Conserved and novel enhancers regulate embryonic ventral midline gene expression in theAedes aegypti single-mindedlocus

Abstract

ABSTRACTTranscriptionalcis-regulatory modules, e.g., enhancers, control the time and location of metazoan gene expression. While changes in enhancers can provide a powerful force for evolution, there is also significant deep conservation of enhancers for developmentally important genes, with function and sequence characteristics maintained over hundreds of millions of years of divergence. Not well understood, however, is how the overall regulatory composition of a locus evolves, with important outstanding questions such as how many enhancers are conserved vs. novel, and to what extent are the locations of conserved enhancers within a locus maintained? We begin here to address these questions with a comparison of the respectivesingle-minded (sim)loci in the two dipteran speciesDrosophila melanogaster(fruit fly) andAedes aegypti(mosquito).simencodes a highly conserved transcription factor that mediates development of the arthropod embryonic ventral midline. We identify two enhancers in theA. aegypti simlocus and demonstrate that they function equivalently in both transgenic flies and mosquitoes. OneA. aegyptienhancer is highly similar to knownDrosophilacounterparts in its activity, location, and autoregulatory capability. The other differs from any knownDrosophila simenhancers with a novel location, failure to autoregulate, and regulation of expression in a unique subset of midline cells. Our results suggest that the conserved pattern ofsimexpression in the two species is the result of both conserved and novel regulatory sequences. Further examination of this locus will help to illuminate how the overall regulatory landscape of a conserved developmental gene evolves.AUTHOR SUMMARYThe expression patterns and roles of genes, especially those involved in core developmental processes, are often conserved over vast evolutionary distances. Paradoxically, the DNA sequences surrounding these genes, which contain thecis-regulatory sequences (enhancers) that regulate gene expression, tend to be highly diverged. The manner and extent to which enhancers are functionally conserved, and how the overall organization of regulatory sequences within a locus is preserved or restructured, is not well understood. In this paper, we investigate these questions by identifying enhancers controlling expression of a master nervous system regulatory gene namedsimin the mosquitoAedes aegypti, and comparing their functions and locations to those in the well-characterizedsimlocus of the fruit flyDrosophila melanogaster. Our results suggest that the two species generate identical patterns ofsimexpression through a mix of conserved and novel regulatory sequences. Continued exploration of thesimlocus in these two species will help to build a comprehensive picture of how a regulatory locus for a master developmental regulator has evolved.