GABRIEL, Willow; PATEL, Sapna; GOLDSTEIN, Bob; Univ. of North Carolina, Chapel Hill: Developing Tardigrades as a New Model for Studying the Evolution of Development

How morphological diversity evolves is a central question in biology. We aim to develop tardigrades as a model for studying the evolution of developmental processes that determine morphology. Tardigrades are phylogenetically ideal for this since they belong to the protostome superclade Ecdysozoa, as do Drosophila and C. elegans, and we can utilize many of the scientific tools developed for these organisms to study tardigrade development. We have traced the cell lineage of the tardigrade Hypsibius dujardini through seven rounds of division. The dorsal-ventral axis can be identified by the 4-cell stage. Two cells of the four cell stage undergo a series of unequal stem cell-like divisions, producing two small cells with significantly extended cell cycles at the 32 cell stage. These cells ingress 7 hours post-fertilization (hpf), followed by one of their sister cells. We are testing whether the ingressing cells form an equivalence group using laser ablation techniques. An epithelium forms at 10-11 hpf; embryonic elongation starts 18 hpf, muscle twitching at 54 hpf, and embryos hatch at 110 hpf. We are developing histochemical and immunological staining methods to determine when tissues begin to differentiate. Staining for alkaline phosphatase (AP) activity, which marks endoderm, is enriched in cells found in the interior of the embryo before elongation begins. AP staining remains strong throughout subsequent development and, in adults, is restricted to the gut. Muscle differentiation, visualized with fluorescent phalloidin, is first detected after elongation, at 22 hours. These data begin to give us a picture of how tardigrade development proceeds. We are continuing to use new techniques to study tardigrade embryogenesis, in order to develop this species as a model to understand how developmental processes have evolved.