Kirk D L
Department of Biology, Washington University, St Louis, MO 63130.
Ciba Found Symp. 1994;182:2-15; discussion 15-30. doi: 10.1002/9780470514573.ch2.
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.
团藻以形象清晰的方式阐释了魏斯曼关于不朽种系的概念，即种系产生一个终有一死的体细胞，体细胞会在一段时间内携带种系，但随后会消亡。每个成年团藻由位于球体内部的约16个无性生殖细胞（生殖细胞）组成，球体表面由约2000个双鞭毛体细胞组成。成熟时，每个生殖细胞分裂形成一个具有相同细胞组成的幼体。在幼体成熟过程中，它们从亲本体球体中孵化出来，此时亲本体细胞经历程序性死亡，而幼体生殖细胞则为新一轮繁殖做准备。团藻种系 - 体细胞分化的第一个可见步骤是不对称分裂，它将大型生殖细胞初始细胞与小型体细胞初始细胞区分开来。实验分析表明，决定一个细胞将成为生殖细胞还是体细胞的是大小差异，而非细胞质质量的任何差异。突变和分子研究得出了以下关于种系 - 体细胞二分法遗传控制的模型：首先，gls基因座起作用导致不对称分裂；然后大型细胞激活一组lag基因座，抑制体细胞基因的表达，而小型细胞激活抑制生殖细胞基因的regA基因座。
