Origin and evolution of the enhancer of split complex

Peter K. Dearden

The Enhancer of split complex is an unusual gene complex found in Arthropod genomes. Where known this complex of genes is often regulated by Notch cell signalling and is critically important for neurogenesis. The Enhancer of split complex is made up of two different classes of genes, basic helix-loop-helix-orange domain transcription factors and bearded class genes. The association of these genes has been detected in the genomes of insects and crustaceans. Tracing the evolution of the Enhancer of split complex in recently sequenced Arthropod genomes indicates that enhancer of split basic helix-loop-helix orange domain genes arose before the common ancestor of insects and Crustacea, and before the formation of the complex. Throughout insect and crustacean evolution, a four-gene cluster has been present with lineage specific gene losses and duplications. The complex can be found in the vast majority of genomes, but appears to be missing from the genomes of chalcid wasps, raising questions as to how they carry out neurogenesis in the absence of these crucial genes. The enhancer of split complex arose in the common ancestor of Crustacea and insects, probably through the linkage of a basic helix-loop-helix orange domain gene and a bearded class gene. The complex has been maintained, with variations, throughout insect and crustacean evolution indicating some function of the complex, such as coordinate regulation, may maintain its structure through evolutionary time.