Relative Heterothallism and Production of Hybrid Perithecia by Auxotrophic Mutants of Glomerella cingulata from Apple

Glomerella cmgulata is a homothallic species but produces a ridge of fertile perithecia at a frontier between certain wild-type strains on agar. To account for the presence or absence of perithecia earlier workers suggested that alleles at A and B loci control the formation of perithecia at mycelial frontiers in + and – strains. We propose that G. cingulata actually demonstrates “relative heterothallism”. Of 7 induced nutritionally deficient mutants (auxotrophs) in 2 wild-type strains from apple, only one methionine (met-1) and one arginine (arg) mutant in only one wild-type strain gave a heavy ridge of perithecia at their junctures. Neither the met-l nor arg mutations have been identified as those in the A or B locus. The perithecia were either homozygous (selfs) for met-1 or arg, or heterozygous (hybrids). Paired met-1 and arg segregants from hybrid perirhecia as well as diauxotrophic strains from met-l or arg mutants also gave hybrids of selfs. Specific nutritional deficiencies in certain wild-type strains which can direct sexuality are not yet known. Genetic studies are now feasible in G. cingulata to define enzymatic factors responsible for pathogenicity.