The Nuclei and Parasexuality in Phymatotrichum Omnivorum

An active parasexual cycle in Phymatotrichum omnivorum is indicated by the presence of haploid, diploid, aneuploid and other rare polyploid nuclei in the multinucleate mycelial cells, migration of nuclei through the central pore of the transverse hyphal septum, anastomosis of the hyphae, and strain differences. The haploid nuclei divide by mitosis, and the haploid chromosome number is four. Diffuse nuclei, each containing many very small dark‐staining bodies, are also common in the mycelium. As the “spore mat” matures vesicles are extruded through the walls of its conidiophores and hyphae. One, two, or rarely three nuclei migrate into each enlarging vesicle to form conidia. Almost all the nuclei migrate progressively from the hyphae into the conidiophores and then into the conidia. The nuclei of the ungerminated strands and sclerotia are dark‐staining spheres and ovals, although a few are similar to those mycelial nuclei which have evident chromosomes. As the sclerotia germinate, nuclei of the latter appearance become common in the pseudoparenchymatous cells, align in a row, and apparently reduce in size. Hyphae form in the pseudoparenchymatous cells and contain nuclei similar to those of the vegetative mycelium. The hyphae of the Sclerotium form a mycelium. The nuclei of the various stages of the life cycle were stained by one to three procedures (Feulgen, aceto‐orcein, and haematoxylin). Treatment of some sections of mycelium with deoxyribonuclease prior to staining by Feulgen technique resulted in the absence of staining or observable nuclei, while similar treatment with ribonuclease had no effect on the appearance of the nuclei.