Cytoskeleton Structure and Composition in Choanoflagellates

Abstract

The structure and composition of the cytoskeleton has been studied in Monosiga ovata (Protozoa: Order Choanoflagellida Kent 1880) using a combination of methods in association with light and electron microscopy. Supplementary observations are included for Desnmrellu moniliformis. The basal body of the single anterior flagellurn is subtended proximally and at right angles by a second, nonflagellar basal body. The edges of the two basal bodies are connected by a fibrillar bridge. A long, narrow, striated. fibrillar rootlet extends posteriorly from the lower edge of the non-flagellar basal body towards the Golgi apparatus. It is associated throughout most of its length with the surface of a flattened sac. Rootlet microtubules pass radially from a ring of electron dense material which encircles the distal end of the flagellar basal body. These microtubules extend outwards for about one-third of the length of the cell. Within each collar tentacle is a longitudinal bundle of microfilaments composed of actin as illustrated by rhodamine-phalloidin staining for fluorescence microscopy. The base of each microfilament bundle is associated with one or more rootlet microtubules by fine fibrillar bridges. The attachment between microtubules and tentacle microfilaments is further demonstrated by their coordinated displacement when the cytoskeleton becomes dislodged. The role of the cytoskeleton in maintaining the position of the collar tentacles during interphase and cell division is discussed.Supplementary key words. Actin. basal bodies. epithelial microvilli, fibrillar rootlet, microfilaments. microtubules. rhodaminephalloidin. S 1 fragment myosin.Received 1 -15-97, 1-5-98: uccepml 1-28-98 ErratumIn the paper by J. F? Dubey, J. Euk. Micrubiul. 44 (6):592-602, 1997, "Bradyzoite-induced murine toxoplasmosis: Stage conversion, pathogenesis, and tissue cyst formation in mice fed bradyzoites of different strains of Tuxuplamia gondii" there was a typing error in the last column (in bold) in Table 6. The corrected table is below.

Kinetid structure of choanoflagellates and choanocytes of sponges does not support their close relationship

“…The choanoflagellates are well studied in this respect and it has been shown in many papers that the single celled and colonial, sedentary and swimming, marine and freshwater, naked and thecate or loricate -all of them have very conservative internal cell structure and flagellar apparatus (Laval, 1971;Leadbeater, 1972Leadbeater, , 1977Leadbeater, , 1983Leadbeater, , 1991Leadbeater, , 2015Leadbeater and Morton, 1974;Hibberd, 1975;Karpov, 1981Karpov, , 1982Karpov, , 1985Zhukov and Karpov, 1985;Karpov and Leadbeater, 1998;Karpov and Zhukov, 2000;Leadbeater and Thomsen, 2000;Wylezich et al, 2012). The choanoflagellates have a highly conservative and unique kinetid structure: long transition zone with central filament, microtubular bands (roots) radiating from a dense circle (MTOC) around a kinetosome.…”

Exploring the evolutionary history of centrosomes

“…Like in chytrids, the flagellar base consists of a mature centriole nucleating the flagellum and an immature centriole in its immediate vicinity. Cytoplasmic microtubules are organized from a ring of electron-dense material surrounding the mature centriole and run laterally under the plasma membrane, likely to support the collar of microvilli [67]. The flagellum does not disassemble during mitosis and in certain species the flagellar apparatus is duplicated prior to mitosis [89].…”

“…The basic architecture of choanoflagellates is highly reminiscent of chytrid zoospores, with a single posterior flagellum that can serve for both motility and feeding [67,[88][89][90]. In addition, a collar of actin-based microvilli surrounds the base of the flagellum and helps capture bacterial preys.…”

Molecular phylogeny of choanoflagellates, the sister group to Metazoa

“…This comprises a spherical to ovoid cell with a single anterior flagellum surrounded by a collar of narrow actin-based microvilli (Fig. 1A) (10). In contrast to the uniformity of the choanoflagellate cell, the morphology of the external covering (periplast) is varied and sometimes striking, ranging from simple organic sheaths to complex silica ''cages'' up to a 100 m or more in length.…”