The effect of glucose concentration and light on thallospore ultrastructure in Ellisomyces anomalus (Thamnidiaceae, Mucorales)

Abstract

The development of chains of thallospores in the mucoraceous fungus Ellisomyces anomalus has been examined using scanning, thin section, and freeze-fracture electron microscopical techniques. The development and cytology of spores formed on high-glucose (HG, 4.0%) and low-glucose (LG, 0.06%) media, under both light and dark conditions, have been compared. The vegetative hyphae of spores on LG media accumulate glycogen particles in their cytoplasm but contain very little lipid. The cytoplasm of HG hyphae is packed with lipid globules. The thallospores are delimited by septa, which as in most other mucoraceous fungi are perforated by plasmodesmatalike pit connections. A thick secondary (spore) wall layer is accreted as the spores swell. Although mature thallospores on LG and HG media are similar in their external morphology, they show internal differences. HG spores are thicker walled and contain abundant lipid and glycogen reserves. LG spores contain relatively little lipid and are more highly vacuolate. The only structural differences observed between the carotene-rich, light-grown spores and their dark-grown counterparts is an increase in lipid globule electron density and an overall enhancement of membrane staining properties. The mature thallospores secede from the parent hyphae by both shizolytic (septum splitting) and rhexolytic (circumcissile hyphal splitting) mechanisms. From a developmental standpoint it is suggested the thallospores of Ellisomyces resemble thallic arthric conidia (arthrospores), simiar to those produced by Mucor rouxii, more closely than true chlamydospores, such as those produced by Gilbertella persicaria and Mucor mucedo.