Download Mollie K. Manier: Evolution Faculty Search

Mollie K. Manier: Evolution Search
STATEMENT OF CURRENT AND FUTURE RESEARCH PLANS
My research explores the causes and consequences of adaptive divergence in reproductive
traits with a focus on the functional significance of morphological variation. My research
program asks (1) if microevolution of sperm competition mechanisms within species can predict
interspecific incompatibilities in hybrid crosses on a macroevolutionary scale, (2) how the
extraordinary diversity in sperm form relates to sperm function as well as male and female
fitness, (3) what is the genetic basis for sperm length diversity and the parallel evolution of
giant sperm in Drosophila, (4) what is the genetic architecture of sperm competition traits, and
(5) what is the genetic basis of sperm competition traits.
My study system exploits the remarkable sperm length diversity in Drosophila, ranging from the
modest D. persimilis (0.32 mm) to the record-holding D. bifurca (5.8 cm). Evolution of giant
sperm is thought to be driven by post-copulatory sexual selection, but our understanding of
what they do during sperm competition has been hampered by our inability to distinguish the
sperm of different males inside the female reproductive tract. Our recent development of
transgenic Drosophila with GFP- or RFP-labeled sperm heads has transformed our view of
sperm biology and allows us to test previously untestable hypotheses about sexual selection
and evolution of reproductive traits. Transformations of several closely-related species allow
these questions to be addressed within a comparative framework.
In my current research, I am using three different sets of experimental lines of Drosophila,
which will organize my projects and goals in this research statement. The first are GFP and RFP
lines of D. melanogaster, D. simulans and D. mauritiana (“GFP/RFP Stock Lines”), which I have
used to compare sperm precedence mechanisms within species with events during conspecific
sperm precedence between species. The second set of lines (“Selection lines”) have been
artificially selected for long or short sperm and long or short female sperm storage organs
(seminal receptacle or SR). Both of the sperm lines have been transformed to express GFP or
RFP in sperm heads. The third set (“Isofemale lines”) is comprised of 100 moderately inbred isofemale lines generated from the D. melanogaster GFP and RFP stocks that have been
phenotyped for a number of ejaculate and sperm precedence-related traits.
GFP/RFP Stock Lines
Project One: Do mechanisms of sperm competition within species play a role in conspecific
sperm precedence in hybrid matings? An important and unresolved question in evolutionary
biology is the degree to which macroevolutionary processes involved in speciation are a
function of microevolutionary patterns within species. Intraspecific variation in sperm length
and SR length has been shown to influence sperm competition in D. melanogaster, and postcopulatory sexual selection may have played an important role in species divergence in the
Drosophila lineage. In D. melanogaster (Manier et al. 2010. Science), I have resolved detailed
mechanisms of sperm precedence that provide a backdrop in which to ask if divergence in
sperm competition patterns between closely-related species can predict mechanisms of postmating pre-zygotic reproductive isolation. Drosophila simulans and D. mauritiana diverged
~300,000 years ago and have developed conspecific sperm precedence (CSP) in which hybrid
single matings are highly successful but double matings involving both a conspecific and
heterospecific male result primarily in conspecific-sired progeny.
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Sperm transferred
Mollie K. Manier: Evolution Search
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Proportion displaced
s x s,s
s x m,m
s x s,m
s x m,s
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0.8
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s x s,s s x m,m s x s,m s x m,s
Min to ejection
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I have found that D. simulans males transfer fewer sperm if a
conspecific female was previously mated to a D. mauritiana
male. Despite a smaller ejaculate size, D. simulans males are
much better at displacing heterospecific sperm than conspecific
sperm from storage. This result can be explained by interspecific
differences in ejaculate traits, because D. simulans sperm are
longer and swim faster, which we found to increase paternity
success in D. melanogaster (using Isofemale lines described
below). Furthermore, D. simulans males transfer fewer sperm to
virgin than to mated conspecific females, demonstrating that
ejaculate tailoring occurs within species as well as in hybrid
crosses. Female D. simulans eject heterospecific sperm faster if
previously mated to a conspecific male, suggesting a role for
sequential mate preference and cryptic female choice in CSP. We
predict that female ejection should play a role in cryptic female
choice against low quality males within species as well. At least
three publications from this work will be submitted Fall 2011.
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Selection lines
Project 2: How does sperm form relate to sperm function within
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and between species? Virtually nothing is known about how
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s x s,s s x m,m s x s,m s x m,s
sperm motility and behavior are affected by sperm morphology
Fig. 1. Sperm transfer, percent
and female reproductive tract morphology, although these two
sperm displaced, and timing of
characters are tightly correlated across species in many diverse
ejection in D. simulans (red) and
taxa. I am using D. melanogaster populations previously selected
hybrid matings. ♀×♂1♂2,
for long or short sperm and long or short SR’s (four distinct lines)
s=simulans, m=mauritiana
to examine how sperm length and SR length interact to affect sperm motility and behavior.
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I will also compare sperm motility and behavior across our transgenic lines of D.
pseudoosbscura, D. mauritiana, D. simulans, D. melanogaster, and D. bifurca, with sperm
lengths ranging from 360 to 58,000 μm and corresponding SR lengths from 410 to 80,000 μm.
This comparative approach will allow me to characterize how sperm and SR lengths influence
sperm performance across species that encompass the gamut of diversity in these two
reproductive traits within Drosophila. This experiment will be conducted Fall 2011.
Project 3: What genes control sperm morphology? A large part of understanding how giant
sperm have evolved within Drosophila is knowing how variation in genotype relates to variation
in phenotype. However, very little is known about the molecular basis for natural variation in
sperm morphology or the cellular processes involved in sperm morphogenesis. I have applied a
RAD (restriction-site associated DNA) Illumina QTL mapping approach using the sperm length
selection lines described above to identify genes/genomic regions associated with sperm
length. I will then estimate rates of divergence for candidate loci and determine the molecular
basis of parallel evolution of giant sperm across the Drosophila lineage. I am a Co-PI on an NSF
grant that was funded to do this work in collaboration with Scott Pitnick (Syracuse University),
John Belote (Syracuse University) and José Andrés (University of Saskatchewan). Data will be
delivered Fall 2011, and a follow-up NSF pre-proposal will be submitted January 2012.
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Mollie K. Manier: Evolution Search
Isofemale lines
Project 4: What is the genetic architecture of ejaculate traits and their fitness consequences
during sperm competition? To understand and predict the evolutionary response of sperm traits
to sexual selection, it is important quantify their phenotypic and genetic variances and
covariances within the natural selective environment of the female reproductive tract. Our lab
has generated a set of 100 moderately inbred isofemale lines that have been phenotyped
across multiple generations for sperm length, velocity, ejaculate size, sperm storage patterns,
displacement, female ejection, and paternity success. We found significant repeatability and a
strong genetic signal for each of these traits, both within lines and across generations. We will
assess male- and female-mediated genetic variation and covariation in these sperm
competition traits and determine the extent of male x female and male x male x female
interactions on these traits. Finally, we will measure condition-dependence of these traits. I am
a co-PI on an NSF grant proposal for this project in collaboration with Scott Pitnick, Stefan
Lüpold, and John Belote at Syracuse University that is currently pending. This project will be
conducted over the next three years.
Project 5: What is the genetic basis of sperm competition traits? We have established that all of
the sperm competition traits measured in the isofemale lines have a significant genetic
component. We will fully sequence all isofemale lines and use whole genome association
mapping on the suite of sperm competition traits to identify loci that are associated with
ejaculate traits like sperm morphology (complementing Project 3), velocity and motility, and
competitive success. We will additionally obtain the genetic basis of heritable patterns of sperm
storage, displacement, and ejection. Finally, we will be able to examine sequence variation in
accessory gland proteins and their association with sperm competition traits. These data will
supplement an ongoing collaboration with Tracey Chapman (University of East Anglia) to
quantify levels of sex peptide in these lines. These data will generate years of further work
following up candidate genes with transcriptional and proteomic studies as well as genetic
manipulations. This project will be the subject of our next grant proposal to the NIH.
Summary
My research investigates the evolution of reproductive traits along three complimentary tracks.
The first uses the GFP/RFP Stock Lines to ask what are the patterns of sperm precedence
mechanisms within species, and how can they inform the evolution of reproductive isolating
mechanisms between species? The second uses the Selection Lines to investigate the functional
significance of sperm length variation in Drosophila and its genetic basis. The third track uses
the iso-female lines to investigate the genetic architecture and genomics of ejaculate
characteristics and their fitness consequences during sperm competition. My extensive
research program will mesh well with the Department of Ecology and Evolutionary Biology at
the University of Michigan by continuing to shed new light on the evolution of sexually selected
traits within functional, genetic, and comparative frameworks. I anticipate fruitful
collaborations with a number of research groups, including those of Patricia Wittkopp, Lacey
Knowles, and Alex Kondrashov.
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