SummaryRepeat-induced point mutation (RIP) is a process that detects DNA duplications and peppers their sequences with C:G to T:A transitions in the sexual phase of the life cycle. So far, this unique mechanism has been identified as a currently active process in only two fungal species, Neurospora crassa and Podospora anserina . To determine whether a RIP-like process operates in the plant pathogenic fungus Magnaporthe grisea , the retrotransposon MAGGY and the hygromycin B phosphotransferase gene were introduced into the fungus as multiple transgenes and examined for sequence alterations after a cross. Frequent C:G to T:A transitions in the transgenes were found in the descendants, preferentially in (A/ Tp)Cp(A/T) contexts, suggesting that a process similar to RIP functions in M. grisea . We also examined the sequence of another retrotransposon Pyret in six field isolates of M. grisea. Even though no perfect stage has been known in M. grisea under field conditions to date, RIP-like transitions were found in all the field isolates tested. Interestingly, the frequency of the transitions mostly correlated with the fertility of the isolates examined under laboratory conditions. These results imply that the sexual cycle of this fungus exists or existed in the natural field context.

Fungal isolates from gray leaf spot on perennial ryegrass (prg isolates) were characterized by DNA analyses, mating tests, and pathogenicity assays. All of the prg isolates were interfertile with Triticum isolates and clustered into the crop isolate group (CC group) on a dendrogram constructed from rDNA-internal transcribed spacer 2 sequences. Since the CC group corresponded to a newly proposed species, Magnaporthe oryzae, all of the prg isolates were designated M. oryzae. However, DNA fingerprinting with MGR586, MGR583, and Pot2 showed that the prg isolates are divided into two distinct populations, i.e., TALF isolates and WK isolates. The TALF isolates were virulent only on Lolium species, whereas the WK isolates were less specific, suggesting that gray leaf spot can be caused not only by Lolium-specific isolates but also by less specific isolates. We designated the TALF isolates as Lolium pathotype. The TALF isolates showed diverse karyotypes in spite of being uniform in DNA fingerprints, suggesting that theyare unstable in genome organization.

A Triticum isolate (pathogenic on wheat) of Magnaporthe oryzae was crossed with an Oryza isolate (pathogenic on rice) to elucidate mechanisms of their parasitic specificity on wheat. When the pathogenicity of their F (1) cultures (hybrids between a Triticum isolate and an Oryza isolate) was tested on wheat, avirulent and virulent cultures segregated in a 7:1 ratio. This result suggests that three loci are involved in avirulence of the Oryza isolate on wheat. One of the three loci conditioned papilla formation, whereas the others conditioned the hypersensitive reaction. Allelism tests revealed that the locus conditioning papilla formation is Pwt2 while one of the two loci conditioning the hypersensitive reaction is Pwt1. The other locus conditioning the hypersensitive reaction was different from any other known loci and, therefore, was designated as Pwt5.