Cytoskeleton structure and composition in choanoflagellates

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 Desmarella moniliformis. The basal body of the single anterior flagellum 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.