<p>Abstract</p> <p>Background</p> <p>Functional studies in model organisms, such as vertebrates and <it>Drosophila</it>, have shown that basic Helix-loop-Helix (bHLH) proteins have important roles in different steps of neurogenesis, from the acquisition of neural fate to the differentiation into specific neural cell types. However, these studies highlighted many differences in the expression and function of orthologous bHLH proteins during neural development between vertebrates and <it>Drosophila</it>. To understand how the functions of neural bHLH genes have evolved among bilaterians, we have performed a detailed study of bHLH genes during nervous system development in the polychaete annelid, <it>Platynereis dumerilii</it>, an organism which is evolutionary distant from both <it>Drosophila </it>and vertebrates.</p> <p>Results</p> <p>We have studied <it>Platynereis </it>orthologs of the most important vertebrate neural bHLH genes, i.e. <it>achaete-scute, neurogenin, atonal, olig</it>, and <it>NeuroD </it>genes, the latter two being genes absent of the <it>Drosophila </it>genome. We observed that all these genes have specific expression patterns during nervous system formation in <it>Platynereis</it>. Our data suggest that in <it>Platynereis</it>, like in vertebrates but unlike <it>Drosophila</it>, (i) <it>neurogenin </it>is the main proneural gene for the formation of the trunk central nervous system, (ii) <it>achaete-scute </it>and <it>olig </it>genes are involved in neural subtype specification in the central nervous system, in particular in the specification of the serotonergic phenotype. In addition, we found that the <it>Platynereis NeuroD </it>gene has a broad and early neuroectodermal expression, which is completely different from the neuronal expression of vertebrate <it>NeuroD </it>genes.</p> <p>Conclusion</p> <p>Our analysis suggests that the <it>Platynereis </it>bHLH genes have both proneural and neuronal specification functions, in a way more akin to the vertebrate situation than to that of <it>Drosophila</it>. We conclude that these features are ancestral to bilaterians and have been conserved in the vertebrates and annelids lineages, but have diverged in the evolutionary lineage leading to <it>Drosophila</it>.</p
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