EVIDENCE OF PARASEXUAL EXCHANGE OF DNA IN THE RICE BLAST FUNGUS CHALLENGES ITS EXCLUSIVE CLONALITY

We applied DNA markers to determine whether parasexual recombination m ay contribute to the extreme genetic diversity and variability observe d in Magnaporthe grisea, the causal agent of rice blast disease. Dispe rsed repetitive elements and mapped, low-copy restriction fragment len gth polymorphism (RFLP) probes were used to detect transfers of DNA be tween cultured isolates of M. grisea. Low-copy RFLP probes also were u sed to detect putative recombinants among isolates from well-character ized field populations of the pathogen. Microscopic examination of tuf ted mycelium between cocultured isolates revealed frequent hyphal fusi ons. Hyphal tips and conidia were recovered without selection from tuf ted zones in two separate vegetative pairings involving isolates with dissimilar haplotypes, based on the repetitive element MGR586. Haploty pic changes were observed at a higher frequency in tuft derivatives th an in subcultures of each isolate alone. From 136 tuft derivatives ana lyzed, 5 putative recombinant haplotypes were identified. Introgressio n was demonstrated with two independent repetitive elements, fosbury a nd MGR586, as probes on DNA digested with several restriction enzymes. Introgressions were characterized by addition of 1 to 10 MGR586 bands , and 1 to 3 fosbury bands from one parent into the background of the other. Polymorphic single-copy probes were used to analyze putative re combinants. One probe detected an introgression event as predicted by analysis with MGR586. To assess the possible role of parasexual recomb ination in field populations of the pathogen, isolates in the Philippi nes previously grouped based on DNA fingerprinting were analyzed with low-copy RFLP markers. Polymorphism in single-copy loci typically was seen between, but not within, putative pathogen lineages. One lineage (designated lineage 4), however, was polymorphic for several probes. F or some isolates, alleles at these loci comigrated with alleles charac teristic of other lineages, suggesting the transfer of DNA fragments b etween lineages. One isolate was apparently a merodiploid, carrying an allele typical of lineage 4 plus another allele characteristic of a d ifferent lineage. In a survey of isolates from the Indian Himalayas, a merodiploid also was found with single- or low-copy probes. Examinati on of MGR586 profiles of the putative recombinant and its putative don or strains showed the expected introgression of MGR586 bands. The dete ction of parasexual DNA exchanges in wild-type strains under unselecte d conditions and the existence of merodiploids in nature suggest that parasexual recombination occurs in field populations of M. grisea. Thi s raises questions concerning exclusive clonality in the blast fungus.