Asymmetric division, cell size and germ-soma specification in Volvox

Volvox carteri is a spheroidal green alga in which each young adult has ∼2000 small, terminally differentiated somatic cells at the surface and ∼16 large, potentially immortal asexual reproductive cells, or gonidia, on the interior. When mature, each gonidium initiates a stereotyped cleavage program, during which prospective gonidia and somatic cells of the next generation are set apart by asymmetric divisions. At the end of cleavage, prospective gonidia are ∼30 X the volume of prospective somatic cells, and a variety of studies lead to the conclusion that it is this difference in size, not an accompanying difference in cytoplasmic quality, that triggers entirely different programs of gene expression and differentiation in the two cell lineages. Genes that appear to play key roles in differentiation include gls, which is required for asymmetric division, regA, which suppresses reproductive development in small cells, and the lag genes which suppress somatic development in large cells. The prospects for analysing the nature of these genes and their actions have been brightened by recent molecular-genetic developments, but the mechanism by which differences in cell size are transduced into differences in gene expression remains obscure.