Author
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Submitted to: Heredity
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/25/2009 Publication Date: 10/1/2009 Citation: Jia, Y. 2009. Artificial introgression of a large fragment around the Pi-ta rice blast resistance gene in backcross progenies and several elite rice cultivars. Heredity. 103(4):333-339. Interpretive Summary: Study of the size of genomic introgressions should lead to a better understanding of the inheritance and evolution of linkage blocks in crop breeding. Historically the size of genome introgression has been estimated by modeling. However now rice presents a unique opportunity to examine the size of introgressions due to the availability of its genome sequence and the abundance of simple sequence repeat (SSR) markers. In the present study, we investigated the introgression size of genomic fragments around Pi-ta, a blast disease resistance gene on chromosome 12, in 43 BC6F2 individuals and five modern elite rice cultivars. It was found that large introgression segments ranging from half (14Mbp) to the entire chromosome (27 Mbp) were still from the donor after six generations of backcrossing. Similarly, large segments of the Pi-ta genomic region originating from a landrace indica cultivar, Tetep from Vietnam, were also identified in Pi-ta containing US rice cultivars, Katy, Madison, Kaybonnet and Drew. It was determined that Tetep has an identical chromosome 12 as another landrace cultivar 'Tadukan' from the Philippines. The most widely grown indica cultivar IR64 was found to contain the same 2.6 Mbp around Pi-ta. These observations suggest that a large portion of the chromosome 12 can be maintained by artificial selection for Pi-ta mediated blast resistance during backcrossing crop breeding. Technical Abstract: Study of the size of genomic introgressions should lead to a better understanding of linkage disequilibrium in crop breeding. Rice presents a unique opportunity to examine the size of introgressions because of the availability of abundant simple sequence repeat (SSR) markers. In the present study, progenies of a cross of a tropical japonica rice cultivar Katy that contains the rice blast resistance gene Pi-ta with a temperate japonica cultivar M202 (pi-ta) were inoculated with race IB49 of Magnaporthe oryzae that recognizes Pi-ta, and the resistant progenies were identified for backcrossing for six generations. Two progeny of each of 22 BC6F1 were genotyped using 12 SSR around the Pi-ta genomic region on chromosome 12. Unlinked DNA in 43 BC6F2 individuals was found from the recurrent parent M202 as expected. However, unexpectedly various sizes of genomic fragments around Pi-ta from half (14Mbp) to the entire chromosome (27 Mbp) were found from the donor. Similarly, large segments of the similar 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 has an identical chromosome 12 as another landrace cultivar 'Tadukan' from the Philippines. The most widely grown indica cultivar IR64 was found to contain the same 2.6 Mbp around Pi-ta. The presence of Pi-ta in backcross progeny and cultivars were verified by the codominant marker for Pi-ta and the expected resistant reaction to IB49. This study demonstrates that a large portion of the chromosome 12 has been maintained by artificial selection for blast resistance during crop breeding. |
