Abstract
Field isolates of the heterothallic Ascomycete, Magnaporthe grisea Barr (anamorph, Pyricularia oryzae Cav. or P. grisea), include pathogens of many grasses, with rice (Oryza sativa) being the most important example from an economic perspective. Individual field isolates have a limited host range, parasitizing one or a few grass species (MACKILL and BONMAN 1986). We have recently shown that rice pathogens from around the world contain a family of repeated DNA sequences, MGR sequences, that appear to be absent from or present in low copy number in field isolates that infect grasses other than rice (HAMER et al. 1989a). We have hypothesized that the correlation between MGR sequence conservation and rice pathogenicity is due to genetic isolation and independent evolution of rice pathogens descended from a small ancestral population. As we will discuss, MGR sequences have become useful as tools for strain identification and as genetic markers for cloning host specificity genes of interest.
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