Self-incompatibility alleles from Physalis: implications for historical inference from balanced genetic polymorphisms

Area of Science:

• Population genetics
• Evolutionary biology
• Molecular evolution

Background:

• Balanced genetic polymorphism offers insights into population history, complementing neutral polymorphism studies.
• Coalescent theory, used for inferring population size, faces challenges with empirical data from S genes and MHC genes.
• Gene genealogies under balancing selection often show longer terminal branches, suggesting slower new allele origination.

Purpose of the Study:

• To investigate the impact of population size reduction on balanced genetic polymorphism.
• To examine how selective retention of divergent S alleles in Physalis cinerascens affects gene genealogies.
• To assess the implications for coalescent-based effective population size estimations.

Main Methods:

• Comparative analysis of S gene genealogies in Physalis cinerascens and Physalis crassifolia.
• Investigating allele retention patterns during population size changes.
• Evaluating deviations from coalescent theory expectations.

Main Results:

• Divergent S alleles were selectively maintained in Physalis cinerascens despite a population size reduction.
• This retention resulted in longer terminal branches in the S gene genealogy compared to Physalis crassifolia.
• The observed pattern violates assumptions of standard coalescent models for effective population size.

Conclusions:

• Nonrandom sorting is a characteristic of balanced genetic polymorphisms.
• Studies inferring the absence of population bottlenecks from balanced polymorphism may overestimate effective population size.
• The findings highlight the need to refine models for population history inference under balancing selection.