Synergistic regulation of vertebrate muscle development by Dach2, Eya2, and Six1, homologs of genes required for Drosophila eye formation

We have identified a novel vertebrate homolog of the Drosophila gene dachsh und, Dachshund2 (Dach2). Dach2 is expressed in the developing somite prior to any myogenic genes with an expression profile similar to Pax3, a gene pr eviously shown to induce muscle differentiation. Pax3 and Dach2 participate in a positive regulatory feedback loop, analogous to a feedback loop that exists in Drosophila between the Pax gene eyeless (a Pax6 homolog) and the Drosophila dachshund gene. Although Dach2 alone is unable to induce myogene sis, Dach2 can synergize with Eya2 (a vertebrate homolog of the Drosophila gene eyes absent) to regulate myogenic differentiation. Moreover, Eya2 can also synergize with Six1 (a vertebrate homolog of the Drosophila gene sine oculis) to regulate myogenesis. This synergistic regulation of muscle devel opment by Dach2 with Eya2 and Eya2 with Six1 parallels the synergistic regu lation of Drosophila eye formation by dachshund with eyes absent and eyes a bsent with sine oculis. This synergistic regulation is explained by direct physical interactions between Dach2 and Eya2, and Eya2 and Six1 proteins, a nalogous to interactions observed between the Drosophila proteins. This stu dy reveals a new layer of regulation in the process of myogenic specificati on in the somites. Moreover, we show that the Pax, Dach, Eya, and Six genet ic network has been conserved across species. However, this genetic network has been used in a novel developmental context, myogenesis rather than eye development, and has been expanded to include gene family members that are not directly homologous, for example Pax3 instead of Pax6.