Distinct mechanisms decommission redundant enhancers to facilitate phenotypic evolution

Abstract

The evolutionary loss of morphological traits is often driven by changes in gene regulation. Many developmental genes are controlled by multiple, redundant enhancers, raising the question of how robust regulatory systems can be dismantled to permit phenotypic transitions. We show that the loss of larval trichomes in Drosophila sechellia resulted from the independent inactivation of four embryonic enhancers of the shavenbaby gene. Each enhancer was extinguished by a distinct mechanism: (1) a large deletion that removed essential sequences, (2) the loss of activator sites and gain of repressor sites, (3) the acquisition of a long-range silencer, and (4) the unmasking of pre-existing repression. Notably, three of these mechanisms relied on repression, pointing to repression as a rapid route for the evolutionary loss of robust regulatory elements. These results show that robustness in gene regulation does not prevent morphological change but instead provides multiple opportunities for mutations to reduce enhancer activity, giving selection many paths to reshape form.