Nuclear behaviour during meiosis and ascus formation in Penicillium striatum

Abstract

Meiosis and ascus formation in Penicillium striatum were examined by aid of light- and electron microscopy. The vegetative mycelium is haploid. In early stages of ascus formation two nuclei may partly fuse to a double haploid nucleus (n + n) with a membrane between them. When the intranuclear membrane breaks down, meiosis begins. After nuclear fusion prophase seems similar to that in Neurospora. Four to five bivalent associations occur arranged in a ring at first metaphase. Before anaphase, the bivalents arrange on line. The chromosome separation is almost synchronous. Some sort of interconnecting material occurs between the chromosomes especially during the second and third meiotic division. Typical centrioles did not occur in either the light- or in the electronmicroscopical preparations. Centrioles in vegetative nuclei lack microtubuli, thus differing considerably from the usual type. A typical nuclear membrane for eucaryote nuclei occurs. The central body is usually located near the nuclear membrane and changes shape during nuclear division. In sections it takes the form of a ring which disappears before metaphase. Ascospores are first delimited by a double membrane and the spore-wall seems to increase in thickness by deposition of material between the membranes.