Totipotent migratory stem cells in a hydroid

Hydroids, members of the most ancient eumetazoan phylum, the Cnidaria, harbor multipotent, migratory stem cells lodged in interstitial spaces of epithelial cells and are therefore referred to as interstitial cells or i-cells. According to traditional understanding, based on studies in Hydra, these i-cells give rise to several cell types such as stinging cells, nerve cells, and germ cells, but not to ectodermal and endodermal epithelial cells; these are considered to constitute separate cell lineages. We show here that, in Hydractinia, the developmental potential of these migratory stem cells is wider than previously anticipated. We eliminated the i-cells from subcloned wild-type animals and subsequently introduced i-cells from mutant clones and vice versa. The mutant donors and the wild-type recipients differed in their sex, growth pattern, and morphology. With time, the recipient underwent a complete conversion into the phenotype and genotype of the donor. Thus, under these experimental conditions the interstitial stem cells of Hydractinia exhibit totipotency.