Cell size for commitment to cell division and number of successive cell divisions in multicellular volvocine green algae Tetrabaena socialis and Gonium pectorale

Volvocine algae constitute a green algal lineage comprising unicellular Chlamydomonas, four-celled Tetrabaena, eight to 32-celled Gonium, and others up to Volvox spp., which consist of up to 50,000 cells. These algae proliferate by multiple fissions with cellular growth up to several fold in size and subsequent successive cell divisions. Chlamydomonas reinhardtii cells produce two to 32 daughter cells by one to five divisions, depending on cellular growth in the G1 phase. By contrast, in this study, we found that Tetrabaena socialis and Gonium pectorale cells mostly produced four and eight daughter cells by two and three successive divisions, respectively. In contrast to C. reinhardtii, which is committed to cell division when the cell has grown two-fold, T. socialis and G. pectorale are committed only when the cells have grown four- and eight-fold, respectively. Thus, our results suggest that evolutionary changes in cellular size for commitment largely contributes to the emergence and evolution of multicellularity in volvocine algae.