The evolution and maintenance of plant sexual diversity

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• The evolution and maintenance of plant sexual
  diversity

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Why study polymorphic sexual systems?
Why is there high sexual diversity in flowering
plants?

• Immobility (rely on pollen vectors)
• Hermaphoditism (self-fertilization)
• Modular growth - get bigger by producing
  repeating organs via apical meristems
• (clonality and inter flower selfing)
• Closed carpel (mate selection e.g. SI)
• Life history diversity (mating patterns depend on
  longevity, plant size etc.)

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Outline

• Definitions and examples
• The evolution and function of stylar
  polymorphisms
• heterostyly
• enantiostyly
• flexistyly
• Major transitions
• The evolution of separate sexes from
  hermaphroditism (cosexuality)
• The evolution of self-fertilization from
  outcrossing (Mating system evolution)-next class

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sexual systems

• Sexual system the particular deployment of
  sexual structures within and among plants and the
  physiological mechanisms governing mating
• Sexual interference conflict in maternal and
  paternal functions resulting in gamete wastage
  and reduced fitness.
• (may or may not be associated with
  self-pollination)

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Examples of plant sexual systems

• Dichogamy differences in the timing of pollen
  dispersal from anthers and stigma receptivity of
  flowers.

• protandry male phase comes before the female
  phase

• protogyny female phase comes before the male
  phase

Herkogamy the spatial separation of the anthers
and stigmas within a flower.
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mating systems
Mating system the mode of transmission of genes
from one generation to the next through sexual
reproduction (e.g. maternal selfing rate)
Selfing rate (s) the proportion of seeds that
are self fertilized Outcrossing rate (t1-s)
the proportion of seeds that are
outcrossed Inbreeding depression the reduction
in viability and fertility of inbred offspring
compared with outbred offspring.
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polymorphic sexual systems

• The co-occurrence within a population of
  morphologically distinct mating groups
  distinguished by differences in their sexual
  organs

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Why study polymorphic sexual systems?
Why study plant polymorphic sexual systems?

• simple inheritance
• sexual morphs easily identified in the field
• under strong frequency-dependent selection
• theoretical models provide predictions
• manipulative experiments possible

Sagittaria latifolia
Cyanella alba
Long-styled
Short-styled

• Primula polyneura

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Floral design and pollen transfer heterostyly
Heterostyly two (distyly) or three (tristyly)
style morphs differ in the reciprocal placement
of anthers and stigmas.

• reciprocal sex-organ placement
• heteromorphic
• self-incompatibility
• (disassortative mating)
• genetic polymorphism

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Floral design and pollen transfer
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Pollen transfer and equilibrium morph ratios in
typical tristyly
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Pollen transfer and equilibrium morph ratios in
typical tristyly
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Pollen transfer and equilibrium morph ratios in
typical tristyly
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Pollen transfer and equilibrium morph ratios in
typical tristyly
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Equilibrium morph frequencies

• Disassortative mating results in negative
  frequency-dependent selection
• Equal morph ratios are predicted
• 111 found in many tristylous populations

R.A. Fisher
Lythrum salicaria
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Why study polymorphic sexual systems?
The maintenance of sexual polymorphisms

• disassortative mating among morphs
• negative frequency dependent selection
• selection for equal morph ratios

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Floral design and pollen transfer enantiostyly
Enantiostyly mirror image flowers in which the
style bends either to the left or the right side
of the floral axis-deposits pollen on the left or
right side of the bee.
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Floral design and pollen transfer

• Created straight styled, monomorphic and
  dimorphic arrays from Solanum rostratum
  (monomorphic)
• Highest outcrossing rate in dimorphic arrays
• Most of the mating was intermorph in the
  dimorphic array
• (negative frequency dependent selection)

Jesson and Barrett 2002 Nature
Inter-morph mating
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Floral design and pollen transfer

• Herkogamy reduces self pollination (and other
  forms of sexual interference)
• Separation reduces precision of cross pollination
  (lower male and female fitness-pollen wastage and
  pollen limitation)
• Reciprocal herkogamy improves pollen transfer
  efficiency
• Polymorphisms is generally maintained by
  disassortative mating at equal frequency

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Flexistyly

• Flexistyly populations contain two floral morphs
  that differ in there temporal patterns of style
  growth and orientation

Alpinia

• combines herkogamy and dichogamy
• found in tropical gingers

protandrous protogynous
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The evolution of separate sexes

• Gender is the relative contributions that plants
  make to the next generation as a male and female
  parent (quantitative measure)

• Monomorphism - continuous variation in gender
• Dimorphism - two distinct sexual morphs that
  function primarily as a male or female parent

Dioecy Gynodioecy Androdioecy
Sagittaria latifolia
Mercurialis annua
Silene vulgaris
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Evolutionary pathways to gender dimorphism

• Gynodioecy pathway
• Monoecy pathway
• Distyly pathway
• Heterodichogamy pathway

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Selective mechanisms and the evolution of
separate sexes
The evolution of dioecy from gynodioecy Nuclear
inheritance of male sterility (female)

• Females spread if they produce at least two
  times as many seeds as hermaphrodites
• s ? gt 0.5 (half the seeds of hermaphrodites die
  due to inbreeding depression)
• -resource reallocation from male function to
  female function (females produce twice as many
  ovules)

w for invasion Pollen 1
0 Seed 1 gt2
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Cyto-nuclear control of gender dimorphism
Sterility mutations can occur in the maternally
inherited mitochondrial genome All offspring of
the male sterile mutant with be female Females
can spread with only a slight female fertility
advantage
Dioecy can then evolve from gynodioecy as
hermaphrodites invest in male function
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Selective mechanisms and the evolution of
separate sexes

• Lycium - self incompatibility lost with
  chromosome doubling
• Polyploids are gender dimorphic
• Association between polyploidy and dimorphism
  also found in 12 unrelated genera in other
  families

. Miller and Venable 2000
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Selective mechanisms and the evolution of
separate sexes

• Large plant size (e.g. clonal)-gthigher selfing
  rates
• Geitonogamy (transfer of self pollen between
  flowers)

Sagittaria latifolia Dioecious large
clones Monecious (hermaphrodites) smaller
plants s ? gt 0.5 in some monecious populations
Dorken et al 2002
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The comparative biology of dioecy

• In half of families but only 6 of species are
  dioecious
• Why is dioecy associated with low
  diversification?
• extinction rates
• extinction risk is high in small populations
  (need males and females)
• sexual dimorphism (risk that females may not be
  visited when pollinators are rare)
• speciation rates
• associated with unspecialized pollination systems
  (wind, water, generalist pollinators) which may
  hinder speciation

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Self incompatibility
Two main types of homomorphic incompatibility

-gametophytic incompatibility phenotype is
determined by its haploid genotype e.g. S1 or S2
can not fertilize S1 S2 plants but S3 pollen
can -sporophythic incompatibility governed by
the genotype of the pollen producing parent e.g.
any pollen from and S1S2 plant can not fertilize
an S1_ or S2_ plant
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Self incompatibility
Maintained by negative frequency dependant
selection (balancing selection) Rare SI types
have a fitness advantage as they can mate with
all other plants in the population
Brassica
Many S alleles Low Fst compared to neutral
loci (higher effective migration due to balancing
selection)
Glémin et al 2005