Abstract
A genetic cross between rice-field isolates of Magnaporthe grisea produced progeny segregating for avirulence/ virulence on six rice cultivars among nine race differentials, while on three other cultivars, Shin 2 (Pik-s), Aichi Asahi (Pia) and Ishikari Shiroke (Pii), parental and progeny isolates were all virulent. Based on segregation ratios in 115 progeny isolates, avirulence on Kanto 51 (Pik), Yashiro-mochi (Pita), Fukunishiki (Piz) and Toride 1 (Piz-t) is under monogenic control. On Tsuyuake (Pik-m) and Pi No. 4 (Pita-2), however, a disproportionate ratio in the segregation was observed, suggesting that avirulence on these two cultivars is controlled by two or more genes. Assuming that the avirulence gene AvrPik-m consists of at least two genes, AvrPik-m1 and AvrPik-m2, each of which functions in the whole gene AvrPik-m, and that one of AvrPik-m1 and AvrPik-m2 is AvrPik, we could account for the disproportion in the avirulence/virulence segregation of the progeny. This hypothesis would also be consistently applied for avirulence gene AvrPita-2. There seem to be two types of the avirulence genes : AvrPik-m, that is comprised of the tightly linked genes, AvrPik-ml (=AvrPik) and AvrPik-m2, and AvrPita-2, that is comprised of the loosely linked genes AvrPita-2A (=AvrPita) and AvrPita-2B. As one possible explanation of the rice resistant reaction to blast, multiple specificity was suggested for the first time for the blast fungus. On the contrary, the avirulence genes AvrPiz and AvrPiz-t were inherited independently, despite the corresponding genes for resistance (Piz and Piz-t) being located at the same locus. The cross of rice blast isolates (races 447 and 337) produced only 25 kinds of races in the progeny, although theoretically about 64 kinds of races should be produced if six avirulence genes segregated independently. Because no progeny are with AvrPik (or AvrPita) and without AvrPik-m (or AvrPita-2), the number of races theoretically should be 36 at most. A number of strains, such as races 377 and 737, with a single avirulence gene were obtained from this cross. These strains may be valuable for analysis of resistance genes in rice plant.
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