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

Abstract

Abstract 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.