A self-compatible mutant S allele conferring a dominant negative effect onthe functional S allele in Ipomoea trifida

A spontaneously occurring self-compatible mutant has been identified in Ipo moea trifidia, a species possessing sporophytic self-incompatibility contro lled by a single multiallelic S locus. Analysis of the segregation of compa tibility/incompatibility phenotypes in selfed and crossed progenies of the self-compatible mutant plant indicated that the self-compatibility trait wa s caused by a mutation at the S locus; the mutated S allele was therefore d esignated Sc. RFLP analysis of progeny plants segregating for the Sc allele using the SSP gene (a gene linked closely to the S locus of I, trifida) as a probe confirmed that the mutation was present at the S locus. Self-incom patibility responses were examined in F-1 progenies obtained from crosses b etween the self-compatible mutant and self-incompatible plants homozygous f or one of three S alleles, S-1, S-3 and S-22, where the dominance relations hip is S-22>S-1>S-3. All F-1 progeny plants from crosses with S-22 and S-1 homozygotes were self-incompatible and exhibited the respective phenotypes of each self-incompatible parent (either S-22 or S-1) in both stigma and po llen. However, of the F-1 progeny plants from the cross with the S-3 homozy gote, those carrying the genotype ScS3 were all self-compatible and cross-c ompatible as both female and male parents with the S-3 homozygote. These re sults indicate that the dominance relationship between the four S alleles i s: S-22>S-1>Sc>S-3 and so reveal the unexpected finding that the mutated Sc allele is dominant over a functional S-3 allele. A possible explanation fo r this observation is that the gene product encoded by the Sc allele confer s a dominant negative effect on the S-3 gene product.