The flagellar apparatus structure of the colonial Volvocales has turned out to be unexpectedly diverse. This diversity is especially noteworthy in view of the evidence that members of this group have evolved from a Chlamydomonas-like ancestor with a "typical" chlorophycean flagellar apparatus. The differences in flagellar structure can be hypothesized to result from either: a) independent transitions from the unicellular to the colonial habit, or b) unusual plasticity in flagellar apparatus structure. We cannot yet discriminate between these hypotheses because of the paucity of species examined. This project will study additional taxa of less uncertain affinities by detailed flagellar apparatus reconstruction at the ultrastructural level, and by surveys of rootlet and flagellar positioning with immunofluorescence using a monoclonal antibody that recognized stable microtubules. In addition, correlation of structure of the flagellar apparatus with flagellar functioning will be continued to understand better the effects of possible selection pressures on flagellar apparatus structure. These studies will contribute to understanding of the phylogenetic relationships among algae in the "volvocene line." In addition, they will give information on the evolutionary stability of the flagellar apparatus characters; this is especially important because the structure of the flagellar apparatus is being used increasingly in systematic studies of algae and protozoa at all levels.
