Cell Proliferation and Morphogenetic Compartmentalization in the Phoronid Phoronopsis harmeri : Conserved and Derived Patterns

Cell proliferation is a key driver of morphogenesis and body plan transformation in multicellular animals, yet its spatial organization remains poorly understood in many non-segmented spiralians. In this study, we examine the dynamics of cell division during larval growth and metamorphosis in the larvae and early juveniles of the phoronid Phoronopsis harmeri , using EdU incorporation, anti-phospho-histone H3 immunostaining, confocal laser scanning microscopy, and electron microscopy. Early larval development is characterized by widespread proliferative activity across ectodermal and mesodermal tissues, which becomes progressively compartmentalized as the larva matures. Two structured ring-shaped posterior proliferative zones, pre- and post-telotrochal, emerge within the telotroch and persist through metamorphosis, supporting both larval elongation and the juvenile development of ascending gut branch. In contrast, the metasomal sac and future adult trunk epidermis expand via broadly distributed epithelial proliferation, without forming a localized growth zone. This suggests that P. harmeri combines conserved features, such as a posterior growth zone, with lineage-specific innovations in regional growth. In addition, we identify atypical mitotic characteristics in this species, including unconventional metaphase organization and signs of interkinetic nuclear migration in larval epithelia. Our findings highlight the coexistence of ancestral and derived proliferative mechanisms in phoronids and provide new insights into the evolution of axial elongation and morphogenetic compartmentalization in Lophotrochozoa.