Genotype-dependent differences in S-12-RNase expression lead to sporadic self-compatibility

Sporadic self-compatibility, the occasional fruit formation after otherwise incompatible pollinations, has been observed in some S-12-containing genot ypes of Solanum chacoense but not in others. We have sequenced this S-12 al lele and analyzed its expression in four different genotypes. The S-12-RNas e levels were generally less abundant than those of other S-RNases present in the same plants. In addition, two-fold and five-fold differences in the amount of S-12-RNase and S RNA, respectively, were observed among the genot ypes analyzed. A comparison with the genetic data showed that genotypes wit h the highest levels were fully and permanently self-incompatible, whereas those with the lowest levels were those in which sporadic self-compatibilit y had been observed. The mature protein contains four potential glycosylati on sites and genotype-specific differences in the pattern of glycosylation are also observed. Our results suggest the presence of modifier genes which affect, in a genotype-dependent manner, the level of expression and the po st-translational modification of the S-12-RNase.