FINE STRUCTURE OF THE FLAGELLAR APPARATUS OF UROGLENA AMERICANA (CHRYSOPHYCEAE)1

Abstract

ABSTRACTThe three‐dimensional structure of the flagellar apparatus of Uroglena americana Calkins (Uroglenopsis americana [Calkins] Lemmerman) was determined using serial section reconstruction. The three microtubular rootlet systems (R2, R3, and R4) follow the general pattern found in other chrysophytes. The R2 rootlet originates between the basal bodies of the mastigoneme‐bearing long flagellum (F1) and the short smooth flagellum (F2) and is attached to the former by a fibrous connection. The R3 rootlet system originates as a trough‐shaped band of six microtubules spanning the distance between the proximal ends of the F1 and F2 basal bodies. The six‐membered rootlet splits into two parts (designated R3 and R3) which circle the depression from which the F2 flagellum emerges in counter‐clockwise direction. These two rootlets pass beneath the F2 basal body and descend into the cell alongside the chloroplast. The R4 rootlet originates in fibrous material which passes diagonally over the F2 basal body, forms a clockwise loop about three‐quarters of the way around the depression, and ends in the cytoplasm.In place of a typical chrysophyte R1 rootlet, U. americana has a different array of microtubules attached to the F1 basal body which we have designated the descending rootlet (DR). This rootlet is a hairpin‐shaped structure lying just below the surface of the cell; its longitudinal axis is predominantly parallel to the longitudinal axis of the cell. The DR resembles the bypassing rootlet which occurs in phaeophyte zoospores. Other chrysophytes may possess rootlets which are similar to the DR found in Uroglena.