2023 Volume 35 Issue 1 Pages 69-75
The evolution from unicellular to multicellular organisms has occurred many times in the history of life and many eukaryotic lineages have acquired their multicellular complexity, such as three-dimensional body plans or germ-soma differentiation, independently. Volvocine green algae are one of suitable model organisms for investigation into the evolution of multicellularity. Within this group, Volvocaceae and Astrephomene exhibit parallel evolution of spheroidal colonies from ancestral flattened colonies and germ-soma differentiation. However, little is known about these multicellular traits of Astrephomene in molecular and cellular levels, in contrast to Volvox or Volvocaceae. Therefore, we first established a new strain of Astrephomene and conducted developmental analyses. We found rotation of daughter protoplasts during successive cell divisions in Astrephomene, which is a tactic of spheroidal colony formation different from inversion in Volvocaceae. We also analyzed the embryogenesis of volvocine genera Gonium and Tetrabaena, which form ancestral flattened colonies, and revealed that the ancestor of Astrephomene and Volvocaceae might have newly acquired the cellular mechanisms for spheroidal colony formation respectively. In addition, we conducted de novo whole genome sequencing and cell-type RNA-seq analysis of Astrephomene. Though regA, which is a master regulator of somatic cell differentiation in Volvox, was absent in Astrephomene genome, we found another somatic-specific transcription factors in Astrephomene. On the other hand, differentiated gene expression patterns between somatic and reproductive cells in Astrephomene were similar with those in Volvox.
