Ultrastructure and 18S rRNA gene sequence of a small heterotrophic flagellate Siluania monomastiga gen et sp nov (Bicosoecida)

Siluania monomastiga gen. et sp. nov. - one of the smallest free-living het erotrophic eukaryotes (1.5-3.0 mu m length) has been investigated using SEM and TEM, and by gene sequence analysis of the small subunit rRNA. The flag ellate is covered by only the plasmalemma. The single flagellum has a unila teral array of tubular mastigonemes; the flagellar basal body is only compo sed of doublets of microtubules. One or two additional very short basal bod ies may also be present, composed of single microtubules, of amorphous elec tron dense material. There is a well developed cytostome with permanent pha rynx, supported by rootlet microtubules and a fibrillar sheet. One mitochon drion with tubular cristae contains a spherical body in the centre. Develop ing mastigonemes are located within the perinuclear space. The Golgi appara tus consists of a single dictyosome. The rootlet system of the flagellum is represented by three microtubular bands, numbered as r1, r3 and r4 in acco rdance with the protocol accepted for biosoecids, and by the fibrillar root let having two strands. The cytoskeleton structure and organelle dispositio n of S. monomastiga are similar to those of bicosoecids, particularly Cafet eria roenbergensis. Molecular data confirm this view. The presence of a wel l developed permanent cytostome/cytopharynx complex distinguishes S. monoma stiga from other bicosoecids. A new family Siluaniaceae (according to IBCN) and Siluaniidae (according to IZCN) is erected for this species.