Abstract: In grasses, comparative genome analyses have demonstrated that there were widespread syntenies and colinearities of the genes among different species within a family. As a model genome, analysis of the rice ( Oryza sativa L. ) genome has allowed us to reveal the basic evolutionary units of the cereal genomes. The authors used a rice genomic clone of the gene Xa21 inserted in the plasmid pBluescript as a probe to screen rice bacterial artificial chromosome (BAC) library. A contig of three BAC clones was constructed. One of the BAC clones was adopted as the tested probe. The tested plants were O. sativa subsp, indica cv. "Guang Lu Ai 4" and the inbred line of Zea mays cv. Huang Zhao 4. The biotin-labeled BAC clone was hybridized onto the rice and maize metaphase chromosomes by fluorescence in situ hybridization (FISH). The Xa21 cloned in Bluescript was also hybridized onto the maize chromosomes. The results obtained from the BAC and plasmid clones were the same. In rice, the Xa21 was located in the long arm of the chromosome 11, the percent distance from the centromere to the signal was about 24. This result was in accordance with the rice genetic map. In maize, three hybridized sites were detected ant located on the long arms of chromosomes 1, 3, and 8 respectively. It suggested that the homologous sequences of Xa21 were triplicated in maize. By BAC-FISH technique, the detection rate was over 40 %, much higher than that by FISH with plasmid clones, for which it was only about 15%. Moreover, the rate of signals which were detected simultaneously on two members of a homologous chromosome pair and sister chromatids was higher by BAC-FISH technique than that by FISH with plasmid clones. It demonstrated the feasibility and advantage of BAC-FISH technique in gene comparative physical location. Cot I DNA was necessary for blocking the nonspecific DNA because the tested results were interfered seriously by repeated sequences. The signals could be found on many chromosomes without blocking.

Key words: Rice, BAC, FISH, Xa21, Comparative physical map