The swallowtail butterfly family, Papilionidae, contains over 550 species. The females of them are known to have features that mimic other types of butterflies, in shape and markings, which have the effect of dissuading predators who mistake them for their toxic doppelgangers.
In a study published in the journal Nature Communications, a team led by Marcus Kronforst of the University of Chicago in the US set out to probe the genetic key to this extraordinary fancy dress, and to discover why some swallowtail species don’t exhibit mimicry.
They discovered that far from a process involving the gradual accumulation of genetic changes allowing an ever-broadening range of mimicked forms, the ability to change wing colour and markings arose just once, around two million years ago.
Until this paper, the dominant theory regarding the development of mimicry in butterflies centred on the idea of a “supergene”: a tightly knit collection of individual genes always inherited as a clump.
By sequencing the genomes of Papilio polytes, the Asian swallowtail butterfly, and several similar species, and discovered that only a single gene was involved.
The gene is known as doublesex and is related to both mimicry restricted to females and polymorphism – the ability for shape to vary among members of the same species – again found only in females.
“In butterflies with one colour pattern, we have a gene in a normal orientation on the chromosome,” explains Kronforst.
“In the butterflies with the unusual, alternate colour pattern, that gene was spliced out, flipped, and then spliced back into the chromosome at some point.
“That flip, or inversion, keeps the two genes from recombining if those two different kinds of butterflies mate, so they’ve kept both copies of the gene over evolutionary time, since they split from their common ancestor two million years ago.”
Kronforst and colleagues opt to keep the descriptor “supergene” for doublesex, albeit a little modified. Since the gene-flip occurred, various other pressures have been brought to bear on different swallowtail populations, leading to different outcomes over time.
One of these is selective balancing – where predatory species gradually learn that a particularly patterned butterfly variant does not represent a toxic insect and start to eat them in earnest. This results in the decline of the affected insects while prompting the success of new wing designs.
Another pressure is simply genetic drift, or noise – changes in the genome among populations that gradually affect the rate and type of change to markings and wing shape.
As a result, some swallowtail species exhibit a wide range of possible female shapes, with the assumed “original”, pre-flip, form having disappeared. Others retain the original form among a small or large suite of possibilities.