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

Abstract

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 (14 Mbp) to the entire chromosome (27 Mbp) 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 6.4 Mbp around Pi-ta. This study demonstrates that a large portion of the chromosome was maintained by artificial selection for blast resistance during crop breeding.

Diversification and evolution of the avirulence gene AVR-Pita1 in field isolates of Magnaporthe oryzae

“…Despite that, it is still difficult to determine if a balanced or an unbalanced selection had occurred at the Pi-ta region. These findings do suggest that the Pi-ta region has evolved under extensive selection pressure during crop breeding for resistance (Lee et al, 2009a;Jia, 2009).…”

Evolutionary Dynamics of the Genomic Region Around the Blast Resistance Gene Pi-ta in AA Genome Oryza Species

“…Significantly, one of the largest linkage blocks of resistant Pi-ta was identified in backcrossing and elite rice cultivars (Jia 2009). The identification of a large linkage block around Pi-ta raised at least two possibilities.…”

“…At the Pi-ta locus, another gene Ptr(t) was found to be essential for Pi-tamediated resistance ( Jia and Martin 2008). The possible artificial selection of the large Pi-ta genomic region has been reported for maintaining the broad spectrum of Pi-ta-mediated blast resistance ( Jia 2009). Taken together with other studies, this study suggests that other components such as Ptr(t) or R genes for the Pi-ta-mediated resistance may occur within at least 5 Mb of the Pi-ta region.…”

“…Tadukan was confirmed to be the Pita donor for various Asian japonica cultivars (Rybka et al 1997) whereas Tetep was the Pi-ta donor for the U. S. cultivars (Gravois et al 1995;Moldenhauer et al 1998;McClung et al 1999;Gibbons et al 2006;Moldenhauer et al 2007). Recently, the large introgressed chromosomal segments surrounding the Pi-ta locus were identified in backcross BC 5 progenies and elite rice cultivars ( Jia 2009). This suggests that the broad spectrum of the Pi-ta resistance in the United States may include the effects of other loci in the Pi-ta region, inherited as a ''superlocus.''…”

Genetic Variation of Rice Blast Resistance Genes inOryza sativaand Its Wild Relatives

“…Selection pressure may be responsible for maintaining this introgression in place. Jia (2009) was also able to show that a large chromosome segment around Pita was maintained intact during a backcrossing program.…”