Using the mating type genes (MAT-1 and MAT-2) from Cochliobolus heterostrophus (a pathogen of maize) as probes, MAT genes from C. carbonum (also a maize pathogen) and C. victoriae (an oat pathogen) were cloned. Sequence analyses and functional tests showed that C. carbonum MAT-1 and C. victoriae MAT-2 are structurally similar to their C. heterostrophus homologues, and all three genes function comparably when expressed in C. heterostrophus. Gel blot analyses of DNAs from C. heterostrophus or C. carbonum, probed with C. heterostrophus MAT-specific fragments, revealed that field isolates from diverse geographical locations contained either MAT-1 or MAT-2, but never both. The entire currently existing collection of C. victoriae isolates, however, contained only MAT-2, and all members of the collection were found to be either female sterile or completely infertile. Thus, crosses between isolates of C. victoriae were not possible, although crosses between C. victoriae and C. carbonum (but not C. heterostrophus) were readily made, confirming previous reports that these two species are interfertile. These observations, combined with careful scrutiny of the literature, have led to the hypothesis that MAT-1 female fertile isolates of C. victoriae have never existed. All available evidence is consistent with the notion that the entire C. victoriae field population was derived from a single progenitor strain, which arose as the result of horizontal transfer of genes for pathogenicity to oats into a strain of C. carbonum that was MAT-2 and female sterile.