is an evolutionarily conserved stem cell gene influencing cell proliferation, animal growth and regeneration in the hydrozoan .

() is a vertebrate gene preferentially expressed in stem and cancer cells, which acts to regulate cell cycle progression, genome stability and ribosome biogenesis. and its paralogous gene, (), arose in the vertebrate clade after a duplication event from their orthologous gene, (). Research on invertebrate , however, has been limited. To gain a greater understanding of the evolution and functions of the gene, we have performed studies in the hydrozoan cnidarian , a colonial hydroid that continuously generates pluripotent stem cells throughout its life cycle and presents impressive regenerative abilities. We show that is expressed in stem and germline cells. The knockdown of reduces the number of mitotic and S-phase cells in larvae of different ages. Genome editing of via CRISPR/Cas9 resulted in colonies with reduced growth rates, polyps with impaired regeneration capabilities, gonadal morphological defects, and low sperm motility. Collectively, our study shows that is an evolutionarily conserved stem cell and germline gene involved in cell proliferation, animal growth, regeneration and sexual reproduction in , and sheds new light into the evolution of and of stem cell systems.