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 co mbination 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 proximal ly 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 flat tened sac. Rootlet microtubules pass radially from a ring of electron dense material which encircles the distal end of the flagellar basal b ody. These microtubules extend outwards for about one-third of the len gth of the cell. Within each collar tentacle is a longitudinal bundle of microfilaments composed of actin as illustrated by rhodamine-phallo idin staining for fluorescence microscopy. The base of each microfilam ent bundle is associated with one or more rootlet microtubules by fine fibrillar bridges. The attachment between microtubules and tentacle m icrofilaments is further demonstrated by their coordinated displacemen t when the cytoskeleton becomes dislodged. The role of the cytoskeleto n in maintaining the position of the collar tentacles during interphas e and cell division is discussed.