Figure 3: Phylogenetic analysis of the Platynereis achaete-scute-related bHLH gene. The phylogenetic tree has been constructed by Maximum Likelihood (ML) as described in the Methods section. Similar tree topologies are obtained using other phylogenetic reconstruction methods (not shown). Statistical supports for the internal branches that define these groups are indicated (first number: bootstrap support in Neighbour-joining (NJ) analysis; second number: bootstrap support in ML analysis; third number: posterior probability in Bayesian inference analysis). The tree has been rooted using non-achaete-scute bHLH genes as outgroup. The Platynereis gene is indicated in bold red. The "?" indicates a group of divergent achaete-scute-like genes found in some species and that cannot be related to either of the two bilaterian achaete-scute families (see [31] for more details). Species abbreviations are as in Figure 2.

Mentions: By sequence similarity searches on an expressed sequence tag (EST) collection (40,000 ESTs from normalized cDNA libraries of mixed larval stages, corresponding to more than 10,000 unigene clusters) [34], we identified several Platynereis dumerilii bHLH genes among which some show strong sequence similarity to either achaete-scute- or atonal-related genes. The predicted amino acid sequence of the bHLH domains were aligned with those of a sample of metazoan bHLH genes identified in a previous study [31]. Multiple phylogenetic reconstructions show that we identified orthologs of the neurogenin/biparous, achaete-scute/ASCL/ASH, olig, and NeuroD genes (Figures 2 and 3). We named these genes Pdu-Ngn, Pdu-ASH, Pdu-Olig, and Pdu-NeuroD, respectively. For each family, we found a single Platynereis member but we cannot exclude that additional members do exist, although duplicated evolutionary-conserved genes are rare in Platynereis [34]. We also included in our analysis the previously identified atonal ortholog (Pdu-ATH; not found in the EST collection) [35,37] in order to study Platynereis representatives for all the main families of bHLH genes involved in neural determination and specification in vertebrates and Drosophila [2,4]. We also identified Beta3 and mist genes in Platynereis (Figure 2), but these genes were not further characterized as their Drosophila and vertebrate orthologs do not have well defined functions in neurogenesis. We used WMISH to monitor the expression of Pdu-Ngn, Pdu-ASH, Pdu-Olig, Pdu-ATH, and Pdu-NeuroD during Platynereis development and focused on possible expressions during trunk nervous system formation for which a good characterization exists [35]. All the genes are also expressed in the head, probably in the brain and/or sensory organs, but these expressions were not further characterized.

Simionato E, Kerner P, Dray N, Le Gouar M, Ledent V, Arendt D, Vervoort M - BMC Evol. Biol. (2008)

Bottom Line: Our data suggest that in Platynereis, like in vertebrates but unlike Drosophila, (i) neurogenin is the main proneural gene for the formation of the trunk central nervous system, (ii) achaete-scute and olig 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 Platynereis NeuroD gene has a broad and early neuroectodermal expression, which is completely different from the neuronal expression of vertebrate NeuroD genes.Our analysis suggests that the Platynereis bHLH genes have both proneural and neuronal specification functions, in a way more akin to the vertebrate situation than to that of Drosophila.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 Drosophila.

Affiliation: Evolution et Développement des métazoaires, Centre de Génétique Moléculaire-UPR 2167 CNRS, 1, av. de terrasse, 91198 Gif-sur-Yvette, France. simionato@cgm.cnrs-gif.fr

Abstract: Functional studies in model organisms, such as vertebrates and Drosophila, 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 Drosophila. 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, Platynereis dumerilii, an organism which is evolutionary distant from both Drosophila and vertebrates.We have studied Platynereis orthologs of the most important vertebrate neural bHLH genes, i.e. achaete-scute, neurogenin, atonal, olig, and NeuroD genes, the latter two being genes absent of the Drosophila genome. We observed that all these genes have specific expression patterns during nervous system formation in Platynereis. Our data suggest that in Platynereis, like in vertebrates but unlike Drosophila, (i) neurogenin is the main proneural gene for the formation of the trunk central nervous system, (ii) achaete-scute and olig 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 Platynereis NeuroD gene has a broad and early neuroectodermal expression, which is completely different from the neuronal expression of vertebrate NeuroD genes.Our analysis suggests that the Platynereis bHLH genes have both proneural and neuronal specification functions, in a way more akin to the vertebrate situation than to that of Drosophila. 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 Drosophila.