ULTRASTRUCTURE AND DEVELOPMENT OF THE FLAGELLAR APPARATUS AND FLAGELLAR MOTION IN THE COLONIAL GREEN-ALGA ASTREPHOMENE-GUBERNACULIFERA

  • 1 January 1983
    • journal article
    • research article
    • Vol. 63  (SEP) , 21-41
Abstract
Immediately following embryonic cleavage, the cells of Astrephomene have 4 equal-sized basal bodies, 2 of which are connected by a striated distal fiber and 2 striated proximal fibers. The 4 microtubular rootlets, which alternate between having 3/1 and 2 members, are arranged cruciately. The 2 basal bodies that are connected by the striated fibers then extend into flagella, while the 2 accessory basal bodies are now markedly shorter. At this stage the flagellar apparatus has 180.degree. rotational symmetry and is very similar to the flagellar apparatus of the unicellular Chlamydomonas and related algae. Development proceeds with a number of concurrent events. The basal bodies begin to separate at their proximal ends and become nearly parallel. Each striated proximal fiber detaches at one end from one of the basal bodies. Each half of the flagellar apparatus, which consists of a flagellum and attached basal body, an accessory basal body, 2 rootlets and a striated fiber (formerly one of the proximal striated fibers), rotates about 90.degree., the 2 halves rotating in opposite directions. An electron-dense strut forms near one 2-membered rootlet and grows past both basal bodies. During this time a fine, fibrous component appears between newly developed spade-like structures and associated amorphous material connected to each basal body. The basal bodies continue to separate as the distal fiber stretches and finally detaches from one of them. These processes result in the loss of the 180.degree. rotational symmetry present in previous stages. Although the flagella continue to separate, there is no further reorganization of the components of the flagellar apparatus. In the mature cell of Astrephomene, the 2 flagella are inserted separately and are parallel. The 4 microtubular rootlets are no longer arranged cruciately. Three of the rootlets are nearly parallel, while the 4th is approximately perpendicular to the other 3. A striated fiber connects each basal body to the underside of the strut. These fibers run in the direction of the effective stroke of the flagella and might be important either in anchoring the basal bodies or in the initiation of flagellar motion. Unlike the case in the unicellular Chlamydomonas, the 2 flagella beat in the same direction and in parallel planes. The flagella of a given cell may or may not beat in synchrony. The combination of this type of flagellar motion and the parallel, separate flagella appears to be suited to the motion of this colonial organism.