Transcript Plant mating systems

Plant mating systems
• Plants have a much wider variety of
mating patterns than animals
• Markers in population genetics are very
useful
Autogamy
• Self-fertilization
• Pollen transfer within or
among flowers of same
individual
• ~20% of angiosperms are
habitual selfers
• ~40% of angiosperms
can self-fertilize
Advantages of Autogamy
• Reproductive assurance.
• Selectively advantageous by transmitting
both sets of genes to offspring.
• Only single colonizing individual needed.
• Cost-saving on male expenditure.
Disadvantages of Autogamy
• Decreases genetic variability.
• Inability to adapt to changing conditions.
• Increases inbreeding depression.
– Reduces heterozygosity and increases
homozygosity of deleterious alleles.
– Loss of vigour in offspring!
Loss of Heterozygosity from
Selfing
Aa x Aa
1/4 AA
1/2 Aa
1/4 aa
A
a
A
AA
Aa
a
Aa
aa
A selfed heterozygote will yield offspring that are 50% heterozygous.
Loss of Heterozygosity from
Selfing
Proportion of heterozygotes is 1/2 in each successive generation.
S1: 50% of offspring heterozygous from original parent (Aa).
S2:
S3:
S4:
S5:
S6:
25%
12.5%
6.2%
3.1%
1.5%
Cleistogamy (CL)
• Flowers never open and
self-fertilize
• Small, bud-like flowers
without petals that form
directly into seed capsules
• Common: 488 species, in
212 genera and 49 families
Cleistogamy (CL)
• Mixed mating
systems -can produce
both CL and
chasmogamous (CH)
on an individual
• CL fls are a “back-up”
in case pollinators
scarce
Characteristics of predominantly
self-pollinating species
• 1. Reduced "male" investment
– fewer pollen (lower pollen/egg ratio)
– smaller/fewer attractive structures (corollas, flowers)
• 2. Phenological changes
– more uniform distribution of seed and pollen cones
– simultanous pollen shed and stigma receptivity
• 3. Loss of self-incompatibility (angiosperms)
• 4. Reduced inbreeding depression
– self-pollen is vigorous
– adult plants derived from selfing are vigorous
Monkeyflower (Mimulus)
• Stigma and anther (with
mature pollen) can be seen
to often touch each other
within the flower
• If you grow them in the
greenhouse without bees,
they still set some seed
• Do they self-fertilize in the
wild?
Molecular analysis of selffertilization rates
• Genetic markers (isozymes, microsatellites,
AFLPs) can be used to estimate rates of selffertilization
• Two approaches:
– Deviations from Hardy-Weinberg
• Selfing creates excess homozygosity like the Wahlund effect
– Patterns of segregation in progeny arrays
• Given maternal genotype, selfing creates excess of
homozygous progeny
Molecular analysis of
self-fertilization rates
• Deviations from Hardy-Weinberg
– Work with inbreeding coefficient F
• Probability that a locus is homozygous by descent
• We estimate it as F=(S-J)/(1-J), just like pairwise relatedness
(S=observed homozygosity, J=expected homozygosity)
– Recursion for F with total selfing
• Start with F=0
• After one generation of selfing, F=1/2 (example)
• Ft+1 = .5(1-Ft) + Ft = (1+Ft)/2
– Recursion for F with partial selfing
•
•
•
•
•
Population has a fraction of selfing (s) and outcrossing (1-s)
Ft+1 = s (1+Ft)/2 +(1-s)(0)
At equilibrium, Ft+1 =Ft
F = s (1+F)/2
s=2F/(1+F)
Mimulus guttatus species complex
• Yellow monkeyflowers
• Mostly annual herbs
• Selfing evolved several
times
• Intercrossible
Estimates of mating system parameters for the four Mimulus taxa.

M. nasutus
M. micranthus
M. nudatus
M. lacinatus
F
0.109 (0.055)
0.724 (0.104)
0.219 (0.033)
0.787 (0.053)
s
0.642 (0.040)
0.736 (0.140)
0.718 (0.025)
0.916 (0.035)

Are these populations at inbreeding
equilibrium? (is s=2F/(1+F))
M. nasutus s=2(0.109)/1.109 =0.196
M. micranthis s=2(0.724)/1.724=0.840
M. nudatus s=2(0.219)/1.219 = 0.359
M. lacinatus s=2(0.787)/1.787 = 0.880
Molecular analysis of self-fertilization rate
– Patterns of segregation in progeny arrays
• Given maternal genotype, selfing creates excess of
homozygous progeny
– Consider maternal parent “AA”
• Population is a mixture of “A” and “a” alleles, with frequencies
p and q
• If the parent outcrosses, expected progeny are:
– p of AA
– q of Aa
• If the parent selfs, all progeny are AA
• For selfing rate s, the expected frequency of AA progeny
from AA parents is fAA|AA = (1-s)p + s
• Solve for s, estimate frequency of selfing as s=(fAA|AA-p)/(1-p)
Progeny array model
• Several possible parent genotypes
• Probability matrix of progeny conditioned upon
parents:
– s=selfing rate; p,q are gene frequencies of A, a
Parent genotypes
AA
Progeny
genotypes
Aa
aa
AA
s+(1-s)p s/4+(1-s)p/2
0
Aa
(1-s)q
½
(1-s)p
aa
0
s/4 + (1-s)q/2
s+(1-s)q
Progeny array analysis
• ij = probability of progeny i, given parent j
– (previous table)
• Xij = observed number of progeny i of
parent j
– (isozyme or SSR data)
• Likelihood of data is L=  ijXij
• Use “numerical procedures” to maximize
likelihood “L”
Advantages of progeny arrays
• No need to assume equilibrium
• Maternal parent doesn’t need to be assayed
(can be inferred from progeny segregation
pattern), thus tissue differences are irrelevant
• Separate estimation of pollen gene frequencies
(pattern of paternity)
• Family structure also useful for many other
population genetic inferences (next week)
– Linkage disequilibrium
– Haplotype structure
– Association genetics
A study of inbreeding depression in monkeyflowers
•Measured as fitness of selfed progeny relative to outcrossed
progeny
•Large reduction in survival of progeny from selfing compared to
outcrossing, in two different populations
Selfing and inbreeding depression
• Self-fertilization causes progeny to exhibit
reduced fitness (inbreeding depression)
• Inbreeding depression is a tradeoff with
reproductive assurance
• Exposure of recessive deleterious genes
tends to remove inbreeding depression
over the long term
Genetics of inbreeding depression
• Longer term evolution of inbreeding depression
depends upon its genetic expression
• Is it caused by overdominance, or partial
dominance? (example)
• Expression of inbreeding depression can
depend on the stage of life cycle
– early vs. late acting genes (next)
Markers and inbreeding
depression

Would to know levels in nature, not greenhouse

Fixation index
 Level
of observed homozygosity
 Affected by inbreeding depression
Inferring inbreeding depression using
changes of the inbreeding coefficient
Ritland 1990
Mimulus guttatus and M. platycalyx

Co-occurring along meadows and streams of
North coastal California

M. platycalyx has large flower like guttatus, but
is very autofertile

Recently derived from M. guttatus?

Has inbreeding depression been reduced in M.
platycalyx?
Dole and Ritland 1993
Paternity analyses methods
• Exclusion
• Likelihood: two methods; both use
likelihood in same way
– categorical: assigns the entire offspring to a
particular male
– fractional: splits an offspring among all
compatible males
Example of paternity analysis (two loci)
• Mother
– A1A2, B1B3
• Offspring
– A1A3, B1B2
– (father alleles are A3, B2)
• Potential father 1
– A2A2, B2B3
• Exclude because father doesn’t have A3
• Just one locus can exclude paternity
Paternity analyses methods
• Exclusion
• Likelihood: two methods; both use
likelihood in same way
– categorical: assigns the entire offspring to a
particular male
– fractional: assigns paternity “in probability”,
allows for all possible males
Summary of likelihood
• Total probability is prior probability
(frequency of male parent genotype in
populations, maybe other factors) times
the transmission probability
• Prior probability = genotype frequencies of
alleged male
– perhaps multiplied by female frequencies,
mating distance distribution, male fitness, etc.
Problems with using microsatellites
for paternity analysis
• New mutations
– The mutation rate for microsatellites is estimated
to be between 10-2 - 10-4 per generation; new
mutations can frequency occur resulting in the
true father being excluded.
– This can be overcome operationally by requiring
potential fathers to be excluded at least two loci.
• Null alleles
– If the offspring inherits a null allele (nonamplifying allele) at a locus from the father, then
the true father may be excluded.