Selective sweep of a cis-regulatory sequence in a non-African population of Drosophila melanogaster

Although it is thought that changes in gene expression play an important role in adaptation, the identification of gene-regulatory sequences that have been targets of positive selection has proved difficult. Here, we identify a cis-regulatory element of the Drosophila melanogaster CG9509 gene that is associated with a selective sweep in a derived non-African population of the species. Expression analyses indicate that CG9509 consistently shows greater expression in non-African than in African strains of D. melanogaster. We find that a 1.8 kb region located just upstream of the CG9509 coding region is devoid of DNA sequence polymorphism in a European population sample and that this is best explained by the recent action of positive selection (within the past 4,000-10,000 years). Using a reporter gene construct and phiC31-mediated site-specific integration, we show that the European version of the CG9509 upstream region drives 2-3 times greater expression than the African version in an otherwise identical genetic background. This expression difference corresponds well to that of the native gene and indicates that sequence variation within the CG9509 upstream region can completely account for its high expression in the European population. Selection appears to have favored a quantitative increase in gene expression in the Malphigian tubule, the tissue where CG9509 is predominantly expressed.