Seeking the Ultimate and Proximate Causes of Volvox Multicellularity and Cellular Differentiation1

Abstract Volvox and its relatives provide an exceptional model for integrative studies of the evolution of multicellularity and cellular differentiation. The volvocine algae range in complexity from unicellular Chlamydomonas through several colonial genera with a single cell type, to multicellular Volvox with its germ–soma division of labor. Within the monophyletic family Volvocaceae, several species of Volvox have evolved independently in different lineages, the ultimate cause presumably being the advantage that large size and cellular differentiation provide in competing for limiting resources such as phosphorous. The proximate causes of this type of evolutionary transition are being studied in V. carteri. All volvocine algae except Volvox exhibit biphasic development: cells grow during a motile, biflagellate phase, then they lose motility and divide repeatedly during the reproductive phase. In V. carteri three kinds of genes transform this ancestral biphasic program into a dichotomous one that generates non-motile reproductive cells and biflagellate somatic cells with no reproductive potential: first the gls genes act in early embryos to cause asymmetric division and production of large–small sister-cell pairs; then lag genes act in the large cells to repress the biflagellate half of the ancestral program, while regA acts in the small cells to repress the reproductive half of the program. Molecular-genetic analysis of these genes 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 since they cannot grow, they cannot reproduce.