Cytology and Genetics of Sexual Incompatibility in Didymella rabiei
Mating crosses in all possible combinations between 15 monoascosporic isolates of Didymella rabiei (Ascomycotina, Dothideales), derived from diseased chickpea stems collected at a single location in Genesee, Idaho, were prepared on sterile chickpea stem pieces incubated on moist sterile filter papers in glass petri dishes for 5-6 weeks at 10°C or in nylon mesh bags placed on the soil surface outdoors to over-winter for 6 months. The discharge of large numbers of viable ascospores from mature pseudothecia forming on the chickpea stems was indication of successful matings. Pairings between sympatric isolates demonstrated that the fungus is heterothallic with a unifactorial (bipolar) homogenic mating incompatibility system. Papazian test pairings between tester strains and mating types from three locations in Idaho and Washington indicated the absence of complete interfertility between allopatric populations and provided strong evidence that the fungus is biallelic, lacking multiple alleles at the single mating locus. The nuclear content of somatic and reproductive cells at all phases of the life cycle was examined using Giemsa stain. Ascospores and conidia were commonly multinucleate and appeared to undergo multiple mitotic divisions prior to germination. Somatic hyphae derived from ascospores and conidia were predominately uninucleate. The importance of sexual reproduction, sexual incompatibility, and the teleomorph in the disease cycle is discussed relative to environmental factors required for their occurrence and implications for disease control.