AN ASEXUAL FUNGUS HAS THE POTENTIAL FOR SEXUAL DEVELOPMENT

The availability of cloned genes that control sexual reproduction (mat ing type genes) in higher fungi has allowed us to consider the causes of failure to mate in asexual fungi. We report here that the asexual f ungus Bipolaris sacchari has a homolog of the MAT-2 gene of its sexual ascomycete relative Cochliobolus heterostrophus. The B. sacchari MAT- 2 sequence is highly similar to that of C. heterostrophus MAT-2 and, i n fact, functions in transgenic C. heterostrophus. Thus, the asexual n ature of B. sacchari, is not due to absence or mutation of MAT. When e ither of the C. heterostrophus MAT genes was transformed into B. sacch ari, the recipient could neither self nor cross with other B. sacchari strains, in contrast to transgenic C. heterostrophus. strains which c an do both. Persistent asexuality of B. sacchari, in spite of the pres ence of complementary functional MAT genes, suggests that this fungus lacks genes other than MAT which are essential for mating. Notably, th e transgenic B. sacchari strains were sometimes able to initiate, but not complete, sexual development in interspecific pairings with C. het erostrophus. Transcript analysis showed that the B. sacchari MAT-2 gen e is expressed in transgenic C. heterostrophus and that the C. heteros trophus MAT genes are expressed in transgenic B. sacchari. No transcri pt of the native B. sacchari MAT-2 gene was detected under any growth condition tested.