Expression Dynamics of Neurotransmitter System Genes in Early Sea Urchin Embryos: Insights from a Four-Species Comparative Transcriptome Analysis.

Transmitters such as serotonin, dopamine, noradrenaline, and acetylcholine act as regulators or triggers of numerous processes in the early embryo, including in sea urchins. However, the identity of these mechanisms relative to mature nervous systems remains controversial. The aim of this study was to comprehensively characterize the transcriptomic basis of these as well as glutamatergic, GABAergic and histaminergic systems by comparing publicly available RNA-Seq data across four sea urchin species (Mesocentrotus franciscanus, Lytechinus variegatus, Paracentrotus lividus, Strongylocentrotus purpuratus) during early development (egg to early gastrula). Transcript abundance was normalized using the geometric mean of housekeeping genes (GHG) to facilitate comparative analysis and to use the universal significance threshold. We detected mRNA transcripts encoding numerous components (enzymes, receptors, transporters) for all seven transmitter systems from the earliest stages, suggesting a complex signaling potential prior to neurogenesis. The expression of multiple mRNAs of receptors for the same transmitter indirectly supports our earlier notion of the possibility of simultaneous regulation of different processes by this transmitter even in the single-cell embryo. Notably, transcripts for key synthesis enzymes (TPH, DBH) were often low, indicating limited de novo synthesis, while transcripts for degradation enzymes (MAO, AChE) were abundant. Consistent expression across species was observed for specific receptors such as HTR6, D1-like dopamine, β-adrenergic receptors and the α7 subunit of nicotinic AChR. However, the expression profiles of many components, particularly glutamatergic receptors and metabolic enzymes, showed considerable interspecies variability. These findings indicate that multiple transmitter systems are transcriptionally represented early in development, suggesting substantial molecular overlap with mature systems, while the diversity between species points to possible evolutionary plasticity. This comparative transcriptomic dataset provides a basis for targeted functional studies of the role and interactions of these pre-nervous transmitter pathways in orchestrating embryogenesis.