Context-dependent transcriptional heterogeneity of new markers expands our understanding of stem cells in Hydractinia

Adult stem cells are found in many animals, particularly invertebrates with high regenerative capacities. The stem cell populations in these organisms can exhibit transcriptional and morphological variation across various spatial and biological contexts. We focus on the adult stem cells (i-cells) of Hydractinia symbiolongicarpus , a colonial marine cnidarian, where such heterogeneity in its stem cell compartment remains poorly understood. A cellular atlas was previously generated for H. symbiolongicarpus which revealed two distinct clusters with i-cell signatures: one leading to somatic cell fates and one leading to a germ cell fate. We investigated eight new stem cell marker genes from the atlas, including five expressed in both i-cell clusters ( Pcna , Nop58 , Mcm4 , Ubr7 , and Uhrf1 ) and three expressed in one cluster or the other (somatic i-cells: Pter , FoxQ2-like , and germ i-cells: Zcwpw1 ). We characterized their spatial expression patterns in various tissue locations and during feeding polyp head regeneration, providing insight into the two i-cell clusters. We demonstrate that Hydractinia i-cells exhibit transcriptional and morphological diversity that varies with spatial location and biological context, rather than representing a homogeneous population across the colony. We have expanded our understanding of adult stem cells in this highly regenerative marine invertebrate by characterizing the spatial expression patterns of multiple new i-cell marker genes. It is now clear that i-cells are a transcriptionally heterogeneous population, which includes a subset that is definitively pluripotent. Our data provides important nuances for better understanding Hydractinia ’s incredible adult stem cell population and a solid foundation on which to build future research, allowing for meaningful contributions to the field of regenerative medicine.