Plant mating systems

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Plant mating systems

• Plants have a much wider variety of mating
  patterns than animals
• Markers in population genetics are very useful

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Autogamy

• Self-fertilization
• Pollen transfer within or among flowers of same
  individual
• 20 of angiosperms are habitual selfers
• 40 of angiosperms can self-fertilize

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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.

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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!

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Loss of Heterozygosity from Selfing
Aa x Aa
A
a
AA Aa
Aa aa
A
1/4 AA 1/2 Aa 1/4 aa
a
A selfed heterozygote will yield offspring that
are 50 heterozygous.
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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 25
S3 12.5
S4 6.2
S5 3.1
S6 1.5
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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

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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

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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

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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?

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Molecular analysis of self-fertilization rates

• Genetic markers (isozymes, microsatellites,
  AFLPs) can be used to estimate rates of
  self-fertilization
• 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

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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 (Sobserved homozygosity,
  Jexpected homozygosity)
• Recursion for F with total selfing
• Start with F0
• After one generation of selfing, F1/2 (example)
• Ft1 .5(1-Ft) Ft (1Ft)/2
• Recursion for F with partial selfing
• Population has a fraction of selfing (s) and
  outcrossing (1-s)
• Ft1 s (1Ft)/2 (1-s)(0)
• At equilibrium, Ft1 Ft
• F s (1F)/2
• s2F/(1F)

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Mimulus guttatus species complex

• Yellow monkeyflowers
• Mostly annual herbs
• Selfing evolved several times
• Intercrossible

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Are these populations at inbreeding equilibrium?
(is s2F/(1F)) M. nasutus s2(0.109)/1.109
0.196 M. micranthis s2(0.724)/1.7240.840 M.
nudatus s2(0.219)/1.219 0.359 M. lacinatus
s2(0.787)/1.787 0.880
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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 fAAAA (1-s)p s
• Solve for s, estimate frequency of selfing as
  s(fAAAA-p)/(1-p)

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Progeny array model

• Several possible parent genotypes
• Probability matrix of progeny conditioned upon
  parents
• sselfing rate p,q are gene frequencies of A, a

Parent genotypes
AA 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 genotypes
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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

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Advantages of progeny arrays

• No need to assume equilibrium
• Maternal parent doesnt 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

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• 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

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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

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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)

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Markers and inbreeding depression

• Would to know levels in nature, not greenhouse
• Fixation index
• ?Level of observed homozygosity
• Affected by inbreeding depression

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Inferring inbreeding depression using changes of
the inbreeding coefficient
Ritland 1990
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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?

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Dole and Ritland 1993
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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

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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 doesnt have A3
• Just one locus can exclude paternity

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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

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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.

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Problems with using microsatellitesfor 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
  (non-amplifying allele) at a locus from the
  father, then the true father may be excluded.