Pluripotent, germ cell competent adult stem cells underlie cnidarian plant-like life history

In most animals, pluripotency is irreversibly lost post-gastrulation. By this stage, all embryonic cells have already committed either to one of the somatic lineages (ectoderm, endoderm, mesoderm) or to the germline. The lack of pluripotent cells in adult life may be linked to organismal aging. Cnidarians (corals, and jellyfish) are an early branch of animals that do not succumb to age, but the developmental potential of their adult stem cells remains unclear. Here, we show that adult stem cells in the cnidarian Hydractinia symbiolongicarpus (known as i-cells) are pluripotent. We transplanted single i-cells from transgenic fluorescent donors to wild type recipients and followed them in vivo in the translucent animals. Single engrafted i-cells self-renewed and contributed to all somatic lineages and to gamete production, co-existing with and eventually displacing the allogeneic recipient’s cells. Hence, a fully functional, sexually competent individual can originate from a single adult i-cell. Given that some of their cells remain pluripotent beyond embryogenesis and throughout life, we conclude that Hydractinia embryos never complete gastrulation. Pluripotent i-cells underlie a regenerative, plant-like life history in these animals.