Sporophytic Self-incompatibility in Ipomoea trifida, a Close Relative of Sweet Potato

The genus Ipomoea (Convolvulaceae) has a sporophytic self-incompatibility system that is under the genetic control of a single multiallelic S-locus. Self-incompatibility reactions occur on the stigma surface at an early stage after pollination, and result in the complete arrest of pollen germination such that no seed is set. In a genetic analysis of 224 plants of diploid I. trifida, collected from six native populations in Central America, we identified 49 different S-alleles that showed a linear dominance-recessive hierarchy. We also obtained a spontaneous self-compatible mutant and showed that this self-compatibility trait is due to mutation at the S-locus. To investigate the molecular basis of sporophytic self-incompatibility in Ipomoea, we analysed stigma proteins and mRNAs extracted from several different S-genotypes, using two-dimensional gel electrophoresis (2D-PAGE) and AFLP-based mRNA fingerprinting (AMF), respectively. From the 2D-PAGE analyses, S-locus-linked stigma proteins (SSPs) were identified. The amino acid sequences of these proteins have a high homology to non-metallo short-chain alcohol dehydrogenases reported in several plant species. The SSP gene was mapped at a distance of 1·2 cM from the S-locus. In the AMF analysis, we obtained a clone homologous to the Brassica SRK, which showed no genetic linkage to the S-locus of Ipomoea. This suggests that the sporophytic self-incompatibility system of Ipomoea is mediated through a different molecular mechanism to that of Brassica.