Chourrout and Technau - Press
History of Hox Genes:
A Key to Apprehend Changes of Animal Morphology
Darwin and others proposed that all living organisms share a common ancestor, whose morphology gradually changed to generate their remarkable diversity of forms. The morphology of most animals is determined by complex genetic programs, among which some genes named Hox exert master control on how body parts will become different from the most anterior to the most posterior tip. Hox genes are found in all animals except the sponges, and therefore we assume that changes in their organisation and expression have been a source of morphological changes. Almost all studies of Hox genes have been performed on so-called bilaterians, animals exhibiting a left-right symmetry, which include most animals we know, including insects and vertebrates. However, other modern animals are not bilaterians, and we believe that they preceded the bilaterians during evolution. To understand how evolution proceeds to generate morphological diversity, it was therefore essential to examine Hox genes of non-bilaterian animals. This has been done by two research groups of the Sars Centre (Daniel Chourrout and Ulrich Technau), with support from several foreign collaborators. Their observations after comparing their Hox genes with those of bilaterians are reported in the last issue of the journal Nature (10 August).
Non bilaterian animals include the cnidarians, a group of mostly marine animals (anemones, polyps, jellyfish etc..) studied at the Sars Centre. The genomes of two cnidarian species (Nematostella and Hydra) have been recently sequenced in the US, allowing detection and characterisation of all Hox and Hox-related genes. Comparing Hox genes from animals that have diverged more than half a billion years ago is not a simple task, but powerful methods have been recently developed to do so and successfully employed in the present work by one of the foreign collaborators (Frederic Delsuc, Montpellier, France). The main result is that the Hox gene toolbox of cnidarians shows similarities with that of bilaterians, but it also substantially differs from it. More precisely, bilaterians posses three main subgroups of Hox genes, but only one of them can be recognised in cnidarians. Cnidarians also have a second subset of Hox genes, which cannot be found in bilaterians.
The major implication of the published work is that variations of Hox genes can indeed be part of the explanation for morphological differences between modern animals. Perhaps even more interesting is that modifications of the Hox gene complement can have changed the body plan during evolutionary times. Here, the published article proposes a scenario in which a far ancestor of both bilaterians and cnidarians only had the subgroup of Hox genes they are sharing today. How was this animal looking like is still an enigma, but it was probably very simple. The Nature article suggests that further gain in complexity had to include a diversification of the Hox genes.
More details can be found in:
Chourrout D, Delsuc F, Chourrout P, Edvardsen RB, Rentzsch F, Renfer E, Jensen MF, Zhu B, de Jong P, Steeele RE, Technau U (2006). Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements. Nature, 442(7103):684-7.