Breakdown of self-incompatibility in a natural population of Petunia axillaris caused by a modifier locus that suppresses the expression of an S-RNase gene

We had previously studied the cause of breakdown of self-incompatibility in a natural population (designated U1) of Petunia axillaris subsp. axillaris (Solanaceae) identified in Uruguay. This population contained mostly self-incompatible and some self-compatible plants. We had found that an S-haplotype, S₁₃, carried by three self-compatible individuals (U1-2, U1-16, and U1-22) was functional in the pollen, but not in the style, and that S₁₃-RNase was not detected in the styles of these individuals. This defective haplotype was designated S₁₃^sps, with "sps" standing for "stylar-part suppression". In this work, we further investigated the molecular and genetic basis of this suppression. We isolated and sequenced cDNAs for S₁₃-, S₁- and S₁₅-RNases, and used them as probes to show that the transcript of the S₁₃-RNase gene was not detectable in U1-2 (S₁S₁₃^sps), U1-16 (S₁₃^spsS₁₅), or U1-22 (S₁₃^spsS₁₃^sps), but the transcript levels of the S₁- and S₁₅-RNase genes in U1-2 and U1-16, respectively, were normal. Genomic blotting analysis revealed that the S₁₃-RNase gene was present in these three self-compatible plants. The S-genotypes obtained in budselfed and normally selfed progenies of these three plants were determined. Analysis of these S-genotypes led us to propose a model invoking a modifier locus, named MDF, to explain the suppression of the expression of the S₁₃-RNase gene in the genetic background of the self-compatible plants carrying the S₁₃^sps-haplotype.