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Background  Volvocine algae provide a suitable model for investigation of the evolution of multicellular organisms. Within this group, evolution of the body plan from flattened to spheroidal colonies is thought to have occurred independently in two different lineages, Volvocaceae and  Astrephomene . Volvocacean species undergo inversion to form a spheroidal cell layer following successive cell divisions during embryogenesis. During inversion, the daughter protoplasts change their shape and develop acute chloroplast ends (opposite to basal bodies). By contrast,  Astrephomene  does not undergo inversion; rather, its daughter protoplasts rotate during successive cell divisions to form a spheroidal colony. However, the evolutionary pathways of these cellular events involved in the two tactics for formation of spheroidal colony are unclear, since the embryogenesis of extant volvocine genera with ancestral flattened colonies, such as  Gonium  and  Tetrabaena , has not previously been investigated in detail.    Results  We conducted time-lapse imaging by light microscopy and indirect immunofluorescence microscopy with staining of basal bodies, nuclei, and microtubules to observe embryogenesis in  G. pectorale  and  T. socialis , which form 16-celled or 4-celled flattened colonies, respectively. In  G. pectorale , a cup-shaped cell layer of the 16-celled embryo underwent gradual expansion after successive cell divisions, with the apical ends (position of basal bodies) of the square embryo’s peripheral protoplasts separated from each other. In  T. socialis , on the other hand, there was no apparent expansion of the daughter protoplasts in 4-celled embryos after successive cell divisions, however the two pairs of diagonally opposed daughter protoplasts shifted slightly and flattened after hatching. Neither of these two species exhibited rotation of daughter protoplasts during successive cell divisions as in  Astrephomene  or the formation of acute chloroplast ends of daughter protoplasts as in volvocacean inversion.    Conclusions  The present results indicate that the ancestor of  Astrephomene  might have newly acquired the rotation of daughter protoplasts after it diverged from the ancestor of  Gonium , while the ancestor of Volvocaceae might have newly acquired the formation of acute chloroplast ends to complete inversion after divergence from the ancestor of Goniaceae ( Gonium  and  Astrephomene ).    Electronic supplementary material  The online version of this article (10.1186/s12862-019-1452-x) contains supplementary material, which is available to authorized users.

Embryogenesis of flattened colonies implies the innovation required for the evolution of spheroidal colonies in volvocine green algae