Growth, Remodeling, and Proliferation in the Phoronid <i>Phoronopsis harmeri</i> : The Posterior, the Regionalized, and the Scattered

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 proliferation is partly regionalized from the outset and becomes progressively more localized toward metamorphosis. We identify a tripartite organization of proliferative activity: (1) posterior ring-shaped domains in the telotroch that persist through metamorphosis and support elongation and anal chamber formation; (2) regional proliferative zones at tentacle bases, preoral and postoral regions; and (3) scattered proliferation driving the expansion of the trunk epidermis. This coexistence of posterior, regional, and scattered patterns underscores the developmental plasticity of phoronids and the diversity of growth strategies within Spiralia. Posterior proliferative domains in phoronids contribute important context to homology-convergence debates on posterior growth across spiralians, but are not decisive by themselves; viewed with the distributed epithelial proliferation, they underscore the coexistence of multiple proliferative programs within a single life cycle. In addition, we identify atypical mitotic characteristics in this species, including unconventional metaphase organization and signs of interkinetic nuclear migration in larval epithelia. Our results suggest that phoronids provide a valuable model for exploring how diverse architectures of cell proliferation contribute to larval growth, body elongation, and morphogenetic compartmentalization in Lophotrochozoa.