Diversification or Collapse of Self-Incompatibility Haplotypes as a Rescue Process

Area of Science:

• Evolutionary Biology
• Plant Reproduction
• Population Genetics

Background:

• Angiosperm self-incompatibility (SI) systems prevent self-fertilization through pollen rejection based on matching alleles at the SI locus.
• Extreme allelic polymorphism in SI is maintained by frequency-dependent selection favoring rare alleles.
• The spread of new SI alleles faces a 'chicken-or-egg' problem due to the collaborative nature of pollen and pistil recognition.

Purpose of the Study:

• To investigate the evolutionary dynamics and spread of novel haplotypes within angiosperm self-incompatibility systems.
• To model the conditions under which new SI alleles can arise, spread, and potentially eliminate existing ones.
• To determine the factors influencing the number of SI haplotypes in a population, such as gene conversion rate and population size.

Main Methods:

• Development of a mathematical model analyzing the spread of novel pollen and pistil function mutations.
• Calculation of probabilities for the increase and collapse of SI haplotype numbers.
• Application of a Markov chain model to predict stable haplotype number distributions under various parameters.

Main Results:

• Novel haplotypes can emerge and spread by complementary mutations in pollen and pistil functions, overcoming initial compatibility barriers.
• New haplotypes can drive the elimination of existing ones, especially those with fewer siring opportunities.
• Expansion of haplotype number is favored by high population gene conversion rates, while contractions are more likely otherwise.
• A Markov chain model predicts a stable distribution of 10-40 SI haplotypes under plausible biological parameters.
• Smaller populations are susceptible to significant haplotype loss beyond random drift during bottlenecks.

Conclusions:

• The study resolves the 'chicken-or-egg' paradox for the evolution of new self-incompatibility alleles.
• Gene conversion rate and population size are critical factors shaping the diversity of self-incompatibility haplotypes.
• The findings provide insights into the maintenance of extreme allelic polymorphism in plant reproductive systems.