Transcript lecture 16 - reproductive isolation - Cal State LA

Reproductive isolation
Reproductive isolation can be:
(1) pre-zygotic (before sperm and egg fuse)
 usually governed by mating behavior
(2) post-zygotic: sperm meets egg, but things go wrong
a) complete: embryo fails to develop
b) incomplete: embryo develops, but is sterile
(at least in one sex)
Reproductive isolation
Reproductive isolation can evolve in two ways:
(1) by genetic drift, gradually + randomly fixing different traits
controlling aspects of mating behavior (pre-zygotic) or
genetic compatibility (post-zygotic)
 not adaptive; happens over time as different alleles fix
(2) as a response to selection against hybrids, often between
populations or species in ecologically different habitats
 adaptive:
Selection against hybrids
Hybrids that have intermediate phenotypes will not be as fit in
either habitat
forest-adapted
specialists
hybrids,
not fit in either
environment
desert-adapted
specialists
Selection against hybrids
Reproductive isolation often evolves very rapidly between
populations adapted to ecologically different habitats
Hybrids have intermediate phenotypes and thus lower fitness
in both habitats, compared to their specialized parents
Selection against hybrids will thus favor traits that contribute to
assortative mating (this is termed reinforcement)
- Why? Because individuals that do not produce hybrid
offspring will have higher fitness (their kids do better)
- therefore, traits that favor assortative mating will confer a
fitness advantage
- any trait that stops you from hybridizing is good
Pre-mating isolation is likely to be favored by selection when
recently diverged species encounter each other
- maybe a barrier has disappeared and species now overlap
- maybe they speciated in sympatry to begin with
As predicted, sympatric pairs of Drosophila species have
higher levels of pre-zygotic isolation than allopatric pairs
amount of
pre-zygotic
isolation
Allopatric
pairs
relatedness
Sympatric
pairs
amount of
pre-zygotic
isolation
Allopatric
pairs
genetic difference
Sympatric
pairs
genetic difference
“Sympatric” pairs live in the same place NOW – they did
not evolve in sympatry (at least, not that we know)
-
amount of
pre-zygotic
isolation
Allopatric
pairs
genetic difference
Sympatric
pairs
genetic difference
The issue is, what does selection do when two distinct species
run into each other and potentially try to interbreed?
Selection AGAINST hybrids favors “mate with your own kind”
individuals – i.e., assortative mating
Therefore, pairs that bump into each other a lot have been under
selection for assortative mating; they ignore each other
 pre-zygotic isolation is high, no matter how similar they are
amount of
pre-zygotic
isolation
Allopatric
pairs
genetic difference
Sympatric
pairs
genetic difference
Selection AGAINST hybrids favors “mate with your own kind”
individuals – i.e., assortative mating
Pairs that have NEVER seen each other have also never been
under selection for assortative mating
- if they have it, they got it via genetic drift – this takes time
- therefore, only the very different allopatric pairs (the ones that
were separated for a very long time) have it
amount of
POST-zygotic
isolation
Allopatric
pairs
genetic difference
Sympatric
pairs
genetic difference
POST-zygotic isolation is NEVER favored by natural selection
-
amount of
POST-zygotic
isolation
Allopatric
pairs
genetic difference
Sympatric
pairs
genetic difference
POST-zygotic isolation is NEVER favored by natural selection
Post-zygotic isolation is not a trait like mating preference;
- it’s the inevitable by-product of divergent gene pools getting so
different, their alleles can no longer work together to build
a fully fertile zygote
Pre-zygotic reproductive isolation
Many traits can potentially contribute to pre-zygotic isolation,
if they determine...
A) I never see you
(1) habitat choice, when it causes direct access to mates
(2) time shifts in adult mating behavior or gamete release
 At what time of night do you...
- release your sperm and eggs?
- open your flowers to pollinators?
- just plain feel sexy?
(3) time/space shifts in adult presence
Pre-zygotic reproductive isolation
Many traits contribute to pre-zygotic isolation by determining...
B) I see you... and find you repulsive
(1) mating rituals – songs, dances, pheromones
(2) size-assortative mating (big likes big, small likes small)
C) We totally did it... but nothing happened
(1) sperm didn’t find the egg (navigational problems)
(2) gamete recognition proteins blocked sperm-egg fusion
Pre-zygotic isolation: Time shifts
Coral genus Monastraea has 3 similar species that broadcast
gametes one night a year- are they reproductively isolated?
Monastraea franksi
Monastraea annularis
Monastraea faveolata
Time of spawning
M. faveolata (grey) spawns at the same time of day as other
two species, but their gametes do not hybridize in the lab
Pre-zygotic isolation: Time shifts
Coral genus Monastraea has 3 similar species that broadcast
gametes one night a year- are they reproductively isolated?
coral colony releasing eggs
Coral genus Monastraea has 3 similar species that broadcast
gametes one night a year- are they reproductively isolated?
Monastraea franksi
Monastraea annularis
Monastraea faveolata
8
9
10
11 PM
Time of spawning (hr after sunset)
M. faveolata (grey) spawns at the same time as other two
species, but their gametes do not hybridize in the lab
Coral genus Monastraea has 3 similar species that broadcast
gametes one night a year- are they reproductively isolated?
Monastraea franksi
Monastraea annularis
Monastraea faveolata
8
9
10
11 PM
Time of spawning (hr after sunset)
M. franksi (black) and annularis (white) gametes do hybridize in
the lab, but they spawn a few hours apart in the field
Coral genus Monastraea has 3 similar species that broadcast
gametes one night a year- are they reproductively isolated?
When related species can still
hybridize, selection against
hybrids drives corals to spawn
at different times of night
Time of spawning
Less related species can’t
hybridize (more fully isolated),
so they aren’t under selection
to spawn at different times
Reproductive character displacement = change in
a reproductive trait like spawning time when two similar species
come into contact with each other; a response to selection
Broadcast spawning in abalone
Males & female abalone snails free spawn into sea water
at the same time along our coast (no time shifting)
- there are 7 co-occurring abalone species
 what stops them from hybridizing back into one species?
Sperm navigation can be species-specific
Red and green abalone sperm only navigate towards eggs
of their own species; ignore eggs of the other species
Pre-zygotic isolation: Gamete recognition
Genes involved in reproduction are often the fastest evolving
- gamete recognition proteins allow sperm to dock with egg
Alleles can be “matched”, meaning one sperm protein allele fits
into a corresponding egg receptor allele of the right shape
 males with one allele can fertilize eggs with matching allele
sperm
egg
egg
receptor
allele
A
sperm
docking
protein
allele B
egg
receptor
allele
a
sperm
docking
protein
allele b
Pre-zygotic isolation: Gamete recognition
Genes involved in sexual reproduction are often fast evolving
- gamete recognition proteins allow sperm to dock with egg
- different alleles often have high % of amino acid changes
compared with alleles of non-reproductive genes
Adaptive evolution results when natural selection promotes
amino acid divergence through positive selection
Differences between alleles
silent substitutions
fixed by
drift
speed
slow
non-synonymous changes
(alter amino acid sequence)
positive
selection
fast
Pre-zygotic isolation: Gamete recognition
Compare genes encoding the same protein in 2 different species
- compare the # of sites where they differ in amino acids
If there are more non-synonymous than synonymous changes,
positive selection has likely favored divergence of that gene
between the two species
Example: the sperm protein sp18 is up to 73% different in amino
acid composition between Californian abalone species
- sp18 exons (coding regions) are evolving 20 times faster
than introns (non-coding regions) !
Pre-zygotic isolation: Gamete recognition
Male seminal proteins in Drosophila are also evolving rapidly
- the fast change in these proteins is partly responsible for
maintaining species barriers
- both sperm and “helper” seminal proteins are required for
successful mating in Drosophila; prevent hybridization
Regions of sperm proteins that directly contact egg receptors
are the fastest evolving parts of the protein
Pre-zygotic isolation: Gamete recognition
2 models for why sperm-egg proteins might evolve so fast:
#1) egg receptors evolve by drift, and sperm proteins quickly
evolve changes to “catch up” and dock with mutant receptors
due to selection (match egg, or no reproduction)
#2) egg receptors evolve away from common sperm alleles,
because selection favors eggs with rare (hard to match)
receptor alleles

Why would it be beneficial for eggs to have receptors that few
sperm can stick to?
no males nearby..
rare egg
receptor allele..
Eggs face 2 problems:
1) they might fail to get fertilized, due to:
A) low density of males
B) bad match between your egg receptor allele and
the common sperm docking protein allele
 you’re too hard to fertilize
Why would it be beneficial for eggs to have receptors that few
sperm can stick to?
no sperm
1 sperm
polyspermy
Eggs face 2 problems:
1) fail to get fertilized, due to low density of males, or a bad
match between your egg receptor allele and common sperm
docking protein allele (i.e., you’re too hard to fertilize)
2) polyspermy: too many sperm dock with you  dead egg
(i.e., you’re too easy to fertilize)
When could it benefit eggs to have receptors that few sperm can
stick to?
no match
reproductive
success
as a pair
partial match
full match
low male
density
high male
density
- selection favors common egg receptor alleles when male
density is low (common = matches most sperm)
- favors rare egg alleles when too many males are around
(rare = few matches, avoid polyspermy)
When could it benefit eggs to have receptors that few sperm can
stick to?
no match
reproductive
success
as a pair
partial match
full match
low male
density
high male
density
 conditions of high male density therefore impose selection
favoring rare female egg receptor alleles
- if rare alleles fix, then sperm will be under selection and
evolve new or better matching alleles; keep changing to
keep up with ever-evolving egg receptors
Pre-zygotic isolation: Adult presence
Pink salmon fish (Oncorhynchus) have a 2-yr breeding cycle:
- mate in streams
- travel downstream to ocean
- return to birth stream 2 yrs later as adult to spawn
Alternate-year brood lines are genetically:
- more different from other-year brood in the same stream
- more similar to same-year broods from different streams
along the coast
Differentiation proceeds as different, random adaptations to
the same average environment (same stream) over time in
different brood-lines (= groups going upstream in a given year)
Pre-zygotic isolation: Adult presence
Cicadas insects (Genus Magicicada)
- 3 pairs of species
- spend many years underground
as larvae
- emerge as adults for a 1-month
mating frenzy
3 species emerge every 17 years
- each closely related to a species
emerging every 13 yrs
Each 17-yr species likely evolved
from its sister 13-yr species,
by adding a 4-yr delay period during the larval stage