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.
Kirk
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Department of Biology, Washington University, St Louis, MO 63130.
D000818:Animals; D017209:Apoptosis; D002454:Cell Differentiation; D002455:Cell Division; D016764:Cell Polarity; D048429:Cell Size; D000460:Chlorophyta / Q000166:cytology / Q000235:genetics / Q000502:physiology*; D004625:Embryo, Nonmammalian / Q000166:cytology; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D005854:Germ Cells / Q000166:cytology / Q000502:physiology*; D006358:Hot Temperature; D008954:Models, Biological; D012100:Reproduction, Asexual; D016823:Volvocida / Q000166:cytology / Q000235:genetics / Q000502:physiology*