Programmed ascospore death in the homothallic ascomycete Coniochaeta tetraspora

Immature asci of Coniochaeta tetraspora originally contain eight uninucleat e ascospores. Two ascospore pairs in each ascus survive and mature, and two die and degenerate. Arrangement of the two ascospore types in individual l inear asci is what would be expected if death is controlled by a chromosoma l gene segregating at the second meiotic division in about 50% of asci. Cul tures originating from single homokaryotic ascospores or from single uninuc leate conidia are self-fertile, again producing eight-spored asci in which four spores disintegrate, generation after generation. These observations i ndicate that differentiation of two nuclear types occurs de novo in each se xual generation, that it involves alteration of a specific chromosome locus , and that the change occurs early in the sexual phase. One, and only one, of the two haploid nuclei entering each functional zygote must carry the al tered element, which is segregated into two of the four meiotic products an d is eliminated when ascospores that contain it disintegrate. Fusion of nuc lei cannot be random-a recognition mechanism must exist. More study will be needed to determine whether the change that is responsible for ascospore d eath is genetic or epigenetic, whether it occurs just before the formation of each ascus or originates only once in the ascogonium prior to proliferat ion of ascogenous hyphae, and whether it reflects developmentally triggered alteration at a locus other than mating type or the activation of a silent mating-type gene that has pleiotropic effects. Similar considerations appl y to species such as Sclerotinia trifoliorum and Chromocrea spinulosa, in w hich all ascospores survive but half the spores in each ascus are small and self-sterile. Unlike C. tetraspora, another four-spored species, Coniochae tidium savoryi, is pseudohomothallic, with ascus development resembling tha t of Podospora anserina. (C) 2000 Academic Press.