A Comparative Morphological and Physiological Study of the Species of Dipodascus

Abstract

All 3 species in the genus, D. albidus Lagerheim, D. uninucleatus Biggs and D. aggregatus Francke-Grosmann (=D. albidus forma minor Korf) are homothallic. A study of the living material of D. aggregatus with light and phase-contrast microscopy revealed its unique method of formation of asci in clusters. Mycelium obtained from either a uninucleate ascospore or a uninucleate meristem-arthrospore bears gametangia which subsequently form asci. Asexual reproductive structures may be borne on the same hypha which bears asci. Gametangiogenic cells (author''s coinage) are uninucleate, with nuclei slightly larger than those of vegetative cells, and give rise to gametangia. These cells are almost always intercalary, in pairs, and are smaller than the adjacent cells. Gametangia are anisogamous and are uninucleate throughout. The male gametangial initial is always smaller and narrower than the female gametangial initial and arises from a contiguous gametangiogenic cell. Neither of the gametangial initials is delimited by septa from its respective gametangiogenic cell until they come in contact with each other. Delimitation of the female gametangium always precedes that of the male gametangium. After the fusion of the 2 gametangia, new gametangia may be initiated from either, or most frequently from both, of the gametangiogenic cells. A gametangiogenic cell supporting one or more female gametangia gives rise only to other female gametangia, and similarly a gametangiogenic cell bearing male gametangia gives rise only to male gametangia. One to 4 gametangia may develop from each gametangiogenic cell. Following plasmogamy, the female gametangium enlarges to form the young ascus in the middle of which fusion of the 2 nuclei takes place. D. albidus and D. uninucleatus do not produce more than 1 gametangium per gametangiogenic cell. D. uninucleatus differs from the other 2 species in having very inconspicuous, isogamous gametangia. Dipodascus spp. are heterotrophic with respect to biotin and only D. uninucleatus can utilize and reduce nitrate. Besides morphological characters, physiological characters such as vitamin requirements, assimilation of carbon compounds, assimilation of nitrogen compounds, fermentation of some sugars, and acid production are also employed in characterizing the apecies. A key to the species is provided.