The evolution of engrailed genes after duplication and speciation events

Members of the engrailed class encode transcription factors involved in major steps of metazoan development. Few developmental regulatory genes have been studied in such a wide range of animals. Furthermore duplications of an ancestral engrailed gene independently generated multiple engrailed paralogues in several organisms. This offers the opportunity to reconstruct the evolution of the engrailed family and to study the processes involved in the functional diversification following speciation or duplication events. The ancestral function of engrailed is very likely involved in neurogenesis. Recent studies in Drosophila and mice have shown its crucial role in neuronal connectivity and neuromuscular targeting. engrailed was probably recruited very early for a role in segmentation through intercalary evolution. Several new functions were acquired later on in specific phyla. Some duplication events have been followed by the loss of one paralogue, whereas others have led to the functional diversification of the paralogues. The Duplication-Degenerescence-Complementation model recently proposed by Force et al. seems to be the main process involved in functional diversification after duplication events. This does not exclude acquisition of new functions for one or both paralogues after duplication. The acquisition of such new functions principally involves the evolution of cis-regulatory sequences, but evolution of the coding sequence has also been revealed. However, in all engrailed duplications studied, even in ancient chromosomal duplications, the paralogues have kept redundant functions. In fact, selection seems to maintain a certain redundancy between engrailed paralogues.