Evidence that intragenic recombination contributes to allelic diversity of the S-RNase gene at the self-incompatibility (S) locus in Petunia inflata.

For Solanaceae type self-incompatibility, discrimination between self and nonself pollen by the pistil is controlled by the highly polymorphic S-RNase gene. To date, the mechanism generating the allelic diversity of this gene is largely unknown. Natural populations offer a good opportunity to address this question because they likely contain different alleles that share recent common progenitors. We identified 19 S haplotypes from a natural population of Petunia inflata in Argentina, used reverse transcriptase-polymerase chain reaction to obtain cDNAs for 15 alleles of the S-RNase gene, and sequenced all the cDNAs. Phylogenetic studies revealed that five of these alleles and two previously identified alleles form a major clade, and that the 5' region of S(19) allele was derived from an ancestor allele closely related to S(2), whereas its 3' region was derived from an ancestor allele closely related to S(8). A similar evolutionary relationship was found among S(3), S(12), and S(15) alleles. These findings suggest that intragenic recombination contributed to the generation of the allelic diversity of the S-RNase gene. Two additional findings emerged from the sequence comparisons. First, the nucleotide sequence of the S(1) allele identified in this work is completely identical to that of the previously identified S(1) allele of a different origin. Second, in the two hypervariable regions HVa and HVb, thought to be involved in determining S allele specificity, S(6) and S(9) alleles differ only by four nucleotides, all in HVb, resulting in two amino acid differences. The implications of these findings are discussed.