Figure 1

Convergent evolution of germ-soma differentiation in Astrephomene and Volvox. (a) Schematic representation of the phylogenetic relationships of the volvocine algae and the parallel evolution of germ-soma differentiation. Volvocine algae include unicellular Chlamydomonas, multicellular Volvox and various intermediate forms and a suitable model for research into the evolution of multicellularity⁵. Germ-soma differentiation are thought to have evolved twice or more independently within this group^24,25. Astrephomene have acquired somatic cells independently of Volvocaceae. The phylogenetic relationships are based on the present phylogenomic analysis (Supplementary Fig. 4) and a previous report³⁸. The genomes of all genera except for Astrephomene have been sequenced in previous studies^7,8,9,10,11. (b) A vegetative colony of Astrephomene gubernaculifera. Scale bar = 20 μm. (c) Four posterior somatic cells (arrowheads) in A. gubernaculifera. Compared to reproductive cells (arrows), somatic cells are small and pale colored. Scale bar = 10 μm. (d) A vegetative colony of Volvox carteri. Scale bar = 50 μm. (e) Somatic cells (arrowheads) and gonidia (reproductive cells, arrow) in V. carteri. While somatic cells are biflagellate, gonidia have no flagella. Scale bar = 10 μm. (f) Schematic diagram of the asexual life cycle of A. gubernaculifera in the synchronous culture. The asexual life cycle is completed in approximately 24 h, and the culture is highly synchronized with the light–dark cycle. While reproductive cells undergo cell growth and embryogenesis to form daughter colonies, somatic cells do not undergo obvious growth or embryogenesis and reach cell death. ZT Zeitgeber time.