The Fine Structure of Ascospore Formation in Pyronema domesticum

Abstract

The details of ascospore delimitationand subsequent development are given. An ascus vesicle composed of a pair of membranes was formed prior to the division of the diploid nucleus of the young ascus. There were no apparent changes in the ascus vesicle during succeeding nuclear divisions. The ascus vesicle was cylindrical and surrounded the 8 haploid nuclei in the young ascus. Cross and longisections of the vesicle indicated it was open at both ends and had occasional gaps in the double membranes. By progressive constriction around the individual nuclei and then fusion the 2 membranes delimited the sporoplasm from the epiplasm of the ascus. Following delimitation of the spore, spore wall-material was deposited between the 2 limiting membranes. The spore wall was composed of 2 primary layers: the endospore and epispore. The endospore was first deposited between the 2 membranes. Then spore matrix material was formed external to the endospore. Finally the epispore was formed between the endospore and spore matrix material. The mature spore was limited externally by an investing membrane. The regular occurrence of endoplasmic reticulum elements near the developing endospore suggested that wall material was synthesized in these elements and subsequently migrated to the external portion of the ascospore.