Construction of a Sorghum propinquum BAC library, toward positional cloning of the sorghum shattering gene (Sh1) and the sorghum photoperiodic gene (Ma1)

This research focuses on the isolation of the sorghum shattering gene (Sh1) and the photoperiodic flowering gene (Ma1). Shattering of the mature inflorescence is an important way for wild plants and weeds to disperse seeds but can cause economically significant grain losses in crop plants. Isolation of shattering genes can enhance understanding of seed dispersal processes and perhaps help in reducing grain losses. Most species in the grass family originate from tropical or subtropical areas with wild genotypes that are photoperiod-sensitive, in which flowering time is regulated by day length. The response of tropical genotypes to photoperiod has hindered the use of exotic germplasm in temperate zones. Isolation of the photoperiodic flowering gene could help us understand how photoperiodic genes regulate flowering in response to daylength, and also help in breeding photoperiod-insensitive species adapted for temperate agriculture.;A large insert Sorghum propinquum (K. Hitchcock) BAC library has been constructed to analyze the physical organization of the sorghum genome and to facilitate positional cloning of Sh1, Ma1, and other sorghum genes and QTLs associated with the early stages of grain crop domestication. This library consists of 38,016 BAC clones with an estimated average insert size of 126 kb and coverage of 6.6 genome-equivalents.;The sorghum shattering gene, Sh1 has been genetically mapped to an interval of 0.8 cM, co-segregating with RZ474 and flanked by pSB097 and BCD1072b. Twelve BAC clones with an average size of 113 kb were identified, and nine of them formed a contig spanning the region of pSB097 and RZ474(Sh1). The largest BAC in the contig, 39E21, has been used to screen a Johnsongrass (Sorghum halepense (L.) Pers.) inflorescence cDNA library, and six candidate cDNAs were identified. Four of the six may represent members of a gene cluster in this region.;The sorghum photoperiodic gene, Ma1, has been genetically mapped by using substitution mapping to an interval of 0.5 cM, flanked by pSB1113 and CDSR084. These two markers hybridized to ten BAC clones with an average size of 190 kb, which set the stage for chromosome walking to clone Ma1.