Homeotic genes and the regulation and evolution of insect wing number.

The evolution of wings catalysed the radiation of insects which make up some 75 per cent of known animals. Fossil evidence suggests that wings evolved from a segment of the leg and that early pterygotes bore wings on all thoracic and abdominal segments. The pterygote body plan subsequently diverged producing orders bearing three, two or just one pair of thoracic wings. We have investigated the role of homeotic genes in pterygote evolution by examining their function in Drosophila wing development and their expression in a primitive apterygote. Wing formation is not promoted by any homeotic gene, but is repressed in different segments by different homeotic genes. We suggest here that wings first arose without any homeotic gene involvement in an ancestor with a homeotic 'groundplan' similar to modern winged insects and that wing formation subsequently fell under the negative control of individual homeotic genes at different stages of pterygote evolution.