Single-nucleus profiling highlights the all-brain echinoderm nervous system

Metazoans comprise diverse tissues and cell types, each essential for the
survival of the organism. Most of these types are established early in
embryogenesis and persist into adulthood. In indirectly developing sea
urchins, however, the continuity between embryonic and adult stages is
interrupted by a planktonic larval stage that undergoes complete
metamorphosis. While gene regulatory networks controlling embryonic and
larval lineages are well studied, the molecular and morphological
identities of post-metamorphic cell types remain poorly understood. Here,
we reconstructed the cell type atlas of post-metamorphic Paracentrotus
lividus juveniles using single-nucleus transcriptomics, revealing
conservation of regulatory mechanisms. We identified signatures of eight
distinct cell type groups and analyzed 29 neuronal families, including 15
unique photoreceptor types. By combining transcriptomics, spatial
analysis, and ultrastructure, we identified vertebrate neuronal and opsin
homologues expressed across the sea urchin juvenile. These findings show
the echinoderm body plan is predominantly head-like and exhibits an
“all-brain” organization.