Stem cells in a colonial animal with localized growth zones Authors

America. CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not. 2 Summary Siphonophores (Hydrozoa) have unparalleled colony-level complexity, precision of organization, and functional specialization between zooids (i.e., the units that make up colonies). Previous work has shown that, unlike other colonial animals, most growth in siphonophores is restricted to one or two well-defined growth zones that are the sites of both elongation and zooid budding. To understand this unique growth at the cellular level, we characterize the distribution of interstitial stem cells (i-cells) in the siphonophore Nanomia bijuga. Within the colony we find that i-cells are present at the tips of the growth zones, at well-defined sites where new zooid buds will arise, and in the youngest zooid buds. As each zooid matures, i-cells become progressively restricted to specific regions until they are mostly absent from the oldest zooids. We find no evidence of the migratory i-cells that have been observed in colonial cnidarian relatives. The restriction of i-cells to particular developing structures and sites of growth suggest a plant-like model of growth for siphonophores, where the growth zones function much like meristems. This spatial restriction of stem cells could also explain the precision of colony-level organization in siphonophores as a consequence of restricted growth potential. Highlights-Siphonophore stem cells are largely restricted to growth zones and developing zooids-No evidence for migratory capacities of these stem cells-The growth zones of siphonophores function much like the meristems of plants-Restriction of stem cells may play a major role in facilitating the precision of siphonophore growth but also lead to reduced developmental plasticity. CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not. 3 Results Colonial animals provide a unique opportunity to investigate general questions about the evolution of development, and to better understand development beyond embryogenesis [1-3]. Animal colonies arise when asexual reproduction is not followed by physical separation [4]. This results in many genetically identical multicellular bodies, known as " zooids " , that are attached and physiologically integrated to form a colony. Each zooid is homologous to a solitary free-living animal, but lives as part of the colony. The complex lifecycles of colonial animals require multiple developmental processes-the embryological development of the zooid that founds the colony, the asexual development of subsequent zooids, and …