Observation and Interpretation of Nuclear Reductions During Maturation and Germination of Entomophthoralean Resting Spores

Abstract

tory ofthe pathogen. It is important to determine if karyogamy and meiosis occur in the life cycle of this fungus, and if so, at what site and time, before any strain improvement through genetic manipulation might be attempted. Conflicting data about the nuclear events associated with resting spore pro? duction and germination in the Entomophthorales have presented difficulty in the correct interpretation of entomophthoralean life cycles. Sawyer (1931) believed the resting spores of E. radicans {=Entomophthora sphaerosperma Fres.) re? mained multinucleate at maturity; Perry et al. (1982) studied the germination of resting spores of this species but did not note the nuclear condition at any time in mature, activated, or germinating spores. Latge (1976) reported a reduction in the number of nuclei to one during resting spore maturation in Conidiobolus thromboides Drechsler {=Entomophthora virulenta Hall & Dunn). Resting spores of E. radicans (strain RS-141, Insect Pathology Research Unit) were harvested from 3% yeast extract agar at various stages of development and stained with haematoxylin following the procedures of Lu and Raja (1970). Resting spores of Entomophaga grylli (Fres.) Batko were recovered from field-collected cadavers of Melanoplus bivittatus Say after several months of storage at 4 C in a sealed container. Confirmatory studies on the nuclear condition of resting spores of C thromboides (strain RS-40) were done with spores produced on egg yolk/ Sabouraud maltose agar (Soper et al, 1975). Nuclear conditions of germinating resting spores in this species were studied from spores stored over silica gel in a sealed container at 4 C for 3 yr; approximately 25% of these spores would ger? minate after being wetted. These 3-yr-old spores were used to determine nuclear events during the germination process. In the early formation of E. radicans resting spores, the prespores may contain as many as 25 nuclei (Fig. 1). The young resting spore forms in less than 24 h inside the parental prespore. During the deposition of the spore wall some nuclei may be trapped between the prespore (parental) wall and newly formed inner (sporangial) wall (Fig. 2). The excluded nuclei rapidly disintegrate leaving a more or less empty space between the two thickened wall layers. As the resting spore matures, many ofthe nuclei degenerate until, after two wk, only two nuclei remain in each spore (Fig. 3). No further reduction in the number of nuclei was observed,