Ultrastructure of the Parabasalid Protist Holomastigotoides

ABSTRACT. The ultrastructure of two species of Holomastigotoides is presented. The basic unit of organization of these large cells is the flagellar band. Each flagellar band consists of a row of flagellar basal bodies linked by three fiber systems. The number of flagellar bands is species dependent. The flagellar bands originate at the cell apex and are arranged in parallel spirals of increasing gyre, thus defining the conical shape of the cell. In the cell apex a striated root called a parabasal fiber is juxtaposed with the basal bodies of each flagellar band. Linear extensions of two parabasal fibers function as the spindle poles for the persistent extra‐nuclear spindle. The nucleus is in close contact with the spindle poles and spindle microtubules. Parallel sheets of microtubules which constitute axostyles are nucleated along the underside of the parabasal fibers. The axostyles extend away from the cell apex, with many reaching the basal region of the cell. Some of the axostyles follow the spiral pattern of the flagellar bands. Numerous Golgi bodies are spaced regularly along the flagellar bands. Together the parabasal fiber, axostyles and Golgi bodies associated with a flagellar band are termed a parabasal complex.