Light and Electron Microscopy ofMortierella IndohiiZygospores

Abstract

Development of the zygospore of Mortierella indohii was investigated by light, scanning electron, and transmission electron microscopy. During development of the zygospore, one suspensor acts as a basal supporting element. Investing hyphae, which either partly or entirely cover the zygospore at maturity, develop from the basal suspensor. The surface of the mature zygospore appears dimpled. The wall of the zygospore comprises a fine outer layer, presumed to correspond with the zygosporangial wall, an electron-opaque layer, a fine electron-translucent layer, and a zoned multilamellate, relatively thick layer. Organelles, which tend to be located in the central cytoplasm of the zygospore, are surrounded by numerous lipid droplets. Mortierella indohii Chien is included in the Mortierellaceae on the basis of zygospore characteristics, the spreading zonate colony with a garlic-like odor, and the apparent resemblance of the spinose chlamydospores to those of M. polycephala Coemans. A sporangial stage has not been observed in M. indohii, M. chlamydospora (Chesters) Plaats-Niterink, and M. echinosphaera Plaats-Niterink, and these species remain isolated in Gams' key (1977). All produce morphologically similar, spinose chlamydospores (Van Der Plaats-Niterink et al., 1976), and both M. chlamydospora and M. indohii are known to produce naked and invested zygospores, respectively. Although Chien et al. (1976) have described zygospore development in M. indohii, no study of the fine structure of the zygospore has been reported for this species. A problem associated with the description of the zygospore occurs in deciding whether the structure should properly be considered as a zygosporangium containing a zygospore, or whether the entire structure should be considered the zygospore proper. Where the temporal origin of the various wall layers is believed to be known, the terms primary, secondary, and so on, have been used to describe them (Hawker and Beckett, 1971; O'Donnell et al., 1976). Thus, the outermost layer of the wall of the zygospore continuous with the wall of the suspensor may be termed the primary layer. On the other hand, in the absence of definitive knowledge of the origin of the various layers the concept of the sexual unit as a zygosporangium may be both convenient for descriptive purposes and may also remain the correct interpretation of the structure. In this paper an account is given of zygospore formation as determined by light and scanning electron microscopy together with notes on zygospore structure as ascertained from ultrathin sections.