Download Selfing and Outcrossing

Selfing and Outcrossing
The ESS and Hermaphroditism
Why Outcross?
Models
Evolution of Selfing
Phenotypic Evolution in a Selfer
Why be an Hermaphrodite?
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Facultative selfing.
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If pollinators are not available.
Male/female shared investment
in attractants and rewards.
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Two for the price of one?
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Attractants affect male and female
fitness gain curves differently.
Temporal separation of
investment in male and female
function.
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Male today, female tomorrow.
Why not be an Hermaphrodite?
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Interference of sexual
function.
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Mechanical interference.
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Favors evolution of herkogamy
or dichogamy.
Self pollination.
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Problematic when physiological
self-incompatibility is lost and
genetic load is high.
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Evolution of dioecy on islands.
Decreased pollination for
females.
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If pollen is a reward for
pollinators.
ESS Models
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Hermaphrodites favored
only when the sum of
fitnesses gained through
male and female function
exceeds the sum of
fitness for the dioecious
strategy.
Male and female
function may be limited
by different resources.
ESS = Hermaphroditism
Female Success »
Shapes of male and
female gain curves.
Male Success »
ESS = Dioecy
Female Success »
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Male Success »
Sex Lability
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Many plant species change sex.
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Plant size (larger plants are female).
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Sugar maples.
Jack in the pulpit.
Environment (females grow in less
stressful microhabitats).
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Salt bush.
Salt grass.
Mosses.
Selfing and Outcrossing
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Distribution of selfing
taxa.
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40% sometimes facultative.
20%
mostly/completely self.
Why self?
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Colonizers (Baker’s
rule).
Unreliable pollinators.
Potential fitness gains
(2x) by fathering your
own seeds.
Selfing and Outcrossing
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Why not self?
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High levels of homozygosity.
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Overdominance for fitness (heterosis).
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Heterozygous genotype is the most fit (e.g., sickle cell
anemia, spots on Clarkia petals [Vince Eckhart]).
Loss of segregation and recombination for the
generation of novel genetic variation.
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Expression of deleterious recessive alleles (DRA’s).
Heterozygous genotypes have high fitness because normal
alleles are dominant.
Selfing populations less able to respond to environmental
change?
The heterozygosity/fitness debate.
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Are high levels of heterozygosity better because of
dominance or overdominance?
Automatic Selfing Advantage
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Invasion of a selfing mutant in an outcrossing
population.
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Gain of fitness through increased transmission of
genes (= 3/2 for a rare selfer in a very large
population of outcrossers).
Loss of fitness through inbreeding depression.
Invasion of an outcrossing mutant in a selfing
population.
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Loss of fitness due to production of outcrossed seeds.
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Transmission is decreased by ½ for an outcrosser in a
population of selfers.
Could be an advantage if inbreeding depression is
severe.
Why Self?
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Reproductive assurance.
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Unpredictable pollinators.
Purging of genetic load may occur when the
selfing rate is high.
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Genetic load = a reduction in fitness from the
maximum possible because of segregation.
Load can be “purged” to the degree that it is due to
DRAs
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Selection to remove DRAs.
With many DRAs of small effect some will become fixed.
Reduces inbreeding depression (Barrett and
Charlesworth 1991).
Selection for Selfing or Outcrossing
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Depends on the environment (Grant
1975).
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Lots of variation in selfing rates within
species.
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Horovitz and Harding (1972) in Lupinus.
Disruptive selection for on the selfing rate.
If d < ½k then selfing favored.
„ If d > ½k then outcrossing favored.
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Where d = inbreeding depression and k = seed set
in outcrossers.
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k = 1 if seed set is not limited by pollinators.
Do selfers become
stuck?
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A bimodal distribution of selfing and outcrossing
is predicted by theory (Lande and Schemske
1985).
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Once genetic load is reduced to a certain
threshold, the mating system should go to
complete selfing.
Maybe supported by empirical observation
(Schemske and Lande 1985).
But, to some degree this depends on the
pollination syndrome….
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Once wind-pollinated plants are removed, animal
pollinated species are not clearly bimodal (Aide
1986).
Animal pollinated species show some bimodality,
but there are still many species with mixed
mating systems (Barrett and Eckert 1990).
The Selfing Lifestyle
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Short-lived weeds and invasive
species.
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Autogamy.
Selection for reduced flower size.
Reduced allocation to male
function.
Cleistogamy.
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Literally “closed fertilization.”
Mixed cleistogamous and
chasmogamous (open) flowers.
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Examples: Jewel weed, peanuts,
and some violets.
Complete cleistogamy is rare.
Variation in Selfing and Outcrossing
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Ecological factors.
Pollinator visitation rates.
„ Plant size (opportunities for geitonogamy).
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Genetic factors – individual plants in a
population may vary in selfing levels.
Flower size – may vary by season.
„ Stigma-anther separation.
„ Levels of cryptic self incompatibility.
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Is Selfing a
Dead End?
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Most phylogenetic studies
indicate that selfing is
derived.
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Predictions (from theory) and
observations indicate that:
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Arises multiple times in a
phylogeny.
Selfing lineages do not
diversify.
Selfing lineages are short-lived.
Exceptions….
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Evolution of dioecy on islands.
From Barrett et al. 1997
Estimating Outcrossing Rates
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We assume that all progeny are produced either through
outcross (t ) or self (s ) fertilization, such that t = 1 – s.
Use genetic markers to estimate t
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Maternal genotype: AA
Possible progeny: AA or Aa
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Assume all Aa progeny are outcrossed.
What if mom (AA) outcrosses with another AA genotype, or with a
heterozygote?
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What is the chance that some AA progeny are actually outcrossed?
Depends on the frequency of the A allele in the population.
These ‘undetected’ outcross events become varnishingly small with
many loci/hypervarible markers such as microsatellites.
Do Autogamous Plants Evolve?
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Example:
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Arabidopsis thaliana in
Europe after the
Pleistocene.
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Nearly autogamous lineages
have spread across Europe
from a few refugia in the
south.
Southern Refuge
Hypotheses:
1. Lineage sorting by
Latitude?
2. Rare outcrossing
and recombination?
Evolution by Mutation Accumulation
within Selfing Lineages
Individual selfing
lineages display as
much as 30%
change in
phenotypic
characters.
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Over 10,000 –
13,000 generations.
If 5% of mutations
are advantageous,
this works.
110
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Days to Bolting
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Spain
S France
N France
Region
Netherlands
Sweden