Supergene defines butterfly patterns
Scientists have discovered the gene enabling multiple female morphs that give the Common Mormon butterfly its very tongue-in-cheek name. Doublesex, the gene that controls gender in insects, is also a mimicry supergene that determines diverse wing patterns in this butterfly, according to a recent study published in Nature. The study also shows that the supergene is not a cluster of closely linked genes as postulated for nearly half a century, but a single gene controlling all the variations exhibited by the butterfly's wings.
Thanks to a medley of shapes, sizes, colors and patterns, butterfly wing variations have helped scientists understand evolution in numerous ways. Intense predator pressures have forced butterflies to evolve different tactics to ward them off. Some butterflies have evolved to feed on toxic plants, and consequently, are inedible and toxic themselves. Predators think twice before attacking them. In response to this, palatable butterflies have developed a rather devious tactic: they imitate the wing patterns of toxic butterflies found in the region, a phenomenon known as Batesian mimicry. Usually, females of such species mimic a toxic model, as they are more prone to predation than males. Sexual dimorphism (marked differences in gender) in mimetic butterflies is often a result of this, such as the Danaid eggfly female, which imitates the Plain tiger.
In some cases, females of a single mimetic species imitate not one, but several toxic species. This results in multiple female forms or morphs: what scientists call polymorphism. The widespread Asian butterfly, the Common Mormon (Papilio polytes) is a perfect example of this. The male has a single non-mimetic wing pattern but the female has four forms: a non-mimetic morph that resembles the male, and three other forms that mimic the toxic butterflies in the genus Pachliopta found around them. These females mimic the colors, wing patterns and even shape of their Pachliopta models, right down to their small tail-like projections on the hindwings.
This undoubtedly impressive natural phenomenon drew the attention of geneticists in the 1960s. They speculated that alternative wing patterns are controlled by a supergene, which had to be a cluster of tightly linked genes located near each other. These genes would control specific attributes of wing patterning such as the presence or absence of tails, color differences on the forewing and so on.
So which supergene controls the numerous wing pattern variations of the Common Mormon? What genes constitute this supergene? And how do these genes produce the mimetic morphs? These were the questions scientists sought to answer.
Read more at ENN affiliate Mongabay.
Common Mormon image via Shutterstock.