Plant Genome IV Conference Plant Genome IV Conference
Recent genome comparative mapping has demonstrated that the content and order of genes in grass genomes are conserved. Rice is the key grass species because it is a true diploid, has the smallest grass genome, the highest density molecular maps, and a well-developed transformation system. Thus, rice has been considered the model species for genetic and biological studies of grass species. To facilitate rice and other grass genome research, we previously constructed two rice BAC libraries from the genomes of two major cultivated rice subspecies, indica and japonica (Zhang et al., Molecular Breeding, 1995). We are using the Hd3a gene locus for rice heading date to optimize procedures of using the two rice BAC libraries for map-based gene cloning, genome analysis and physical mapping. We previously used a DNA marker (RG348) closely linked to the Hd3a locus as a probe and isolated 5 RG348-associated BAC clones from the two rice BAC libraries. The BAC DNAs were isolated, physically mapped by fluorescent in situ hybridization (FISH), and fingerprinted to construct a contig for these BACs. The ends of the BACs were isolated by inverse PCR (IPCR) and plasmid rescue. The single-copy ends that gave RFLPs between the parents of the mapping population were genetically mapped. The single-copy ends of the contig were then used as probes to rescreen the BAC libraries for the next step of the chromosome walk toward the Hd3a locus. We report here the progress of chromosome walking toward the Hd3a gene.