HOX GENES, HOMEOSIS AND THE EVOLUTION OF SEGMENT IDENTITY - NO NEED FOR HOPELESS MONSTERS

Significant changes have occurred in the developmental role of Hox gen es, even within groups of arthropods that already have complex body pl ans and many different segment types. This is hard to reconcile with t he 'selector gene' model for Hox gene function. Selector genes act as stable binary switches that direct lineages of cells to adopt alternat ive developmental fates. This model suggests that the regulation of se lector genes can only evolve through mutations that alter the identity of whole developmental compartments -in the case of Hox genes, whole segments. Once segments have evolved distinct morphology and function, such mutations will result in dramatic homeotic transformations that are unlikely to be tolerated by natural selection. Thus we would expec t the developmental role of these ''master control genes'' to become f rozen as body plans become more complex. I argue for a revised model f or the role and regulation of the Hox genes. This provides alternative mechanisms for evolutionary change, that may lead to incremental chan ges in segment morphology. The summation of such changes over long per iods of time would result in differences in Hox gene function between taxa comparable to the effects of gross homeotic mutations, without th e need to invoke the selective advantage of hopeful monsters.