Molecular cloning of two Solanum chacoense S-alleles and a hypothesis concerning their evolution

The polymerase chain reaction (PCR) has been used to clone two S-alleles (S13 and S14) from Solanum chacoense. The two alleles do not cross-hybridize on genomic Southern blots or on northern blots using stylar RNA. Although the S14 message was not detected in a stylar cDNA library prepared from mature flowers, a full-length copy of the S13 coding sequence was isolated by screening with the PCR fragment. We have analysed the sequences of the S13 cDNA and the S14 PCR fragment (60% of the mature protein coding sequence) in the context of S-RNase evolution, and propose that random point mutations may be sufficient to generate new S-alleles. Based on a phylogenetic tree composed of RNase sequences containing the conserved RNase motifs HGLWP and KHGXC, we suggest that gametophytic self-incompatibility genes are RNase genes that have acquired a new function in the gametophytic self-incompatibility system early in the evolution of flowering plants.