Polyploidy is widespread within flowering plants, with 35% of extant species being recent polyploids. Whether and how polyploidy modulates the action of natural selection remains debated, and the particular case of balancing selection has been poorly explored. This study investigates the impact of autopolyploidy on sporophytic self-incompatibility in plants, a striking example of a genetic system evolving under a special form of balancing selection (strong negative frequency-dependent selection). Stochastic simulations reveal that under strict co-dominance, the number of S-alleles maintained in tetraploid populations is expected to double as compared with diploid populations. However, under a model with strict hierarchical dominance among alleles, the number of S-alleles increases only slightly, but gene diversity and observed heterozygosity are substantially reduced in tetraploids because of enhanced dominance masking effects. Empirical data on Arabidopsis arenosa and Arabidopsis lyrata confirm the latter predictions, showing similar levels of allelic diversity but dramatically lower observed and expected heterozygosity at the self-incompatibility locus in tetraploids compared with diploids. The study highlights the significant impact of autopolyploidy on patterns of diversity at the self-incompatibility locus, emphasizing the increased dominance effect in tetraploids. The results also allow us to reject a scenario of strong founder effects associated with the evolution of the polyploid lineages. This article is part of the theme issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.
Vekemans et al. (Thu,) studied this question.