Fig. 1

Schematic representations of cell sheet configurations of volvocine algae before and after embryonic inversion mapped on a phylogenetic tree. Blue arrows lead from the cell sheet configurations of embryos right after cleavage (before inversion) to the cell sheet configurations of adults (after inversion). The flagellar/apical side of the cell sheet is shown in brown color and the basal side of the cell sheet is shown in green color. Cell sheets of volvocine algae either are spherical, bowl-shaped or flat. Inversion processes with lower complexity are shown more to the left side and inversion processes with increased complexity are shown more to the right side. The background shading pools species with the same cell sheet configuration before and after embryonic inversion. Light micrographs on the right side of the figure show wild-type phenotypes of some representative volvocine species at adult stages. The evolutionary tree is based on the nucleotide sequences of five chloroplast genes. The phylogenetic analysis indicates that multicellularity evolved only once in this group. In contrast, a partial germ-soma division of labor evolved independently in three different lineages and was lost twice [3, 6, 8, 84, 107]. A full germ-soma division also evolved three times. There are two fundamentally different sequences through which embryos of the genus Volvox turn right-side out: type A and type B inversion [38, 108]. Letters A or B behind names of Volvox species indicate which inversion sequences embryos of these Volvox species undergo (type A or B). The meanings of symbols are given at the left edge of the figure. This tree was adapted from Herron and Michod [6] and others [3, 8, 35, 55] and some additional information was added [38, 56, 57]