Meeting Abstract
Hypotheses of origins of neurons and synapses are controversial, mostly due to limited comparative data. Here, we investigated the genome-wide distribution of the bilaterian ‘synaptic’ and ‘neuronal’ protein-coding genes in basal metazoans (Ctenophora, Porifera, Placozoa, Cnidaria). (1) There are no recognized genes uniquely expressed in neurons across all metazoan lineages. (2) Our analysis of ~200 genes encoding canonical presynaptic and postsynaptic proteins in bilaterians suggests that there are no true ‘pan-synaptic’ genes or genes uniquely and specifically attributed to all classes of synapses. The majority of these genes encode receptive and secretory complexes in a broad spectrum of eukaryotes. (3) The majority of genes encoding ion channels and receptors are broadly expressed in unicellular eukaryotes and non-neuronal tissues in metazoans. Therefore, they cannot be viewed as neuronal markers. However, the co-expression of multiple types of ion channels and receptors does correlate with the presence of neural and synaptic organization. Analysis of transcriptomes from 24 different ctenophores and 4 ctenophore genomes indicate that this lineage has a distinct ‘synaptic’ organization compared to other animals. We did not detect recognized orthologs of synthetic enzymes encoding several classical, low-molecular-weight (neuro)transmitters: glutamate signaling machinery is one of the few exceptions. Novel peptidergic signaling molecules were predicted for ctenophores, together with the diversity of putative receptors, many of which could be examples of a lineage-specific expansion within this group. In summary, our analysis supports the hypothesis of independent evolution of neurons and, as corollary, parallel evolution of synapses. Both factors and models underlying origins of synapses will be discussed. We suggest that the formation of synaptic machinery might occur more than once over 600 million years of animal evolution. Supported by NSF, NIH NASA
