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

Abstract

A spontaneously occurring self-compatible mutant has been identified in Ipomoea trifida, a species possessing sporophytic self-incompatibility controlled by a single multiallelic S locus. Analysis of the segregation of compatibility/incompatibility phenotypes in selfed and crossed progenies of the self-compatible mutant plant indicated that the self-compatibility trait was caused by a mutation at the S locus; the mutated S allele was therefore designated 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-incompatibility responses were examined in F1 progenies obtained from crosses between the self-compatible mutant and self-incompatible plants homozygous for one of three S alleles, S1, S3 and S22, where the dominance relationship is S22>S1>S3. All F1 progeny plants from crosses with S22 and S1 homozygotes were self-incompatible and exhibited the respective phenotypes of each self-incompatible parent (either S22 or S1) in both stigma and pollen. However, of the F1 progeny plants from the cross with the S3 homozygote, those carrying the genotype ScS3 were all self-compatible and cross-compatible as both female and male parents with the S3 homozygote. These results indicate that the dominance relationship between the four S alleles is: S22>S1>Sc>S3 and so reveal the unexpected finding that the mutated Sc allele is dominant over a functional S3 allele. A possible explanation for this observation is that the gene product encoded by the Sc allele confers a dominant negative effect on the S3 gene product.