Germ cell specification in Volvox carteri.

Volvox carteri illustrates with diagrammatic clarity Weismann's concept of an immortal germline that produces a mortal soma that will carry it for a time, but then perish. Each V. carteri adult consists of about 16 asexual reproductive cells (gonidia) in the interior of a sphere that consists at its surface of about 2000 biflagellate somatic cells. When mature, each gonidium divides to form a juvenile with this same cellular composition. Half-way through their maturation, juveniles hatch out of the parenteral spheroid, whereupon parental somatic cells undergo programmed death while juvenile gonidia prepare for a new round of reproduction. The first visible step in V. carteri germ-soma differentiation is asymmetric cleavage, which sets apart large gonidial initials from small somatic initials. Experimental analysis indicates that it is a difference in size, not any difference in cytoplasmic quality, that determines whether a cell will become germinal or somatic. Mutational and molecular studies lead to the following model for the genetic control of the germ-soma dichotomy: first, the gls locus acts to cause asymmetric division; then large cells activate a set of lag loci that suppress expression of somatic genes, while small cells activate the regA locus that suppresses gonidial genes.