DROSOPHILA EVOLUTION CHALLENGES POSTULATED REDUNDANCY IN THE E(SPL) GENE-COMPLEX

The Enhancer of split [E(spl)] gene complex belongs to the class of ne urogenic loci, which, in a concerted action, govern neurogenesis in Dr osophila. Two genetically distinct functions, vital and neurogenic, re side within the complex defined by lethal mutations in the l(3) gro ge ne and by the typical neurogenic phenotype of deletions, respectively. Such deletions always affect several of the many embryonically active genes in the region, which cannot be mutated separately to lethality. Seven of these genes are extremely similar at the transcription and s equence level sharing the basic helix-loop-helix (bHLH) motif of trans criptional regulators. While these E(spl) bHLH genes seem to be requir ed collectively for neurogenesis, they are nonessential individually, suggesting functional redundancy of the encoded gene products. No spec ific functions could yet be ascribed to any of the other genes located within the complex. One might expect these apparently dispensable gen es, as well as the supposedly redundant bHLH genes, to be under little evolutionary constraint and, thus, to evolve most rapidly. However, w e find the entire E(spl) gene complex highly conserved during Drosophi la evolution, indicating that all the genes as well as their organizat ion are of functional importance.