Genetic analysis of cross fertility between two self-sterile strains of Glomerella graminicola

The ascomycete Glomerella cingulata has an unusual and complex mating syste m which is controlled by multiple, multiallelic loci. Cross fertility betwe en different isolates occurs via complementation of mutated fertility genes , a process known as unbalanced heterothallism. We have examined the herita bility of cross fertility among progeny of crosses between two self-sterile strains of G. graminicola to determine if unbalanced heterothallism also o ccurs in this species. Both random spore and tetrad progeny were analyzed. One-half of the progeny mated with one parent, one-fourth mated with the se cond parent, and one-fourth did not mate with either parent None mated with both parents, and none were self fertile. The data support a model in whic h cross fertility between strains is regulated by two unlinked loci. One lo cus may be a component of the pathway for self fertility The other locus ap pears to be involved in self/nonself recognition, because alternate functio nal alleles must be present in the parents. However, this second locus is u nlikely to be a typical ascomycete MAT idiomorph, since the HMG box of a pu tative MAT2 idiomorph from G. graminicola was found in both parent strains at a locus which is not linked to the self/nonself recognition locus identi fied in this study. The data reported here suggest that cross fertility in G. graminicola is genetically complex, and that it functions at least in pa rt via unbalanced heterothallism.