The life cycle and cytology of an apomictic Podospora

Abstract

An isolate, provisionally identified as Podospora arizonensis (Griff.) Cain, differs from the type in having asci with four large black and four small, hyaline spores. The small spores do not germinate, but the large spores, after a dormant period, show a low percentage of germination and form a mycelium which produces perithecia with asci containing dimorphic spores. There are numerous hyphal fusions, but no microconidia. The perithecium develops from an ascogonial coil and shows the typical Diaporthe -type of development. Asci arise from the binucleate, penultimate cell of a crozier, but the two nuclei do not fuse; instead they submit to two mitoses to give the eight haploid ascospores. The fungus is therefore apomictic. The haploid chromosome number is two. The four large, black and four small, hyaline spores show all seventy of the possible spore arrangements in the ascus. Since more than 98% of the asci produce four large, black and four small, hyaline spores it is suggested that the two nuclei which initiate the ascus are normally of different genotype. Some asci with an abnormal number of ascospores may have arisen by the inclusion of varying numbers and types of nuclei at their initiation. If this hypothesis is correct, there must be mutation to the ‘small spore’ type during the course of development from a large spore, and also preferential selection of unlike nuclei in the initiation of the dicaryophase.