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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.09.055 Byline: Willow N. Gabriel (a), Robert McNuff (b), Sapna K. Patel (a), T. Ryan Gregory (c), William R. Jeck (a), Corbin D. Jones (a), Bob Goldstein (a) Keywords: Development; Evolution; Ecdysozoa; Tardigrade; Lineage; Model system Abstract: Studying development in diverse taxa can address a central issue in evolutionary biology: how morphological diversity arises through the evolution of developmental mechanisms. Two of the best-studied developmental model organisms, the arthropod Drosophila and the nematode Caenorhabditis elegans, have been found to belong to a single protostome superclade, the Ecdysozoa. This finding suggests that a closely related ecdysozoan phylum could serve as a valuable model for studying how developmental mechanisms evolve in ways that can produce diverse body plans. Tardigrades, also called water bears, make up a phylum of microscopic ecdysozoan animals. Tardigrades share many characteristics with C. elegans and Drosophila that could make them useful laboratory models, but long-term culturing of tardigrades historically has been a challenge, and there have been few studies of tardigrade development. Here, we show that the tardigrade Hypsibius dujardini can be cultured continuously for decades and can be cryopreserved. We report that H. dujardini has a compact genome, a little smaller than that of C. elegans or Drosophila, and that sequence evolution has occurred at a typical rate. H. dujardini has a short generation time, 13-14 days at room temperature. We have found that the embryos of H. dujardini have a stereotyped cleavage pattern with asymmetric cell divisions, nuclear migrations, and cell migrations occurring in reproducible patterns. We present a cell lineage of the early embryo and an embryonic staging series. We expect that these data can serve as a platform for using H. dujardini as a model for studying the evolution of developmental mechanisms. Author Affiliation: (a) Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA (b) Sciento, 61 Bury Old Road, Whitefield, Manchester, M45 6TB, England, UK (c) Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Article History: Received 3 July 2007; Revised 12 September 2007; Accepted 28 September 2007