Studying Butterfly Colors: A Flip of the Genetic Switch
By LIDA TUNESI
Spread across southeast Asia is a surprisingly variable species of butterfly, called Elymnias hypermnestra, or the common palmfly. Not only does this species imitate the appearance of other butterflies—but sometimes the males and females mimic completely different species.
A new study has begun to discover the evolutionary history and genetics behind this butterfly’s appearance. The results, and the new questions the study prompts, will clarify how the females and males of a species sometimes look significantly different, a phenomenon called sexual dimorphism.
The E. hypermnestra males are mimics of poisonous Euploea butterflies, which are dark brown or black. This mimicry is an evolved survival trick that wards off potential predators. The females of some populations imitate Euploea as well, but others imitate poisonous Danaus species, which are orange. No E. hypermnestra population has both orange and brown females.
It might seem that there are two different lineages in this species—one with brown females and one with orange. However, the researchers found the species split into three different genetic groups. The first group has only brown females, while the second and third both have a geographic mosaic of orange populations and brown populations. Why evolution of orange females repeated itself in groups two and three but not group one, however, is still a mystery.
Digging into the genetics a little more, the researchers found two sites in this butterfly’s DNA that are responsible for their color variation by regulating a gene called WntA. If the right changes in DNA (called “mutations”) happen at both sites, the female butterflies will be orange rather than brown.
WntA seems to act like a “master gene” that controls a coordinated unit of other genes in order to change the wing color, Lohman explained. This allows the butterflies to change color just by flipping a genetic switch, so to speak, rather than evolving brand-new set of color-related genes each time. That said, the switch likely happens over a long period of evolutionary time.
These answers lead to more questions: Why did orange females come about in the first place? And since the males also have these regulatory sites, why have they never evolved to be orange? E. hypermnestra butterflies are a unique opportunity for researchers to study dual mimicry, and to understand the genetic and evolutionary forces behind it.