Sexuality of self-sterile strains of Glomerella graminicola

Abstract

The mating system in the ascomycete Glomerella graminicola is unlike that in heterothallic ascomycetes where mating is controlled by a single locus with two alternate specificities. G. graminicola has multiple mating types that are determined by more than one genetic locus. Crosses between a melanin-deficient (Mel-) strain and a wild type (Mel+) strain of G. graminicola produced both darkly pigmented and lightly pigmented perithecia. In heterothallic ascomycetes, it is the ascogonial parent that produces the perithecial wall and the cytoplasm of the ascospores. Given the unusual nature of the mating system in G. graminicola, it was important to test the hypothesis that perithecium color is an indicator of parentage in matings of this fungus. Although Mel+ and Mel- progeny were recovered from both types of perithecia, mitochondrial DNA RFLPs were correlated with the color of the perithecium from which the progeny were derived. Crosses involving the Mel- strain and a second Mel+ strain produced Mel+ and Mel- progeny, but only one type of mitochondrial DNA which was like that of the Mel- parent, indicating that this Mel+ strain was female sterile. The putative female sterile strain, which displays normal sensitivity to potassium chlorate (Chls), was further examined by crossing it with a Mel+, female fertile, chlorate resistant (ChlR) strain. Among the progeny, the ChlR and Chls phenotypes segregated 1:1 but only the type of mitochondrial DNA found in the ChlR parent was detected. These results indicate that sexual differentiation and cytoplasmic inheritance in matings of G. graminicola are like those processes in heterothallic fungi, in spite of the fundamental differences between the genetic mechanisms that regulate mating specificity in these two groups.