S-allele diversity in Lycium andersonii: Implications for the evolution of S-allele age in the Solanaceae

We evaluate competing explanations for striking differences in the average age of self-incompatibility (S-) alleles in population samples. The age of alleles is inferred from evidence for trans-generic evolution (TGE): in which an allele sampled from one species is more closely related to an allele found in another genus than any con-generic allele, as determined by phylogenetic analysis. Whereas some species exhibit extensive TGE, indicating very long persistence of allelic lineages, in others limited TGE suggests extensive extinction and origination of alleles. We consider two explanations for inter-specific differences in TGE and allelic turnover: (1) bottleneck event(s) which have accelerated the loss of allelic diversity in some species; and (2) differences among species in the origination rate of new allelic specificities. We used data on S-allele diversity in Lycium andersonii (Solanaceae), a self-incompatible perennial shrub of deserts of southwestern North America, to estimate the presumed change in origination rate. We find that predicted allelic turnover assuming a change in the origination rate of new S-allele specificities is insufficient to account for inter-specific differences in allele turnover. (C) 2000 Annals of Botany Company.