A cis-regulatory sequence of the wing selector gene, vestigial, drives the evolution of scaling relationships in Drosophila species

Scaling between specific organs and overall body size has long fascinated biologists because they are a primary mechanism through which organismal shapes evolve. Yet, the genetic mechanisms that underlie the evolution of allometries remain elusive. Here we measured wings and tibia lengths in four Drosophila species (D. melanogaster, D. simulans, D. ananassae, and D. virilis) and show that the first three of them follow a single evolutionary allometry. However, D. virilis exhibits a divergent wing-to-tibia allometry due to a dramatic underscaling of their wings with respect to their bodies compared to the other species. We asked whether the evolution of this scaling relationship could be explained by changes in a specific cis-regulatory regulatory region of the wing selector gene, vestigial (vg), whose function is broadly conserved in insects and its expression pattern determines wing size in D. melanogaster. To test this hypothesis directly, we used CRISPR/Cas9 to replace the DNA sequence of the predicted Quadrant Enhancer (vgQE) from D. virilis for the corresponding vgQE sequence in the genome of D. melanogaster. Strikingly, we discovered that D. melanogaster flies carrying the D. virilis vgQE sequence have wings that are much smaller with respect to controls, partially rescuing the wing-to-tibia ratio observed in D. virilis. Our results show that this cis-regulatory element in D. virilis contributes to the underscaling of wings in this species. This provides evidence that scaling relationships may be unconstrained and may evolve gradually through genetic variations in cis-regulatory elements. Summary statementUsing CRISPR/Cas9 replacement of a cis-regulatory element, this study suggests that changes within the vestigial Quadrant Enhancer sequence are responsible for the evolution of wing allometries in Drosophila species.