Fine structure of Telonema subtilis Griessmann, 1913: a flagellate with a unique cytoskeletal structure among eukaryotes.

Telonema is a genus of heterotrophic flagellates with two flagella that occurs in marine environments. Although some aspects of the morphology and ultrastructure of Telonema have been reported in previous studies, several characters have been described incompletely or not at all. In the present study, we identify and describe several of these characteristics, such as extrusomes, telonemosome, adhesive fibers and the intricate cytoskeleton structure of T. subtilis using serial ultra-thin sections in transmission electron-microscopy. The extrusomes are scattered throughout the cell. Their structure in transverse section is similar to those of several monadofilosan cercozoa, but are distinguished by a longitudinal element. The telonemosome is an enigmatic organelle surrounded by a single membrane. It contains many thin tubular structures and resembles the K-bodies found in oomycetes. The complex cytoskeletal structure is multilayered and unique among eukaryotes, although the posterior half resembles the penetrating/feeding apparatus found in apicomplexans, protoalveolates and kathablepharids. The proposed function and distribution pattern of the adhesive fibers in Telonema resemble those of the fibrous structures of Microheliella maris (Heliozoa). Our observations provide a more complete understanding of the characteristics of Telonema and support the conclusion from molecular studies that Telonema is a lineage without a clear sister group among the eukaryotes.