Structure and function of mating type genes in Cochliobolus spp. and asexual fungi

Mating type (MAT) genes of Cochliobolus heterostrophus have homologs in other heterothallic Cochliobolus spp., in homothallic Cochliobolus spp., and in asexual fungi thought to be taxonomically related to Cochliobolus (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. heterostrophus MAT-1 strain, and transgenic strains mated with C. heterostrophus MAT-1. Thus the cause of asexuality in B. sacchari is not absence or lack of a functional MAT gene. When the C. heterostrophus MAT genes were expressed in B. sacchari, however, no sexual development occurred, suggesting that this asexual fungus lacks an attribute, other than the mating type gene, which is required for mating. Although cloned MAT genes function upon transformation into recipient strains, they do not confer 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 self and cross to tester strains of either mating type. However, any transgenic strain carrying both a resident MAT gene and an homologous or heterologous MAT transgene develops normal perithecia but few ascospores in a cross that requires function of the transgene. To determine if the resident MAT gene interferes with function of the transgene, the MAT locus was deleted from the genome of C. heterostrophus and then replaced with the MAT gene of C. heterostrophus, C. carbonum, C. victoriae, or B. sacchari. Interference was eliminated and abundant ascospores were formed when the four transgenic strains were crossed to C. heterostrophus strains of opposite mating type. Key words: asexual fungi, DNA-binding proteins, heterologous expression, transformation.