Genealogical Structure Among Alleles Regulating Self-Incompatibility in Natural Populations of Flowering Plants

A method is proposed for characterizing the structure of genealogies among alleles that regulate self-incompatibility in flowering plants. Expected distributions of ratios of divergence times among alleles, scaled by functions of allele number, were generated by numerical simulation. These distributions appeared relatively insensitive to the particular parameter values assigned in the simulations over a fourfold range in effective population size and a 100-fold range in mutation rate. Generalized least-squares estimates of the scaled indices were obtained from genealogies reconstructed from nucleotide sequences of self-incompatibility alleles from natural populations of two solanaceous species. Comparison of the observed indices to the expected distributions generated by numerical simulation indicated that the allelic genealogy of one species appeared consistent with the symmetric balancing selection generated by self-incompatibility. However, the allelic genealogy of the second species showed unusually long terminal branches, suggesting the operation of additional evolutionary processes.