Assortative Mating in Sunflowers

Ecological Speciation

  • Ecological speciation is driven by the strength of natural selection and disruptive selection, leading to speciation in a previously homogeneous population.
  • Consider a flowering plant distributed along a salinity gradient as a hypothetical scenario.
    • Low salinity soils exist at one edge of the species range.
    • High salinity soils exist at the opposite edge.
    • These differing environments create selective regimes, causing subpopulations to differentiate.
  • Sufficient differentiation and adaptation can lead to new species formation if selection acts on traits relevant to reproduction (physiology, phenology, morphology).

Reinforcement

  • Differentiated populations occurring sympatrically may produce hybrids.
  • These hybrids often have low fitness due to intermediate phenotypes maladaptive to either environment.
  • The formation of maladaptive hybrids leads to reinforcement, where natural selection selects against hybridization, strengthening prezygotic reproductive barriers.
  • Reinforcement patterns show stronger reproductive isolation in co-occurring species pairs compared to allopatric pairs.
  • Reinforcement often follows a period of temporary allopatry.
  • The steps for reinforcement are:
    1. Incipient speciation
    2. Secondary contact of incipient species
    3. Production of low-fitness hybrids due to gene flow
    4. Selection against hybrids favoring divergence in mating-associated traits (prezygotic isolation)
    5. True speciation once barriers are strong enough, forming distinct species under the biological species concept

Helianthus pedialaris subspecies phallics: A Case Study

  • Also known as the prairie sunflower, this species is found at Great Sand Dunes National Park in southwestern Colorado.
  • Great Sand Dunes is the largest and most complex dune system in North America, characterized by a harsh environment:
    • Nutrient-poor soil
    • Limited water availability
  • Two main ecotypes exist:
    • Nondune ecotype: Found on the nondune sand sheet surrounding the dune complex, where conditions are more amenable to plant life (more nutrients, more water).
    • Dune ecotype: Found exclusively within the dune complex.
  • The dune ecotype colonized the dune habitat within the last 10,000 years.
  • Significant genetic and phenotypic distinction has already occurred between the ecotypes in this short time.

Ecotype Differentiation

  • Several traits distinguish the ecotypes, most notably seed size.
    • Dune seeds are significantly larger (2-3 times the weight) than nondune seeds.
    • Larger seeds likely provide more resources for seedling establishment in the harsh dune environment, enabling the development of a large root system to access deep water.
  • Genetic differentiation exists as well, particularly in chromosomal inversions.
    • Multiple inversions exist within the species.
    • Genome-wide association studies (GWAS) show that these inversions are associated with adaptive traits like seed area, timing of reproduction, and tolerance to nutritionally poor soils.
  • How GWAS works:
    1. Collect individuals from the species of interest.
    2. Extract DNA and perform whole-genome sequencing on each individual.
    3. Collect phenotypic (e.g., seed area, days to bud) and environmental data (e.g., soil nutrient content) for each individual.
    4. Upload genome sequence data and trait data into a program.
    5. The program scans the genome to identify SNPs significantly associated with the trait of interest.
  • Manhattan plots are used to visualize GWAS results, with SNPs above a significance threshold (magenta line) being strongly associated with the trait.
  • Inversions (purple bars) contain a large majority of SNPs associated with adaptive traits.

Inversion Frequency and Adaptation

  • Significant differences exist in the frequency of inversions between the dune and nondune habitats.
    • The dune habitat has a higher frequency of certain inversions compared to the nondune habitat, and vice versa.
  • These adaptive inversions likely enable the dune ecotype to adapt to the harsh dune environment.

Reproductive Isolation and Assortative Mating

  • Reproductive isolation exists between the ecotypes, despite only diverging for 10,000 years.
    • Selection against hybrids occurs, as they perform poorly in both environments.
    • Assortative mating is observed, with dune plants preferentially mating with other dune plants, and nondune plants with nondune plants.
  • The strength of assortative mating differs based on location in the landscape, potentially reflecting reinforcement.
    • Plants at the dune system's edge show a stronger preference for mating with their own ecotype compared to plants in the dune core.

Research Project: Genetic Basis of Assortative Mating

  • The current research project aims to:
    • Expand the sample size to confirm significant differences in assortative mating strength based on landscape.
    • Map the loci underlying assortative mating.
    • Determine if these loci lie within previously mapped inversions.
  • Steps:
    1. Crossing experiment to assess assortative mating strength.
    2. GWAS to determine the genetic basis of assortative mating.
    3. Compare GWAS results to known inversions.
    4. Simulation data to see if loci associated with assortative mating more often lie within inversions than would otherwise be expected by random chance.
  • Crossing design:
    • Mix dune and nondune pollen in a roughly 50/50 pool.
    • Use the mixed pollen pool to fertilize both dune and nondune parents.
    • Determine the parentage of the resulting seeds to assess preferential mating.
  • GWAS:
    • Upload whole-genome sequence data and assortative mating data.
    • Scan the genome to find SNPs strongly associated with mate choice.
    • Overlay loci associated with assortative mating onto the location of known inversions.
    • Determine if the association is more frequent than expected by chance.
  • Significance:
    • Better understanding of how and why assortative mating evolves in sympatry.
    • Suggests that inversions may be important for sympatric speciation by linking locally adaptive alleles with prezygotic isolating alleles.
    • Sheds light on the role of inversions in facilitating divergence despite gene flow.
    • Determines which regions of the genome underlie mate choice and pollen success in sunflowers, contributing to our knowledge of pollen-pistil interactions in flowering plants.