map hybrid sterility and more markers for
both traits. Future work could include combining elements of Wu and Beckenbach (1983) and this
work by using SR D. persimilis individuals with visible mutations linked to the QTLs to analyze
individuals more likely to be informative.
The association between hybrid incompatibilities and segregation distortion continues to
strengthen as more genes are discovered that influence both phenotypes: tmy (Tao et al. 2001),
Ovd (Phadnis and Orr 2009), nad6/PPR gene (Case and Willis 2008; Barr and Fishman 2010), and
other hybrid incompatibility genes suggested to be influenced by segregation distortion
(reviewed in Johnson 2010). This association also strengthens the link to speciation, providing
more evidence for Frank’s (1991) and Hurst and Pomiankowski’s (1991) hypothesis that
segregation distortion can cause hybrid sterility in some cases. In further support of their
hypothesis, Bastide et al. (2011) provides evidence that a possible arms race between SR
segregation distorters and suppressors in the D. simulans Paris system is ongoing.
3.4.2 QTL Mapping
Wu and Beckenbach (1983) failed to observe the SR trait in any of their isolated segments
of the SR XR of D. persimilis when in a D. pseudoobscura background, suggesting that the presence
51
of all four segments were required for segregation distortion to be expressed. They mapped a
region causing hybrid sterility linked to the sepia (se) gene, which is located about 0.5 Mb
proximal to our marker DPSX029, which does show an effect on segregation distortion (Figure 6),
but not hybrid sterility (Figure 5). Other work has demonstrated the strong effect of the SR
D.persimilis XR on hybrid sterility with D. pseudoobscura (Orr 1989), while the Standard XR from
D. persimilis reduces the proportion of sterile hybrids (Noor et al. 2001a). These previous studies’
results provide additional circumstantial evidence of an association of factors located on the XR
with segregation distortion and hybrid sterility in these species.
The segregation distortion QTL map would be more precise with a larger sample size;
however, given the success rate of crosses, a larger sample size would be extremely difficult (e.g.
100,000 crosses to generate 1000 individuals with 15 or greater progeny). A lower cutoff value
would increase the sample size, but even with a cutoff value of 5 progeny, the sample size only
increases from 109 to 205. Our cutoff of 15 progeny is within the range used by recent studies:
Phadnis (2011) used 5 progeny, Dyer (2011) used 20 progeny, and Bastide et al. (2011) used 70
progeny (though D. simulans, used by Bastide et al., is more fecund than D. pseudoobscura or D.
persimilis). Using Chi-square goodness-of-fit tests, a significantly biased sex ratio is undetectable
until at least 8 progeny are counted, and even then the p-value is marginally significant
(p=0.0455).
3.4.3 Characterization of segregation distortion in D. persimilis
Despite their overall genetic similarity, D. persimilis and D. pseudoobscura each maintain
distinct segregation distortion systems. The SR XR of D. pseudoobscura contains three
nonoverlapping inversions (Dobzhansky 1939; Beckenbach 1996), while D. persimilis only
52
contains one (Sturtevant and Dobzhansky 1936). D. pseudoobscura females homozygous for the SR
inversion have reduced fertility compared to both Standard homozygotes, and inversion
heterozygotes (Wallace 1948, Curtsinger and Feldman 1980, Beckenbach 1983, Beckenbach 1996,
but see Gebhardt and Anderson 1993). The SR D. persimilis from Santa Cruz Island does not
appear to inflict reduced fertility in female SR inversion homozygotes allowing the strain to be
easily used as a stock.
Critical for this study, SR D. persimilis and ST D. pseudoobscura share the same XR
chromosomal arrangement, allowing mapping of SR factors, unlike SR D. pseudoobscura which
carries unique inversion arrangements, preventing recovery of recombinant progeny. While more
is known about the within-species fertility effects of SR in D. pseudoobscura (Wallace 1948;
Beckenbach 1978; Curtsinger and Feldman 1980; Beckenbach 1981; Wu 1983a; Wu 1983b;
Beckenbach 1996; Price et al. 2008a; Price et al. 2008b; Price et al. 2009), SR D. persimilis actually
serves as a better model system for understanding the genetic basis of SR given that it is
homosequential with the Standard D. pseudoobscura XR-chromosome.
In summary, we characterized and developed lines of SR D. persimilis, mapped
segregation distortion within D. persimilis, and mapped hybrid sterility between D. persimilis and
D. pseudoobscura. Our maps suggest an association between both traits, hinting at a possible
causal relationship. This work expands on previous mapping studies and opens opportunities for
other research using SR D. persimilis. Future work in all SR systems will hopefully provide
answers to the various questions surrounding segregation distortion, including its association
with hybrid sterility and speciation.
53
4. Summary
Understanding the genetic basis of speciation, including hybrid sterility and its possible
causes, is extremely important to the field of evolution. Knowing how species changed in the past
can help biologists predict how species are currently changing or will change in the future. Many
studies of hybrid sterility only use single strains of two species to map QTLs; however, if
variation exists within either species, the hybrid sterility QTLs may not have the broad isolating
effect that authors usually assume. My work here tests a prior finding of hybrid sterility QTLs
using Drosophila persimilis and D. pseudoobscura bogotana. We repeated the experiment using
different strains of D. persimilis to test for variation in regions known to confer hybrid sterility.
We observed little variation at these regions in this system; however, new studies continue to
demonstrate within-species variation for hybrid incompatibilities (Bracewell et al. 2011;
Kozlowska et al. 2011; Turner et al. 2012).
A previous study (Chang and Noor 2007) mapped hybrid sterility QTLs on the second,
third and fourth chromosomes of D. persimilis that act in a D. pseudoobscura bogotana background.
We further tested introgressed D. pseudoobscura USA copies of the second chromosome locus for
hybrid sterility effects in the D. pseudoobscura bogotana background. The Bogota allele
(introgressed into D. persimilis) did confer hybrid sterility in the USA background, suggesting
that the second chromosome QTL is ancestral to D. persimilis and D. pseudoobscura USA and was
lost in the Bogota lineage, or gene flow between D. persimilis and D. pseudoobscura USA can
transfer hybrid incompatibilities with a third taxa. Since this work was published, a new study
(Phadnis 2011) mapped D. pseudoobscura USA hybrid sterility alleles in a D. pseudoobscura bogotana
background. Two of the mapped alleles were in the same position as the second and third
54
chromosome QTLs identified in Chang and Noor (2007). Phadnis (2011) confirmed our
introgression experiment:,the same alleles act in both D. pseudoobscura USA and D. persimilis to
cause sterility in the Bogota subspecies. Phadnis (2011) also mapped the third chromosome QTL,
which we could not do in our introgression experiment, since D. persimilis and D. pseudoobscura
bogotana differ in their third chromosome inversion arrangements, making detecting
recombinants across that region impossible.
The association of meiotic drive with hybrid sterility has a controversial history, but
accumulating work (reviewed in Johnson 2010) supports previous hypotheses that drive can
cause hybrid sterility (Frank 1991; Hurst and Pomiankowski 1991). In the first chapter, we
elaborate on a possible mechanism where chromatin remodeling caused by meiotic drive can, in
turn, lead to hybrid sterility (also reviewed in Sawamura 2012). We argue that changes in
chromatin due to meiotic drive may cause failure of meiotic sex chromosome inactivation
(MSCI), where male X chromosomes are inactivated briefly during spermatogenesis. Such failure
has been known to cause sterility within-species, and therefore perhaps between-species as well
(though defects in MSCI would need to be suppressed within-species for this hypothesis to be
supported). MSCI is known to occur in a variety of organisms (Namekawa and Lee 2009), but
recent work demonstrates the controversy over MSCI in Drosophila (Meiklejohn et al. 2011;
Mikhaylova and Nurminsky 2011; Kemkemer et al. 2011). Meiklejohn et al. (2011) does indicate
that while evidence of MSCI and dosage compensation is absent, they still observe differential
expression on the X chromosome before meiosis, suggestive of some mechanism similar to MSCI
and dosage compensation. Since our hypothesis described in the first chapter was dependent
only on some form of chromatin remodeling caused by meiotic drive and leading to hybrid
55
sterility, our hypothesis is not rejected, but it is unlikely that the causal mechanism is MSCI in
particular.
56
Appendix A
Microsatellite markers used in testing variation and allelism. This list does not include markers
used in previous studies. F: forward primer, R: reverse primer. We attached an M13 sequence
(CACGACGTTGTAAAACGAC) to the 5' end of each forward primer.
Marker
Chromosome Size (bp)
DPSX002
XL
150
DPSX046
XL
119
DPSX030
XR
130
DPS2022
2
138
DPS2026
2
144
DPS2_27.05
2
140
DPS2_2395c
2
103
DPS3001
3
150
DPS4G1a
4
110
DPS34G1h
4
99
DPS4033h
4
106
DPS4611966
4
112
Sequence (F)
attcttgtcgctctgttggc
aaatcgcagcggcattgac
gctaacacacactcgcgcaca
ggcgcaaggtccttttttgt
cagactcttactacgagcacggaga
tgtgtgggtggtttgttttgaaacg
gaactggaaaattgcatggccacc
gggaaaccataagaaaatgcc
atggctgacacagaagcatctgtg
taaagcgcaacccatcgaccatatc
ttctcggtggcacttcataagcc
aaaacgactcacgagcaaag
57
Sequence (R)
tcagctgcgtaacaatctgg
aaatgcagagcaagtacacgcatc
tgcacactgtgatggccaaat
tcccgataccgacgaaacatt
gcaaatatccttgaagcagatgca
ccaataactatgtatgacctgacaaccctg
catgccacacagtgagtggc
gtacatgaatcggctacggg
cttatgcatgaaatgtgcgcataattggtc
agcatccacaaagttgatgcctctc
ctggctggtgttgttgctgc
gagcgctttagcatagggt
Appendix B
Variation in percent fertility between lines of D. persimilis at the collinear QTLs, including Q3.
Each line of D. persimilis was backcrossed to D. pseudoobscura bogotana ER white, and the percent
fertility at each genotype is listed for each cross. The p-value listed represents statistical
significance of the difference between the percent fertilities of each cross, and was calculated
using a G-test (see Methods) or aChi-Square if the G-test failed (see Methods). "bog": D.
pseudoobscura bogotana allele, "per": D. persimilis allele, and "het": heterozygous for both D.
pseudoobscura bogotana and D. persimilis alleles. "nc": not able to be calculated.
BCbog (MSH1993)
BCbog (MSH1)
BCbog (MSH3)
BCbog (SCI)
Line
Genotype
% Fertility Sample Size % Fertility Sample Size % Fertility Sample Size % Fertility Sample Size
1
Q2=bog, Q3=bog, Q4=bog
87%
86
90%
31
88%
17
84%
38
0.898
2
Q2=bog, Q3=bog, Q4=het
73%
95
100%
33
93%
15
78%
41
3
Q2=bog, Q3=het, Q4=bog
93%
103
74%
43
71%
21
100%
7
0.004
nc
4
Q2=bog, Q3=het, Q4=het
67%
93
49%
47
67%
24
60%
10
0.222
5
Q2=het, Q3=bog, Q4=bog
65%
67
83%
35
100%
25
52%
21
6
Q2=het, Q3=bog, Q4=het
33%
60
50%
34
53%
17
55%
31
0.0004
0.149
7
Q2=het, Q3=het, Q4=bog
65%
74
38%
42
41%
22
67%
6
0.021
8
Q2=het, Q3=het, Q4=het
30%
69
22%
41
24%
29
14%
7
0.645
58
p-value
a
a
Appendix C
Microsatellite markers used in mapping segregation distortion within D. persimilis SCI and
hybrid sterility between D. persimilis SCI and D. pseudoobscura white. F: forward primer, R:
reverse primer. We attached an M13 sequence (CACGACGTTGTAAAACGAC) to the 5' end of
each forward primer.
Marker Chromosome Size (bp)
DPSX046
XL
119
DPS2026
2
144
DPSX014
XR
118
DPSX029
XR
132
DPSX007
XR
150
DPSX048
XR
121
DPSX063
XR
100
DPSX024
XR
199
DPSX030
XR
130
DPSX012
XR
112
DPSX062
XR
106
DPSX021B2
XR
180
Sequence (F)
aaatcgcagcggcattgac
cagactcttactacgagcacggaga
atgtgtatctgtgcatgtgca
actgtgtgcctgggtgaacct
cactcgaggttattgaacgg
ggaaatgattcagctgctggg
gctctgctctggacagcc
tttgtgaggcagcagcagc
gctaacacacactcgcgcaca
tatgtccctgtgtgcgtgtgt
cagagacagccccaaagaga
gagctaagccgatttcctccct
59
Sequence (R)
aaatgcagagcaagtacacgcatc
gcaaatatccttgaagcagatgca
actccacacccaaggaacaat
gcccagctgagctttcagctt
aatctatggcgggttctaag
ccgagctaatcaaattaccagacg
tgcgttgcctgataaaacct
ttcgtcctccatcctcattcg
tgcacactgtgatggccaaat
acagcacttgcttttgctga
ttagtggcacaaacagacgg
tgccaacaacagacagccga
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Biography
Shannon McDermott was born in West Islip, NY on August 21, 1985. She attended Cedar
Crest College from 2003-2007, and earned her Bachelor’s of Science in Genetic Engineering. After
graduation, she started her Ph.D. at Duke University (2007-2012). She published several papers,
one of which as an undergraduate (“Estimation of isolation times of the island species in the
Drosophila simulans complex from multilocus DNA sequence data”). She also published three
papers during graduate school, “The role of meiotic drive in hybrid male sterility,” “Genetics of
hybrid male sterility among strains and species of the Drosophila pseudoobscura species group,”
and “Hybrid Sterility.” A fourth paper, “Association of within-species segregation distortion in
D. persimilis with hybrid sterility between D. persimilis and D. pseudoobscura” has been submitted
and is awaiting review. While at Duke, she was awarded two fellowships (Marcy Speer Memorial
Fellowship, and Biology Department Fellowship), and the Dean’s Award for Excellence in
Teaching. She has received two research grants: a Grant-in-Aid of Research awarded by Sigma
Xi, and an REU supplement for an undergraduate mentee awarded by the National Science
Foundation. She has also received travel/tuition grants to attend the Summer Institute in
Statistical Genetics, and the Evolutionary Biology Workshop in the Alps.
79