STRUCTURE AND FUNCTION OF MATING-TYPE GENES IN COCHLIOBOLUS SPP AND ASEXUAL FUNGI

Mating type (MAT) genes of Cochliobolus heterostrophus have homologs i n other heterothallic Cochliobolus spp., in homothallic Cochliobolus s pp., and in asexual fungi thought to be taxonomically related to Cochl iobolus (e.g., Bipolaris spp.). To examine the cause of asexuality in B. sacchari, its homolog of C. heterostrophus MAT-2 was cloned. The B, sacchari sequence was 98% identical to that of C. heterostrophus MAT- 2, the gene conferred homothallism when expressed in a C, heterostroph us MAT-1 strain, and transgenic strains mated with C. hererostrophus M AT-1. Thus the cause of asexuality in B. sacchari is not absence or la ck of a functional MAT gene. When the C. heterostrophus MAT genes were expressed in B. sacchari, however, no sexual development occurred, su ggesting that this asexual fungus lacks an attribute, other than the m ating type gene, which is required for mating. Although cloned MAT gen es function upon transformation into recipient strains, they do nor co nfer full fertility. When an homologous or heterologous (e.g., from C. carbonum, C. victoriae, or B. sacchari) MAT gene is transferred into a C. heterostrophus strain of opposite mating type, the strain can sel f and cross to tester strains of either mating type. However, any tran sgenic strain carrying both a resident MAT gene and an homologous or h eterologous MAT transgene develops normal perithecia but few ascospore s in a cross that requires function of the transgene. To determine if the resident MAT gene interferes with function of the transgene, the M AT locus was deleted from the genome of C. heterostrophus and then rep laced with the MAT gene of C. hererostrophus, C, carbonum, C. victoria e, or B. sacchari. Interference was eliminated and abundant ascospores were formed when the four transgenic strains were crossed to C. heter ostrophus strains of opposite mating type.