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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 – PowerPoint PPT presentation

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Title: Plant mating systems


1
Plant mating systems
  • Plants have a much wider variety of mating
    patterns than animals
  • Markers in population genetics are very useful

2
Autogamy
  • Self-fertilization
  • Pollen transfer within or among flowers of same
    individual
  • 20 of angiosperms are habitual selfers
  • 40 of angiosperms can self-fertilize

3
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.

4
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!

5
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.
6
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
7
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

8
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

9
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

10
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11
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?

12
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

13
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)

14
Mimulus guttatus species complex
  • Yellow monkeyflowers
  • Mostly annual herbs
  • Selfing evolved several times
  • Intercrossible

15
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
16
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)

17
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
18
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

19
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

20
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21
  • 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

22
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

23
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)

24
Markers and inbreeding depression
  • Would to know levels in nature, not greenhouse
  • Fixation index
  • ?Level of observed homozygosity
  • Affected by inbreeding depression

25
Inferring inbreeding depression using changes of
the inbreeding coefficient
Ritland 1990
26
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?

27
Dole and Ritland 1993
28
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29
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30
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

31
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

32
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33
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

34
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.

35
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.
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