Stepwise Modification of a Modular Enhancer Underlies Adaptation in a Drosophila Population

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Abstract
Degrees of Darkness: Fruit flies in Africa have a tendency to be darker, the higher the altitude at which they live, because melanization offers a selective advantage. The dark pigmentation seen in some populations of Ugandan Drosophila melanogaster is owing to a lack of expression of the ebony gene, and expression results in yellow cuticle. Rebeiz et al. (p. 1663 ; see the News story by Pennisi ) show that mutations in cis regulatory elements, rather than in the coding region, are responsible for the dark color. A series of five mutations in a modular enhancer element influences the level of ebony expression: Three mutations already existed in fly populations with light cuticle color and a further two, more recently acquired dark-specific substitutions, together have created an allele of large effect, which has been swept to high frequency in this population of flies.