Sexual Reproduction

• Pollination Biology
  • Pollen produced in anthers
  • Transported to (receptive) stigma
    • Various pollination mechanisms
      • Abiotic
        • Anemophily (wind)
        • Hydrophily (water)
      • Biotic
        • Insects
        • Birds
        • Mammals
        • Slugs????, reptiles, etc.
  • Pollen germination
  • Pollen tube growth through style
  • Pollen tube growth into synergid cell in embryo sac (female gametophyte)
  • Fertilization
  • Seed maturation
• Autogamy (results in selfing)
  • Mechanism
  • Floral features promoting autogamy
    • Small flowers
      • Close proximity of anthers to stigma
      • No need to attract pollinators
        • No pollinator reward offered
    • Simultaneous maturation of pollen and stigma
    • Self-compatibility
      • Self-pollen capable of germinating on stigma and pollen tube grows through style
  • Genetic consequences
    • Inbreeding
      • Sexual reproduction involving close relatives
      • Limited genetic diversity among progeny
    • Patterns of genetic variation
      • Limited variation within populations
      • Differences between populations
  • Life history consequences
    • Flowering time not dependent on pollinator availability
    • Little pollen required for successful pollination of stigma
      • Low pollen/ovule ratio
    • Reproductive success independent of population size
      • Good for colonizing species (e.g., weeds)
    • High seed set
    • Limited probability of cross-pollination
      • With conspecific plants
      • With other species (i.e., less hybridization)
• Xenogamy (results in outcrossing)
  • Obligate xenogamy
    • Floral features promoting xenogamy
      • Attraction structures (e.g., petals, odor)
      • Rewards for pollinators (e.g., nectar, pollen, oils, waxes, pheromones)
      • Flowers open when pollinators are active
      • Physical separation of anthers and stigma
      • Different maturation time of pollen and stigma
      • Self-incompatible
        • Gametophytic (most common)
          • Single gene (S gene), but some examples of multiple genes known
          • Many alleles in a population
          • If pollen grain (male gametophyte) has same gene as is found in style (sporophytic tissue), pollen tube growth is halted, and no germination occurs
          • Self pollen will always have a match with one of the two possible alleles in the style tissue
          • Closely related individuals are more likely to have matching alleles between pollen and style than will less closely related individuals
        • Sporophytic (Asteraceae, Brassicaceae, a few others)
          • Single gene (S)
            • Many alleles in a population
            • Only two alleles (S and s) in population
            • One allele typically dominant to other
          • Protein products in wall of pollen grain are derived from sporophytic tissue; are determined by genotype of sporophyte plant
          • Incompatibility reaction occurs between stigma and wall of pollen grain; no germinatioon if incompatible
          • Heterostyly couples incompatibility reaction with structural features (through action of multiple genes tightly linked as a "supergene")
            • Pollen size, pollen wall sculpturing, stigma papillae, style length, stamen length
            • Distyly
              • Long style morph = ss
              • Short style morph = Ss
              • Only one combination is compatible: pollen from one morph on stigma of the other
                • Yields 1:1 ratio of long to short morphs in progeny
    • Genetic consequences
      • Outbreeding (outcrossing)
        • Sexual reproduction involving relatively unrelated individuals
        • Increased (relative to autogamous) genetic diversity among progeny
      • Patterns of genetic variation
        • Significant variation within populations
        • Less significant differences between populations
    • Life history consequences
      • Flowering time dependent on pollinator availability
      • Much pollen required for successful pollination of stigma
        • High pollen/ovule ratio
      • Reproductive success requires multiple individuals
      • Seed set is not assured; may be highly variable among flowers and among individuals
  • Facultative xenogamy
    • Floral features promoting xenogamy
      • Attraction structures (e.g., petals, odor)
      • Rewards for pollinators (e.g., nectar, pollen, oils, waxes, pheromones)
      • Flowers open when pollinators are active
      • Physical separation of anthers and stigma
      • Different maturation time of pollen and stigma
      • Self-compatible
      • Rely on non-physiological mechanisms to promote cross-pollination and cross-fertilization
      • Often self-pollinate to some extent, or self-pollinate as a "last resort"
    • Genetic consequences
      • Mixed mating system
        • Sexual reproduction involving a mix of relatively unrelated individuals and selfing
        • Genetic diversity among progeny exists, but some are more homozygous than others due to selfing
      • Patterns of genetic variation
        • Significant variation within populations
        • Less significant differences between populations
      • Inbreeding depression common when selfed
    • Life history consequences
      • Flowering time dependent on pollinator availability
      • Much pollen required for successful pollination of stigma
        • High pollen/ovule ratio
      • Outcrossing requires multiple individuals, but a single plant can establish a new population (but will suffer inbreeding depression)
      • Seed set may be assured, at expense of reduced fitness of offspring due to inbreeding depression