Chromosomal rearrangements maintain a polymorphic supergene controlling butterfly mimicry

The toxic butterfly Heliconius numata, found in forests across South America, mimics the wing patterns of several species of another family of toxic butterflies, Melinaea sp., in order to deter predators more effectively. This example of Mllerian mimicry is under the control of a classic 'supergene', a tight gene cluster usually inherited as a single unit. H. numata is particularly adept at mimicry, able to copy as many as seven different wing patterns. A study of the individual wing-pattern morphs in H. numata shows that different genomic rearrangements at the single supergene P locus tighten the genetic linkage between loci that are otherwise free to recombine in other closely related species. The resulting supergene acts as a simple switch that once thrown, selects which one of a range of complex adaptive phenotypes the butterfly displays.