ORIGIN OF ALLELIC DIVERSITY IN ANTIRRHINUM S-LOCUS RNASES

In many plant species, self-incompatibility (SI) is genetically contro lled by a single multiallelic S locus. Previous analysis of S alleles in the Solanaceae, in which S locus ribonucleases (S RNases) are respo nsible for stylar expression of SI, has demonstrated that allelic dive rsity predated speciation within this family. To understand how alleli c diversity has evolved, we investigated the molecular basis of gameto phytic SI in Antirrhinum, a member of the Scrophulariaceae, which is c losely related to the Solanaceae, We have characterized three Antirrhi num cDNAs encoding polypeptides homologous to S RNases and shown that they are encoded by genes at the S locus. RNA in situ hybridization re vealed that the Antirrhinum S RNases are primarily expressed in the st ylar transmitting tissue. This expression is consistent with their pro posed role in arresting the growth of self-pollen tubes. S alleles fro m the Scrophulariaceae form a separate group from those of the Solanac eae, indicating that new S alleles have been generated since these fam ilies separated (similar to 40 million years). We propose that the rec ruitment of an ancestral RNase gene into SI occurred during an early s tage of angiosperm evolution and that, since that time, new alleles su bsequently have arisen at a low rate.