[Extreme divergence of a homeotic gene: the bicoid case]

Evolutionary developmental genetics (evo-devo) reveals that the plasticity of development is so important that every developmental biology project should carefully take this point into consideration. The example of bicoid, the first discovered morphogen, illustrates how an essential gene can change its function during evolution. The search for bicoid homologues showed that this gene is surprisingly specific to flies (cyclorraphan diptera) and absent in other insects. In fact, recent studies demonstrate that bicoid is a very derived Hox3 homeotic gene. During insect evolution, the ancestral Hox3 gene lost its homeotic function and acquired new roles in oocytes and embryonic annexes. Then, in the lineage leading to modern flies, a duplication of this new gene, followed by functional divergence, led to the formation of bicoid and zerknüllt. Both genes are located within the Drosophila Hox complex; however, they have no homeotic function. Thanks to the power of Drosophila genetics, it is possible to suggest that torso and hunchback may constitute the insect primitive anterior organizer. The bicoid evolutionary history reveals several fundamental mechanisms of the evolution of developmental genes, such as changes of gene regulation, modifications of protein sequences and gene duplication. It also shows the need for studying a wider range of model organisms before generalisations can be made from data obtained with one particular species.