Genetic and physical mapping of Pi36(t), a novel rice blast resistance gene located on rice chromosome 8.

Blast resistance in the indica cultivar (cv.) Q61 was inherited as a single dominant gene in two F2 populations, F2-1 and F2-2, derived from crosses between the donor cv. and two susceptible japonica cvs. Aichi Asahi and Lijiangxintuanheigu (LTH), respectively. To rapidly determine the chromosomal location of the resistance (R) gene detected in Q61, random amplified polymorphic DNA (RAPD) analysis was performed in the F2-1 population using bulked-segregant analysis (BSA) in combination with recessive-class analysis (RCA). One of the three linked markers identified, BA1126(550), was cloned and sequenced. The R gene locus was roughly mapped on rice chromosome 8 by comparison of the BA1126(550) sequence with rice sequences in the databases (chromosome landing). To confirm this finding, seven known markers, including four sequence-tagged-site (STS) markers and three simple-sequence repeat (SSR) markers flanking BA1126(550) on chromosome 8, were subjected to linkage analysis in the two F2 populations. The locus was mapped to a 5.8 cM interval bounded by RM5647 and RM8018 on the short arm of chromosome 8. This novel R gene is therefore tentatively designated as Pi36(t). For fine mapping of the Pi36(t) locus, five additional markers including one STS marker and four candidate resistance gene (CRG) markers were developed in the target region, based on the genomic sequence of the corresponding region of the reference japonica cv. Nipponbare. The Pi36(t) locus was finally localized to an interval of about 0.6 cM flanked by the markers RM5647 and CRG2, and co-segregated with the markers CRG3 and CRG4. To physically map this locus, the Pi36(t)-linked markers were mapped by electronic hybridization to bacterial artificial chromosome (BAC) or P1 artificial chromosome (PAC) clones of Nipponbare, and a contig map was constructed in silico through Pairwise BLAST analysis. The Pi36(t) locus was physically delimited to an interval of about 17.0 kb, based on the genomic sequence of Nipponbare.