The genome of the colonial hydroid <i>Hydractinia</i> reveals that their stem cells use a toolkit of evolutionarily shared genes with all animals

Abstract

<i>Hydractinia</i> is a colonial marine hydroid that shows remarkable biological properties, including the capacity to regenerate its entire body throughout its lifetime, a process made possible by its adult migratory stem cells, known as i-cells. Here, we provide an in-depth characterization of the genomic structure and gene content of two <i>Hydractinia</i> species, <i>Hydractinia symbiolongicarpus</i> and <i>Hydractinia echinata</i>, placing them in a comparative evolutionary framework with other cnidarian genomes. We also generated and annotated a single-cell transcriptomic atlas for adult male <i>H. symbiolongicarpus</i> and identified cell-type markers for all major cell types, including key i-cell markers. Orthology analyses based on the markers revealed that <i>Hydractinia</i>'s i-cells are highly enriched in genes that are widely shared amongst animals, a striking finding given that <i>Hydractinia</i> has a higher proportion of phylum-specific genes than any of the other 41 animals in our orthology analysis. These results indicate that <i>Hydractinia</i>'s stem cells and early progenitor cells may use a toolkit shared with all animals, making it a promising model organism for future exploration of stem cell biology and regenerative medicine. The genomic and transcriptomic resources for <i>Hydractinia</i> presented here will enable further studies of their regenerative capacity, colonial morphology, and ability to distinguish self from nonself.