Origin and diversification of the basic helix-loop-helix gene family in metazoans: insights from comparative genomics

Elena Simionato^*¹ , Valérie Ledent^*² , Gemma Richards³ , Morgane Thomas-Chollier²^,4 , Pierre Kerner¹ , David Coornaert² , Bernard M Degnan³ and Michel Vervoort¹^,5

¹Evolution et Développement des protostomiens, Centre de Génétique Moléculaire- UPR 2167 CNRS, 1, av. de la terrasse, 91198 Gif-sur-Yvette Cedex, France

²Belgian EMBnet Node – Laboratoire de Bioinformatique, Université Libre de Bruxelles, Institut de Biologie et de Médecine Moléculaires, Rue des Professeurs Jeener et Brachet 12, B-6041 Gosselies, Belgium

³School of Integrative Biology, University of Queensland, Brisbane, Qld 4072, Australia

⁴Vrije Universiteit Brussel, Laboratory for Cell Genetics, Pleinlaan 2, B-1050 Brussels, Belgium

⁵UFR de Biologie et Sciences de la Nature, Université Paris 7 – Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France

author email corresponding author email* Contributed equally

BMC Evolutionary Biology 2007, 7:33doi:10.1186/1471-2148-7-33


Published:	2 March 2007

Abstract

Background

Molecular and genetic analyses conducted in model organisms such as Drosophila and vertebrates, have provided a wealth of information about how networks of transcription factors control the proper development of these species. Much less is known, however, about the evolutionary origin of these elaborated networks and their large-scale evolution. Here we report the first evolutionary analysis of a whole superfamily of transcription factors, the basic helix-loop-helix (bHLH) proteins, at the scale of the whole metazoan kingdom.

Results

We identified in silico the putative full complement of bHLH genes in the sequenced genomes of 12 different species representative of the main metazoan lineages, including three non-bilaterian metazoans, the cnidarians Nematostella vectensis and Hydra magnipapillata and the demosponge Amphimedon queenslandica. We have performed extensive phylogenetic analyses of the 695 identified bHLHs, which has allowed us to allocate most of these bHLHs to defined evolutionary conserved groups of orthology.

Conclusion

Three main features in the history of the bHLH gene superfamily can be inferred from these analyses: (i) an initial diversification of the bHLHs has occurred in the pre-Cambrian, prior to metazoan cladogenesis; (ii) a second expansion of the bHLH superfamily occurred early in metazoan evolution before bilaterians and cnidarians diverged; and (iii) the bHLH complement during the evolution of the bilaterians has been remarkably stable. We suggest that these features may be extended to other developmental gene families and reflect a general trend in the evolution of the developmental gene repertoires of metazoans.