MAPPING OF A MAGNAPORTHE-GRISEA LOCUS AFFECTING RICE (ORYZA-SATIVA) CULTIVAR SPECIFICITY

Magnaporthe grisea causes rice blast, the most important fungal diseas e of rice. The segregation of genes controlling virulence of M. grisea on rice was studied to establish the genetic basis of cultivar specif icity in this host-parasite interaction. Full-sib progeny and parent i solates Guy11 and 2539 of M. grisea were inoculated onto rice (Oryza s ativa) cultivar 'CO39' and five near-isogenic lines (NILs) of C039. Ea ch NIL contained a different single gene affecting resistance to speci fic isolates of M. grisea. No differential interactions between NILs a nd progeny or parents were observed; parents and progeny pathogenic on CO39 were pathogenic on all five NILs. Segregation ratios of 101 full -sib progeny, 117 progeny from full-sib parents, and 109 backcross pro geny, indicated a common single gene affecting pathogenicity on CO39 a nd the five NILs, A subset of the above 327 isolates (43 full-sib prog eny, 37 progeny from full-sib parents, and 32 backcross progeny) were inoculated onto rice cultivar '51583'; all were pathogenic, indicating that cultivar specificity to CO39 was segregating in this population of isolates. The locus controlling cultivar specificity, named avrCO39 , was mapped to chromosome 1 using a subset of the progeny previously used to construct an RFLP map of M. grisea. The closest reported RFLP markers were 11.8 (estimated 260 kb) and 17.2 cM (estimated 380 kb) aw ay and provide starting points on either side of the locus for a ''chr omosome walk'' to clone the locus.