Molecular Evolution within and between Self-Incompatibility Specificities

Genes under multiallelic balancing selection have sharply contrasted evolutionary dynamics across timescales, with much longer coalescence time among functionally distinct allelic lines but much shorter coalescence time among gene copies within allelic lines as compared with the genomic background. In this paper, we combine theoretical and empirical approaches to investigate patterns of molecular evolution within and between self-incompatibility (SI) specificities. We first use numerical simulations to investigate coalescence times within allelic lines in a subdivided population for a sporophytic SI system. We then report on a comprehensive analysis of nucleotide polymorphism among gene copies within five distinct allelic lines in the closely related Arabidopsis halleri and Arabidopsis lyrata. In line with our model predictions, we find that the observed level of polymorphism among gene copies was generally low but differed among allelic lines. The data provide compelling direct evidence for recombination and/or gene conversion not only within the two most recessive allelic lines but also between two closely related but distinct allelic lines, suggesting that recombination at the Arabidopsis SI locus is possible in the absence of large sequence divergence among haplotypes. We observed shared polymorphic sites between the two species in one allelic line and strikingly similar haplotypes in another allelic line. We discuss whether convergent evolution may have led to this pattern and suggest that these observations are consistent with ongoing or very recent introgression, as previously documented.