Comparisons of cell proliferation and cell death across life histories in the hemichordate Schizocardium californicum

Background There are a wide range of developmental strategies in animal phyla, but most insights into adult body plan formation come from direct-developing species. For indirect-developing species, there are distinct larval and adult body plans that are linked together by metamorphosis. Some outstanding questions in indirect-developing organisms include the extent to which larval tissue undergoes cell death during the process of metamorphosis and when the tissue that will give rise to the adult originates. Here we present patterns of cell proliferation and cell death during larval development, metamorphosis, and adult body plan formation, in the hemichordate Schizocardium californium to answer these questions. Results We identified distinct patterns of cell proliferation between larval and adult body plan formation of S. californicum . We found that some adult tissues proliferate prior to the start of any morphological metamorphosis. Additionally, we describe a genetic signature of proliferative cells with an irradiation approach that revealed markers shared between the life history states and others that are unique to larvae or juveniles. Finally, we observed that cell death is minimal in larval stages but begins with the onset of metamorphosis. Conclusions Cell proliferation during the development of S. californicum has distinct patterns in the formation of larval and adult body plans. However, cell death is very limited in larvae and begins during the onset of metamorphosis and into early juvenile development in specific domains. The populations of cells that proliferate and give rise to the larva and juvenile have a genetic signature that is more suggestive of a heterogeneous pool of proliferative progenitors versus a population of pluripotent set-aside cells. Taken together, we propose that S. californicum has a transformative metamorphosis that may be more representative of the development strategies that characterize metamorphosis in many metazoan animals. ### Competing Interest Statement The authors have declared no competing interest.