Download lecture 17 - post-zygotic + hybrids - Cal State LA

Post-zygotic reproductive isolation
Post-zygotic isolation results when hybrid offspring are:
- inviable (fail to develop at all)
- viable, but infertile (one or both sexes cannot reproduce)
Hybrid breakdown is the term for infertility in viable hybrids
- many hybrids, like mules, are viable but cannot reproduce
- often results from male sterility (failure to make sperm, etc)
- hybrids can sometimes backcross with a parent species
Post-zygotic isolation: hybrid breakdown
1st generation (F1) hybrids are often viable (= they live just fine)
because they have inherited one co-adapted gene complex
from each set of parental chromosomes
maternal proteins all fit
together and interact
paternal proteins all fit
together and interact
Post-zygotic isolation: hybrid breakdown
When hybrid offspring reproduce, independent assortment
assures that they will not pass on co-adapted gene complexes
- their offspring get a mix of genes that are not adapted to fit
together, so they don’t form
working protein complexes
Result: hybrids cannot
produce F2 offspring if their
parents were too distantly
related
Post-zygotic isolation: Haldane’s Rule
Haldane’s Rule: usually the heterogametic sex is sterile or
inviable in the F1 generation of hybrids
- XY for mammals, flies (male)
- WZ for birds, butterflies (female)
Female mammals have one complete set of genes from
maternal X, and one from paternal X chromosome
- can produce needed proteins to interact with rest of
maternal and paternal chromosomes
Males only have an X from mom; cannot supply X-linked gene
products needed to interact with rest of dad’s genome
- are unable to make reproductive protein complexes
needed for spermatogenesis (sperm-making)
Post-zygotic isolation & Positive selection
Molecular data indicate that genes under positive selection
are commonly involved in post-zygotic isolation
- genes that have partner-proteins, and rapidly evolve
changes in their respective amino acid sequences, cause
problems when species hybridize
- the protein-partners of different species don’t fit together
Post-zygotic isolation & Positive selection
Example: the Nup160 gene codes for part of the Drosophila
nuclear pore protein complex
- Nup96 and Nup160 alleles from D. simulans are lethal when
combined with some X-linked gene in D. melanogaster
- both genes evolved under strong positive selection, and
are now so different from D. melanogaster alleles
that they are fatal when combined with their partner proteins
from melanogaster
Viable hybrids as a source of genetic variation
In some cases, hybrids may produce offspring that are fertile, at
least when they backcross with one of their parental species
- hybrids may have same or even higher fitness than parents
(if alleles interact favorably in a certain combination, or
ecological circumstances are juuuust right)
- documented among plants, birds
Sometimes, key alleles can be introduced to a species through
hybridization + back-crossing to one of the parental species
- introgression: alleles move from one species into another
- documented in Darwin’s finches
Introgressive hybridization in Darwin’s finches
Rare but important hybridization events have been documented
during a 30-year study of Darwin’s finches (several species)
One case: 2 small G. fortis females mated with scandens males
- male offspring imprinted on paternal songs, bred back into
scandens population (which was only ~200 birds)
Introgression resulted because hybrids were able to breed
back into one of the parental populations
- re-introduced alleles that had been lost from G. scandens
during the speciation process
why were male hybrids able to backcross in these birds?..
Introgressive hybridization in Darwin’s finches
Because fortis is much smaller, the mean beak size of the
scandens population went way down in the following years
- introduction of alleles from fortis strongly affected the genetic
makeup of the scandens population
1972
2001
White bars show the large negative change in beak size, due to
hybrid offspring in the population
Hybridization and species formation
In some plants, hybrids become their own species, genetically
distinct from both parents and viable
In animals, hybridization may result in what appear to be
independent species
- however, if the hybrids cannot produce offspring among
themselves, they are not a true species in their own right
Sometimes hybrids are mistaken for a true species due to a
distinct morphology, different from either parent species
Hybridization and coral reef diversity
As many as 105 coral species may simultaneously spawn on a
single night over the same reef
- despite obvious potential for hybridization, species remain
distinct (no blurring of species boundaries)
Are morphologically different corals really species, or just
unrecognized hybrids?
Vollmer and Palumbi studied 3 corals in the genus Acropora
A. cervicornis, which lives on the back reef
A. palmata, which lives on wave-swept reef tops
sibling
species
A. prolifera, which has two growth forms: bushy and palmate
3 overlapping species of Acropora coral
A. cervicornis
“bushy” morph
A. palmata
“palmate” morph
the two growth morphs of A. prolifera
Hybridization and coral reef diversity
A. cervicornis
A. palmata
A. prolifera
Different alleles fixed between cervicornis and palmata
prolifera were all heterozygotes  they were...?
Hybridization and coral reef diversity
Mitochondria are only inherited from your mother
- mitochondrial DNA haplotypes are maternally inherited
“Bushy” morphs of A. prolifera all had mitochondrial DNA
sequences that matched A. palmate sequences
- thus, bushy prolifera corals are hybrids where the egg came
from A. palmata colony and the sperm from A. cervicornis
“Palmate” morphs of A. prolifera all had mitochondrial DNA
sequences from A. cervicornis
- the reverse case: palmate prolifera are hybrids where
egg came from cervicornis and the sperm from palmata
Hybridization and coral reef diversity
Implications:
(1) Because corals are colonies that grow new polyps
asexually, prolifera colonies may be “immortal mules”
- ecological persistence of diverse morphologies
(2) Coral reef “diversity” may actually reflect the presence of
long-lived hybrids that have little evolutionary potential
(3) Provides the opportunity for rare backcrossing with one of
the two parent species

Hybrids and the Case of the Red Wolf
Conservation effort was underway
to save the red wolf
Encroaching coyotes were breeding with
wild red wolves, and the species was
disappearing
14 remaining individuals were caught and
a captive breeding program established
Then, geneticists at UCLA proved that
the red wolf was actually a hybrid of grey wolves and coyotes
- not a distinct species at all !
Hybrid Speciation
Although common in plants, the formation of new species via
hybridization is rare among animals
For hybrid speciation to occur, hybrids must:
1) be viable and fertile
2) occupy a hybrid-specific ecological niche
- otherwise, will be out-competed by a better-adapted
parental species
3) quickly become reproductively isolated
- otherwise, just serve as a vehicle for introgression between
two parental species; won’t become their own lineage
Hybrid speciation in Lycaeides butterflies
2 butterfly species occur on either side
of Sierras - east, Lycaeides melissa;
west, L. idas
- species are sympatric across part
of their ranges, where they live
mid-way up the mountains
- 3rd form lives on top of mountains,
above the tree line: “alpine” form
Gompert et al. Science 2006
Hybrid speciation in Lycaeides butterflies
genetic studies revealed alpine butterflies are a distinct species
with a mixed genome - a combination of L. melissa and L. idas
Two species of butterflies occur on either side of the Sierras east, Lycaeides melissa; west, L. idas
- genetic data: alpine butterflies are a
distinct species with a mixed genome,
combining alleles of L. melissa + L. idas
 alpine form evolved from a hybrid
of the 2 parent species
- alpine species no longer exchanges
alleles with either parent species
- reproductive isolation comes from at
least 4 different sources...
A) alpine species is highly loyal to one host plant; others aren’t
B) alpines lay non-sticky eggs that fall off leaves
of host plant; other species glue eggs to leaves
- on mountaintops, winds blow leaves away in winter so the
non-glueing strategy is adaptive (gluers have low fitness)
C) alpine butterflies have different color patterns on their
wings used in mate signaling
D) differences in male genital morphology
Case 2: Hybrid speciation by host-shift in Rhagoletis
- new fly race found on introduced honeysuckle plant
- genetically a hybrid, with a mixture of alleles that are fixed in
two parental races (blueberry + snowberry flies)
- honeysuckle flies are not F1 hybrids - no excess heterozygotes
R. mendax
blueberry
maggot fly
Schwarz et al.
Nature 2005
honeysuckle
fly
R. zephyria
snowberry
maggot fly
Speciation by hybridization.. plus some
Among Darwin’s finches on island Daphne Major...
- immigration from other islands is rare:
-- 2 G. fortis and 1 hybrid over 18 yr
-- 1 G. scandens over 24 yr
- hybridization is rare: 13 fortis-scandens hybrids in 21 yr
Mating choice is based on 2 things:
- song that males sing; learned culturally from their dad
- beak shape and size
In 1981, a hybrid immigrant arrived on Daphne and mated
with a resident female, daughter of a hybrid-fortis backcross
- both parents carried some scandens alleles
- immigrant male was unusually large (had a big beak)
- immigrant male sang his own weird song that didn’t match
any existing finch songs
- he came up with a strange, personal variant not sung
by either species on any island...
Grant & Grant, 2009, Proc Nat Acad Sci USA
In 1981, a hybrid immigrant arrived on Daphne and mated
with a resident female, daughter of a hybrid-fortis backcross
Only 2 of their great-great-grandkids survived a 2004 drought
- boy and girl, who mated and started a highly inbred
family line that only mated within itself for 3 generations
(and counting..)
This lineage is reproductively isolated from native G. fortis by
two forms of pre-zygotic reproductive isolation
- unique song sons learn from dad, to which girls respond
- unusually large beaks, which make them look different
from native fortis – a form of size-assortative mating
typical fortis
immigrant lineage
females of immigrant lineage prefer males who sing the funky song
of their great-great-granddad, and who have freakishly big beaks
Grant & Grant, 2009, Proc Nat Acad Sci USA
Summary: evolutionary importance of hybrids
Hybrid lineage persists and is still reproductively isolated –
case of incipient speciation due to hybridization, plus...
- hybrid had exceptional size, due to scandens alleles
(a bigger species – result of natural selection)
- by immigrating his first year, that hybrid evidently came up
with a screwy song all of his own (random event)
- song transmission from father to son  new song lasted as
long as some females responded to it (sexual selection)
- following drought, only 2 large-beaked descendents left on
island, who were siblings (bottleneck + inbreeding)
Summary: evolutionary importance of hybrids
(1) When hybrids have lower fitness than either parent,
selection will favor pre-zygotic isolation to prevent
hybridization
- parents who don’t produce low-fitness offspring have the
highest fitness
(2) Hybrids are often viable but sterile, owing to breakdown
of co-adapted gene complexes in the F2 generation
(3) Persistent hybridization or long-lived hybrids may cause
intermediate phenotypes mistaken for distinct species
(4) In hybrid introgression, alleles move between species
