Flagellar apparatus structure of choanoflagellates.

Phylum choanoflagellata is the nearest unicellular neighbor of metazoa at the phylogenetic tree. They are single celled or form the colonies, can be presented by naked cells or live in theca or lorica, but in all cases they have a flagellum surrounded by microvilli of the collar. They have rather uniform and peculiar flagellar apparatus structure with flagellar basal body (FB) producing a flagellum, and non-flagellar basal body (NFB) lying orthogonal to the FB. Long flagellar transition zone contains a unique structure among eukaryotes, the central filament, which connects central microtubules to the transversal plate. Both basal bodies are composed of triplets and interconnected with fibrillar bridge. They also contain the internal arc-shaped connectives between the triplets. The FB has prominent transitional fibers similar to those of chytrid zoospores and choanocytes of sponges, and a radial microtubular root system. The ring-shaped microtubule organizing center (MTOC) produces radial root microtubules, but in some species a MTOC is represented by separate foci. The NFB has a narrow fibrillar root directed towards the Golgi apparatus in association with membrane-bounded sac. Prior to cell division, the basal bodies replicate and migrate to poles of elongated nucleus. The basal bodies serve as MTOCs for the spindle microtubules during nuclear division by semiopen orthomitosis.