Breakdown of self-incompatibility in a natural population of Petunia axillaris (Solanaceae) in Uruguay containing both self-incompatible and self-compatible plants

Many members of the Solanaceae display a type of gametophytic self-incompatibility which is controlled by a single multiallelic locus, called the S-locus. From our previous survey of more than 100 natural populations of Petunia axillaris (a solanaceous species) in Uruguay, we had found that the majority of the populations of subspecies axillaris were comprised of virtually all self-incompatible individuals. The rest were 'mixed populations' which contained mostly self-incompatible and some self-compatible individuals. In this study, we examined the self-incompatibility behavior and determined the S-genotypes of 33 plants raised from seeds obtained from one such mixed population, designated U1. We found that 30 of the 33 plants (designated U1-1 through U1-33) were self-incompatible and a total of 18 different S-alleles were represented. To determine the S-genotypes of the three self-compatible plants (U1-2, U1-16, and U1-22) and the possible causes for the breakdown of their self-incompatibility, we carried out reciprocal crosses between each of them and each of the 18 S-homozygotes (S₁S₁ through S₁₈S₁₈) obtained from bud-selfed progeny of 14 of the 30 self-incompatible plants. For U1-2 and U1-16, we also carried out additional crosses with U1-25 (with S₁S₁₃ genotype) and an S₁₃S₁₅ plant (obtained from a cross between an S₁₃-homozygote and an S₁₅-homozygote), respectively. Based on all the pollination results and analysis of the production of S-RNases, products of S-alleles in the pistil, we determined the S-genotypes of U1-2, U1-16, and U1-22, and propose that the breakdown of self-incompatibility in these three plants is caused by suppression of the production of S₁₃-RNase from the S₁₃-allele they all carry. We have termed this phenomenon 'stylar-part suppression of an S-allele' or SPS.