From insect eye to vertebrate muscle: redeployment of a regulatory network

The redeployment of the same signaling systems at different places and times during embryogenesis, and the conservation of these systems across species, from invertebrates to vertebrates, has emerged as a major theme of biology. This conservation also applies to transcriptional regulation; the Hox genes are a classic example. More recently a striking illustration of the phenomenon was provided by the demonstration that homologous regulatory genes are responsible for the formation of the ommatidia, which constitute the eye of the fruit fly Drosophila and of the mammalian eye. Furthermore, the transcription factor Pax6, like itsDrosophila homolog eyeless, which is at the top of the regulatory cascade, can induce ectopic eye formation in the fly (Halder et al. 1995). The article by Heanue and collegues in this issue ofGenes & Development, demonstrates that the same combination of transcriptional regulators required for eye formation is redeployed elsewhere during vertebrate embryogenesis, in this case in the somite and its skeletal muscle derivatives.