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

Abstract

The ascomycete Glomerella cingulata has an unusual and complex mating system which is controlled by multiple, multiallelic loci. Cross fertility between different isolates occurs via complementation of mutated fertility genes, a process known as unbalanced heterothallism. We have examined the heritability of cross fertility among progeny of crosses between two self-sterile strains of G. graminicola to determine if unbalanced heterothallism also occurs 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 second 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 which cross fertility between strains is regulated by two unlinked loci. One locus may be a component of the pathway for self fertility. The other locus appears to be involved in self/nonself recognition, because alternate functional alleles must be present in the parents. However, this second locus is unlikely to be a typical ascomycete MAT idiomorph, since the HMG box of a putative MAT2 idiomorph from G. graminicola was found in both parent strains at a locus which is not linked to the self/nonself recognition locus identified in this study. The data reported here suggest that cross fertility in G. graminicola is genetically complex, and that it functions at least in part via unbalanced heterothallism.