The Course of meiosis and centriole behaviour during the ascus development of the ascomycete Gelasinospora calospora

Meiosis and centriole behaviour in the ascomycete Gelasinospora calospora are described. During meiotic prophase, the ascus grows from 15 to 100 . This is accompanied by two marked changes in the primary ascus nucleus: (1) The nucleus increases in size at zygotene, and the paired bivalents elongate considerably from late pachytene-early diplotene until they reach an extremely diffuse diplotene. The time of chiasma formation in relation to these stages is discussed. (2) The nucleolus also increases in size during zygotene but regresses again at the diffuse diplotene stage. The significance of the nucleolar behaviour and the diffuse diplotene stage are discussed. The haploid number of chromosomes in this species is unquestionably demonstrated to be seven. The centrioles were found to increase in size from division I to division III, within the ascus, but to decrease in size again by division IV. At division III they can be seen to be rectangular and flattened in shape, with spindle fibres attached to one side. A three dimensional reconstruction of these organelles and an account of their behaviour is provided.