Population structure at the S-locus of Sorbus aucuparia L. (Rosaceae: Maloideae).

Low sequence divergence within functional alleles is predicted for the self-incompatibility locus because of strong negative frequency-dependent selection. Nevertheless, sequence variation within functional alleles is essential for current models of the evolution of new mating types. We genotyped the stylar self-incompatibility RNase of 20 Sorbus aucuparia from a population in the Pyrenees mountains of France in order to compare alleles found there to those previously sampled in a Belgian population. Both populations returned 20 different alleles from samples of 20 individuals, providing maximum-likelihood estimates of 24.4 (95% CI 20-34) alleles in each. Ten alleles occurred in both samples. The maximum likelihood (ML) estimate of the overlap in the alleles present in both populations was 16, meaning that an estimated eight alleles are private to each population, and a total of 32 alleles occur across the two populations examined. We used Fisher's (1961) missing plot method to estimate that 40 alleles occur in the species. In accord with population genetics theory, we observed at most one synonymous sequence difference between copies of alleles sampled from the different populations and no variation within populations. Phylogenetic analysis shows that nearly every allele in S. aucuparia arose prior to divergence of this species from members of three different genera of the Rosaceae subfamily, Maloideae. Lack of observable sequence variation within alleles, coupled with the slow pace of allelic relative to taxonomic diversification, implies that finding intermediate stages in the process of new allele creation will be difficult in this group.