Artificial introgression of a large chromosome fragment around the rice blast resistance gene Pi-ta in backcross progeny and several elite rice cultivars

Studying the size of genomic introgressions should lead to a better understanding of linkage disequilibrium in crop breeding. In this study, progeny of the cross between a tropical japonica rice cultivar Katy containing the rice blast resistance gene Pi-ta and a temperate japonica cultivar M202 (Pi-ta) were inoculated with the race IB49 of Magnaporthe oryzae that recognizes Pi-ta. The resistant progeny were identified during backcrossing for five generations. Two progeny of each of the 22 BC(5)F(1) were genotyped using 12 simple sequence repeat markers around the Pi-ta genomic region on chromosome 12. Unlinked DNA in 43 BC(5)F(2) individuals was found primarily from the recurrent parent M202 as expected. However, unexpectedly, various sizes of genomic fragments around Pi-ta ranging from half (14Mbp) to the entire chromosome (27Mbp) were found from the donor. Similarly, large segments of comparable sizes of the Pi-ta genomic region originating from a landrace indica variety Tetep from Vietnam were also identified in Pi-ta containing US rice cultivars, Katy, Madison, Kaybonnet, and Drew. It was also determined that Tetep had an identical chromosome 12 to another landrace cultivar Tadukan from the Philippines. The most widely grown indica cultivar IR64 was found to contain the same 6.4Mbp around Pi-ta. This study demonstrates that a large portion of the chromosome was maintained by artificial selection for blast resistance during crop breeding. Heredity (2009) 103, 333-339; doi: 10.1038/hdy.2009.95; published online 22 July 2009