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.

Transmission ratio distortion in Arabidopsis lyrata: effects of population divergence and the S-locus.

We investigated transmission ratio distortion within an Icelandic population of Arabidopsis lyrata using 16 molecular markers unlinked to the S-locus. Transmission ratio distortion was found more often than expected by chance at the gametic level, but not at the genotypic or zygotic level. The gametic effect may be due to meiotic drive or selection acting postmeiotically. At the gametic level, 10.9% of the tests were significant, which is substantially lower than earlier observed in an interpopulation cross (allowing for differences in power)-suggesting that the high level of transmission ratio distortion in the interpopulation cross is due to population divergence. It is also substantially lower than previously observed in intrapopulation crosses at the self-incompatibility locus, suggesting inherent fitness differences of the self-incompatibility alleles. We discuss the possible role of deleterious alleles accumulating at loci under balancing selection. Zygotic effects play a larger role in the interpopulation cross than in the intrapopulation crosses suggesting that Dobzhansky-Muller incompatibilities may be accumulating between the widely diverged populations.