Hallmann Armin
Department of Cellular and Developmental Biology of Plants, University of Bielefeld, Universitätsstr. 25, 33615 Bielefeld, Germany.
Protist. 2006 Oct;157(4):445-61. doi: 10.1016/j.protis.2006.05.010. Epub 2006 Jul 18.
Green algae of the family Volvocaceae provide an unrivalled opportunity to analyze an evolutionary pathway leading from unicellularity to multicellularity with division of labor. One key step required for achieving multicellularity in this group was the development of a process for turning an embryo inside out: a morphogenetic process that is now known as "inversion," and that is a diagnostic feature of the group. Inversion is essential because at the end of its embryonic cleavage divisions, each volvocacean embryo contains all of the cells that will be present in an adult, but the flagellar ends of all cells are pointed toward the interior, rather than toward the exterior where they will need to be to function in locomotion. Inversion has been studied in greatest detail in Volvox carteri, but although all other volvocacean species have to struggle with the same awkward situation of being wrong-side out at the end of cleavage, they do it in rather different ways. Here, the inversion processes of six different volvocacean species (Gonium pectorale, Pandorina morum, Eudorina unicocca, Volvox carteri, Volvox tertius, and Volvox globator) are compared, in order to illustrate the variation in inversion patterns that exists within this family. The simplest inversion process occurs in the plate-shaped alga Gonium pectorale and the most complicated in the spherical alga Volvox globator. Gonium pectorale goes only from a concave-bowl shape to a slightly convex plate. In Volvox globator, the posterior hemisphere inverts completely before the anterior pole opens and the anterior hemisphere slides over the already-inverted posterior hemisphere; during both halves of this inversion process, the regions of maximum cell-sheet curvature move progressively, as radially symmetrical waves, along the posterior-anterior axis.
团藻科的绿藻为分析从单细胞向具有分工的多细胞进化的途径提供了无与伦比的机会。该类群实现多细胞性所需的一个关键步骤是形成一种将胚胎内外翻转的过程：一种现在被称为“反转”的形态发生过程，这是该类群的一个诊断特征。反转至关重要，因为在胚胎卵裂分裂结束时，每个团藻科胚胎都包含了成体中将会出现的所有细胞，但所有细胞的鞭毛端都指向内部，而不是指向它们在运动中发挥功能时所需的外部。在卡特团藻中对反转进行了最详细的研究，但尽管所有其他团藻科物种在卵裂结束时都必须应对同样尴尬的里外颠倒的情况，但它们的做法却大不相同。在这里，比较了六种不同团藻科物种（胸状盘藻、多细胞盘藻、单球空球藻、卡特团藻、第三团藻和球形团藻）的反转过程，以说明该科内存在的反转模式的变化。最简单的反转过程发生在板状藻类胸状盘藻中，而最复杂的发生在球形藻类球形团藻中。胸状盘藻仅从凹碗状变为略凸的平板状。在球形团藻中，后半球在前端开口之前完全反转，前半球滑过已经反转的后半球；在这个反转过程的两个阶段中，最大细胞片曲率区域作为径向对称波沿着后 - 前轴逐渐移动。
