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

开花植物的自交不亲和性(SI)是一种复杂的遗传系统，它阻止自交受精，从而促进异交以维持物种内的属种多样性。在旋花科植物中，旋花科的几个种具有孢子体类型的SI，这种类型导致自花授粉后柱头表面的花粉完全不能萌发。通菜的SI系统由一个称为S的多等位基因座调控。为了鉴定通菜中的S基因，我们采用了一种基于精细连锁图谱的定位克隆策略。从构建的细菌人工染色体文库、λ文库和Fosid文库中筛选出覆盖S基因座的多个基因组克隆。利用这些克隆的末端序列，进一步将S基因座的区域划在0.6 cM以内，跨度约300kb。在S基因座区域检测到明显的遗传重组抑制。通过对313-Kp区的序列分析，预测了43个开放阅读框(ORF)，许多重复序列和5个转座元件。对从生殖器官和营养器官中分离到的mRNAs进行Northern杂交分析，结果表明有5个基因在柱头中特异表达，4个基因在花药花粉中特异表达。本研究表明，这些生殖特异基因与其他植物中已报道的SI基因没有同源性，提示旋花科植物SI系统中存在独特的分子机制。

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.