A cytological study of Aulacomonas submarina Skuja 1939, a heterotrophic flagellate with a novel ultrastructural identity

Abstract

Aulacomonas submarina is a free-living freshwater heterotrophic flagellate, the ultrastructure of which is described here. It has two long, nearly equal flagella that insert apically and beat asynchronously. It engulfs prey as large as itself via a ventral groove. The nucleus is anterior and surrounded by several dictyosomes. The two flagella are similar in structure. The most external part of the transition zone is marked by a constriction of the membrane, and distal to this the periaxonemal space expands for a short distance. The two basal bodies are inclined to each other and connected by roots, in a style reminiscent of some heterokont organisms. The basal body of the dorsal flagellum (Fl) is connected to a dorsal cytoskeletal root that is comprised of 5 superimposed microtubules and which sweeps from right to left in an incomplete loop near the cell apex. This loop is reinforced near its origin by an electron-dense rootlet. Material associated with the dorsal root gives rise to cortical microtubules which form a cape covering the apical region of the cell. The ventral basal body (of flagellum F2) is connected to two microtubular and microfibrillar fibres which support the lips of the ventral groove. More distally the lips of the groove are bordered only by a ridge supported by microfibrillar material. Mitochondria have bleb-shaped tubular cristae similar to those of actinophryid heliozoa. There is no cyst. The shape of the mitochondrial cristae, and the arrangement of basal bodies suggest that this genus is distantly related to the heterokont flagellates. However, most aspects of the ultrastructure are unlike those of other flagellates, such that Aulacomonas cannot be assigned to any familiar subgroup of heterokont organisms or other group of flagellates.