Department of Biology, Washington University, St. Louis, Missouri 63130.
Integr Comp Biol. 2003 Apr;43(2):247-53. doi: 10.1093/icb/43.2.247.
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.
衣藻及其亲缘生物为研究多细胞生物的进化和细胞分化提供了一个极好的综合模型。绿藻门的复杂性从单细胞的衣藻到具有单一细胞类型的几个群体属，再到具有生殖-体分工的多细胞团藻，各不相同。在单系的团藻科中，有几个种的团藻在不同的谱系中独立进化，其最终原因可能是体型较大和细胞分化在竞争有限资源（如磷）方面具有优势。这种进化转变的直接原因正在衣藻中进行研究。除了团藻外，所有的衣藻都表现出二相发育：细胞在运动的、具双鞭毛的阶段生长，然后在生殖阶段失去运动能力并反复分裂。在衣藻中，三种基因将这个祖先的二相程序转化为二歧程序，产生无运动能力的生殖细胞和无生殖潜能的具双鞭毛体细胞：首先是 gls 基因在早期胚胎中作用，导致不对称分裂和产生大-小姐妹细胞对；然后 lag 基因在大细胞中作用，抑制祖先程序的双鞭毛一半，而 regA 在小细胞中作用，抑制程序的生殖一半。这些基因的分子遗传分析正在进行中，将以 regA 为例进行说明，regA 编码一种转录因子，在体细胞中起作用，抑制编码叶绿体蛋白的核基因。叶绿体生物发生的抑制阻止了这些必需的光合自养细胞的生长，由于它们不能生长，也就不能繁殖。
