THE 2 COCHLIOBOLUS MATING-TYPE GENES ARE CONSERVED AMONG SPECIES BUT ONE OF THEM IS MISSING IN COCHLIOBOLUS-VICTORIAE

Using the mating type genes (MAT-1 and MAT-2) from Cochliobolus hetero strophus (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-I and C. victoriae MAT-2 are structurally similar to their C. heterostro phus homologues, and all three genes function comparably when expresse d in C. heterostrophus. Gel blot analyses of DNAs from C. heterostroph us or C. carbonum, probed with C, heterostrophus MAT-specific fragment s, revealed that field isolates from diverse geographical locations co ntained either MAT-I or MAT-L, but never both. The entire currently ex isting collection of C. victoriae isolates, however, contained only MA T-2 and all members of the collection were found to be either female s terile or completely infertile. Thus, crosses between isolates of C. v ictoriae 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 observ ations, 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 t hat the entire C. victoriae field population was derived from a single progenitor strain, which arose as the result of horizontal transfer o f genes for pathogenicity to oats into a strain of C. carbonum that wa s MAT-2 and female sterile.