Dual origins for neural cells during development of the Clytia planula larva.

Adult hydrozoan cnidarians undergo extensive tissue turnover, generating neural cell types that include nematocytes (stinging cells) and gland cells from interstitial stem cells (i-cells) expressing stemness proteins such as Piwi and Nanos. The contribution of i-cells during embryogenesis, however, has been unclear. We address neural cell origins during development of the Clytia hemisphaerica planula larva. In situ hybridisation of nematogenesis genes revealed that Piwi/Nanos1-expressing cells generate a substantial pool of nematoblasts (undifferentiated nematocytes) within the gastrula presumptive endoderm, in line with scRNAseq clustering analysis. Some neurogenic and neuronal markers, however, showed a distinct expression profile, developing within a basal layer of the aboral/lateral ectoderm during gastrulation. Embryo bisection and lineage-tracing experiments support the hypothesis that neurosensory and secretory cell types can be generated from gastrula ectoderm, while nematoblasts and at least some RFamide-expressing ganglionic cells are derived from i-cells. Knockdown and inhibitor treatments revealed steps in neuron and nematocyte development regulated by Wnt-β-catenin. We propose that two distinct neurogenesis pathways operate during Clytia embryogenesis: one involving ectodermal delamination, and one generating many nematoblasts and some neurons from i-cell-like precursors.