Analyses of molecular genetic mechanism involved in the sporophytic self-incompatibility of Ipomoea.

自交不亲和性（Self-incompatibility，SI）是一种阻止植物自花受精从而促进异交以维持植物种群遗传多样性的遗传系统。SI系统通过雌蕊识别自花花粉和非自花花粉后排斥自花花粉而起作用。在旋花科植物中，几种番薯属植物具有由单个多等位基因S-位点调控的孢子体型SI。本研究旨在鉴定与自花花粉识别相关的S位点基因，揭示番薯属植物自花花粉识别系统的分子机制。研究结果总结如下。(1)在S位点附近构建了精细的DNA标记遗传图谱。(2)利用图位克隆技术，从细菌人工染色体（BAC）和粘粒文库中筛选出了几个覆盖S基因座的基因组克隆。(3)分离的克隆的序列分析表明，S-位点被界定为S单倍型特异性的发散区（SDRs）跨越30至95 kb的大小。(4)通过北方杂交和RT-PCR的基因表达分析，确定了SDRs中的3个柱头特异基因（SE 1，SE 2，SEA）和1个花药特异基因（AB 2）分别为SI的雌蕊和花粉决定子的候选基因。(5)导入柱头特异基因的RNAi构建体的转化体SI表型没有变化，表明柱头特异基因可能起互补作用。(6)目前的研究表明，这些柱头特异性候选人没有表现出与其他植物的S-位点基因的同源性，并建议一个独特的分子机制，涉及到在Ipomoea植物的SI系统。

Self-incompatibility (SI) is a genetic system that prevents self-fertilization and thereby promotes out-crossing to maintain genetic diversity in plant populations. The SI system works through the rejection of self-pollen following the recognition of self and non-self pollen by the pistil. In the plant family, Convolvulaceae, several species of Ipomoea possess a sporophytic type of SI that is regulated by a single multiallelic S-locus. The present study was performed to identify the S-locus genes involved in the self-pollen recognition and to reveal molecular mechanism of the SI system in Ipomoea. Research results obtained are summarized as follows. (1) Fine genetic map of DNA markers was constructed in the vicinity of the S-locus. (2) Using a map-based positional cloning strategy, several genomic clones covering the S-locus were screened from bacterial artificial chromosome (BAC) and cosmid libraries of three S haplotypes. (3) Sequence analysis of the isolated clones showed that the S-loci were delimited to the S haplotype-specific divergent regions (SDRs) spanning 30 to 95 kb in size. (4) From gene expression analyses by Northern blot and RT-PCR of mRNAs, three stigma-specific genes (SE1, SE2, SEA) and an anther-specific gene (AB2) in the SDRs were identified as candidates encoding pistil and pollen determinants of the SI, respectively. (5) Transformants introduced with RNAi constructs of the stigma-specific genes showed no changes in the SI phenotypes, indicating that the stigma-specific genes might be functioned as complementary. (6) The present study revealed that these stigma specific candidates showed no homologies with the S-locus genes reported to date from other plants, and suggests that a unique molecular mechanism is involved in the SI system of Ipomoea plants.