Self‐incompatibility RNases from three plant families: homology or convergence?

In the Rosaceae, Scrophulariaceae, and Solanaceae, the stylar product of the self‐incompatibility (S‐) locus is an RNase. Using protein sequence data from 34 RNase genes (three fungal RNases, seven angiosperm non‐S RNases, 11 Rosaceae S‐alleles, three Scrophulariaceae S‐alleles, and ten Solanaceae S‐alleles) we reconstructed the genealogy of angiosperm RNases using the neighbor joining method and two distance metrics in order to assess whether use of S‐RNases in these families is the result of homology or convergence. Four monophyletic groups of angiosperm RNases were found: the S‐RNases of each of the three families and a group comprising most of the angiosperm non‐S RNases. The S‐RNases of the Scrophulariaceae and Solanaceae were found to be homologous but strong inference concerning the homology or convergence of S‐RNases from the Rosaceae with those of the other families was not possible because of uncertain placement of both the root and two of the angiosperm non‐S RNases. The most recent common ancestor of the Rosaceae and both the Scrophulariaceae and Solanaceae is shared by ~80% of dicot families. If the ‐RNases of the Rosaceae are homologous to those of the Scrophulariaceae and Solanaceae, then many other dicot families might be expected to share RNases as the mechanism of gametophytic self‐incompatibility.