Multicellularity arose several times in evolution of eukaryotes. The volvocine algae have full range of colonial organization from unicellular to colonies, and thus these algae are well-known models for examining the evolution and mechanisms of multicellularity. Gonium pectorale is a multicellular species of Volvocales and is thought to be one of the first small colonial organisms among the volvocine algae. In these algae, a cytoplasmic bridge is one of the key traits that arose during the evolution of multicellularity. Here, we observed the inversion process and the cytoplasmic bridges in G. pectorale using time-lapse, fluorescence, and electron microscopy. The cytoplasmic bridges were located in the middle region of the cell in 2-, 4-, 8-, and 16-celled stages and in inversion stages. However, there were no cytoplasmic bridges in the mature adult stage. Cytoplasmic bridges and cortical microtubules in G. pectorale suggest that a mechanism of kinesin-microtubule machinery similar to that in other volvocine algae is responsible for inversion in this species.

The colony formation of Platydorina caudata (Volvocaceae, Chlorophyta) was studied in detail using time-lapse video and scanning and transmission electron microscopy. A unique feature of P. caudata, a horseshoe-shaped flattened colony comprising 16 cells arranged in a single twisted layer, is formed by 'intercalation', the rearrangement of embryo cells within the colony, which proceeds subsequent to the embryo inversion that brings flagellar ends of cells from the concave to the convex surface. Cell fate is precisely decided in colony formation, suggesting that intercalation is a precisely regulated process in which given cells are intercalated into particular parts of the colony. The presence of flaskshaped cells with an elongated stalk and cytoplasmic bridges during the inversion process strongly suggests that a similar mechanism known in Volvox is also involved in the colony formation of Platydorina.