Physical mapping of rice chromosomes 4 and 7 using YAC clones.

Physical maps of rice chromosomes 4 and 7 were constructed by landing yeast artificial chromosomes (YACs) along our high-density molecular linkage map. Using 114 DNA markers, 258 individual YACs were located on chromosome 4. Sixty-two out of 258 YACs carried two or more DNA marker positions and formed 16 contigs which covered a total length of 17.1 cM. The other YACs were arranged to 23 positions. On chromosome 7, 203 individual YACs were landed on 109 DNA markers. Sixty-four out of 203 YACs formed 15 contigs which covered a total length of 21.8 cM and 139 YACs were localized to 26 positions. Chromosomes 4 and 7 were covered with minimum tiling paths of 45 and 48 YACs, respectively. Taking the average size of YAC insert DNA to be 350 kb and the entire genome size to be 430 Mb, about 16-18 Mb of each chromosome or an estimated 50% of their total lengths have been covered with YACs. Physical maps of these 2 chromosomes should be of great help in identifying useful trait genes and unraveling genetic and biological characteristics in rice.