KIRK, David L.: Seeking the Ultimate and Proximate Causes of Volvox Multicellularity

Volvox and its relatives provide an exceptional model for integrative studies of the evolution of an important developmental novelty. The volvocine algae are green flagellates that range in complexity from unicellular Chlamydomonas through several colonial genera with a single cell type, to Volvox with its germ�soma division of labor. Volvox species have evolved independently in five different volvocine lineages. The ultimate cause of this evolutionary transition apparently is the advantage that large size and cytodifferentiation provide in competing for limiting resources; its proximate cause is being studied in V. carteri. In all volvocine algae except Volvox, development is biphasic: cells grow during an initial biflagellate phase, then they lose motility and divide repeatedly. In V. carteri three kinds of genes transform the biphasic program into a dichotomous one that generates motile, post-mitotic somatic cells and non-motile �gonidia� that are specialized for reproduction: First the gls genes act to cause asymmetric division and production of large�small sister-cell pairs. Then lag genes repress the biflagellate half of the ancestral program in the large cells while regA represses the reproductive half of the program in small cells. Molecular-genetic analysis of these regulatory loci is progressing, as will be illustrated with regA, which encodes a transcription factor that acts in somatic cells to repress nuclear genes encoding chloroplast proteins. Repression of chloroplast biogenesis prevents these obligately photoautotrophic cells from growing, and if they can�t grow, they can�t reproduce. Studies of the evolution of developmentally important Volvox genes are in progress.