Fluctuations in heteromorphic self-incompatibility systems

The genetic determination of heterostyly is briefly reviewed and experimental data are presented on the inheritance of tristyly in Oxalis compressa. It is shown that the model appropriate to Lythrum salicaria, of two diallelic loci, is inadequate to explain segregation patterns found in O. compressa, especially the reversal of dominance of the short phenotype. An extended model having an additional allele at the short locus, and a separate modifier gene, is presented and its deterministic genotype frequency dynamics examined numerically. It is shown that fixation of these extra genes or alleles is unlikely. Problems of testing the extended model are considered.