Molecular genetic analysis of the self-incompatibility locus in a wild species of sweet potato

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Self-incompatibility (SI) in flowering plants is an elaborate genetic system that prevents self-fertilization and thereby promotes out-crossing to maintain generic diversity within a species. In a plant family, Convolvulaceae, several species of the genus ipomoea possess a sporophytic type of SI that causes a complete failure of pollen germination on the stigma surface after self-pollination. The SI system of Ipomoea is regulated by a single multiallelic locus called the S-locus. To identify the S-locus genes in Ipomoea, we used a positional cloning strategy based on a fine linkage map of DNA markers. Several genomic clones covering the S-locus were screened from bacterial artificial chromosome (BAC), λ and fosmid libraries that were constructed in the present study. Using the terminal end sequences of these clones, the S-locus region was further delimited within 0.6 cM, spanning ca. 300kb. Remarkable suppression of genetic recombination was detected in the S-locus region. From sequence analysis of the 313-kp region, 43 open reading frames (ORFs), many repetitive sequences and 5 transposable elements were predicted. Northern blot analysis of mRNAs isolated from reproductive and vegetative organs showed that five and four genes were specifically expressed in the stigma and anther-pollen, respectively. The present study made clear that these reproductive specific genes showed no homologies with the SI genes reported to date from other plants and suggests that a unique molecular mechanism is involved in the SI system of Convolvulaceae family.