COPYCAT BUTTERFLY EXPOSED: SHE’S GOT A SUPERGENE


Ever heard of a Papilio polyte? Well she is beautiful. But she is also known to be somewhat of a copycat. The Papilio polyte, also known as the female version of the common mormon butterfly, is known for her mimicry. That is to say, she is a master impressionist. In particular, she impersonates the swallowtail, a butterfly that is more colorful and toxic, in order to trick predators into thinking she is distasteful – and deadly. Explicitly, Miss Mormon is known for imitating the unpalatable – and inedible – red-bodied crimson rose swallowtail.

THE THREE FACES OF…

Beneath her big facade, the common mormon is a lady of real substance. According to Harish Gaonkar, an Indian butterfly specialist at the National History Museum in London, its name is an allusion to the polygamy practiced by members of the Mormon sect (papilio being Latin for butterfly and poly being Greek for many). Says Gaonkar,

“… the first to get such a name was the common Mormon (Papilio polytes), because it had three different females… The name obviously reflected the … Mormon sect in America, which as we know, practiced polygamy.”

Thanks to its uncanny ability to simulate the swallowtail, the aptly named commons are indeed quite common – and bold, as they are not a threatened lot. The female’s three disguises are known as the cyrus, marked by vibrant red crescents, the stichius which most closely resembles the crimson rose swallowtail, and the romulus which is the least adept at mimicry and appears duller in vibrancy than the others. Generally, the common is a jet black butterfly with a row of white spots along the middle part of its hindwing, and measures an approximate 90 -100 mm.

WHAT PUTS THE SUPER IN SUPERGENE

If all of this just feels like one confusing look-alike contest to you, have no worries, there’s no quiz. What you do want to note however, is that for decades, scientists have thought that the female common’s three different masks were controlled by a “supergene”: a cluster of genes, each which manage different parts of the butterfly’s wings. New research has found however, that the once-supposed supergene is actually a single gene called doublesex. By demonstrating different variants of the gene at multiple levels, the butterfly can swap their wing patterns in a quite radical manner. Hitting upon this discovery was a double-play for scientists because the doublesex gene already had a well-established role: to navigate the morphing butterflies down either a male or female trail. That such a well-characterized gene is revealed to carry another function is mind-warping.

The supergene idea was founded in the 1960s by British scientists Sir Cyril Clarke and Philip Sheppard. The duo conducted cross-breeding experiments which showed that common mormons inherit their wing patterns as one – not as separate elements. While this was an amazing unveiling, no one had yet identified the actual cluster of genes that composed the supergene…until a group of scientists at the University of Chicago began tinkering with the flutter-bys. By comparing non-mimetic females that resemble males with those that look like the common rose swallowtail and seeking out parts of the butterfly’s genome that were associated with their methods of imitating, they stumbled upon a tiny section that held five genes, four of which were similar in all of the females, no matter what their wing patterns looked like.

LUCKY NUMBER FIVE

The fifth gene proved to be a hotspot of molecular evolution. The doublesex gene, it differed between the mimetic and the non-mimetic females at more than 1,000 nucleotides. Said Kunte, one of the researchers in Chicago,

“If you look at flies or beetles, this gene is very conserved. Only in mimetic and non-mimetic females of this one species do you see such diversity.”

While these mutations alter the way the doublesex gene works, they have no bearing on the parts of the gene that are responsible for determining the sexes, which allows the gene to simultaneously hold on to the task of wing-controller.

Researchers in Chicago also found that when doublesex is transcribed in RNA, the transcript is divided into four distinct fashions: three identified in females, one in males. The gene showed that the doublesex gene is more strongly exhibited in the burlesque-mormon females, as well as in the small slices of the wing responsible for the insect’s white streaking.

Despite doublesex being a single gene, it appears to be inverted relative to the non-mimetic one so that it is positioned askew in the genome. This prevents different alleles (mutated genes responsible for hereditary variations) from mingling with one another, and for the gene to make certain their thousands of mutations are all inherited in concert. Doublesex is not a cluster of tightly-chained genes. On the contrary, it is a cluster of tightly-chained mutations in one gene.

IT MUST BE GOING AROUND

Amid this flurry of discovery, there are lots of other scientists who, while dabbling in the  supergene scavenge, have discovered it in a slew of other organisms, including other butterflies. One example of a lepidopderist’s lightbulb moment was experienced by Matthieu Joron of the French National Centre for Scientific Research when he unveiled that the varied patterns of the Heliconius butterfly (a genus of the boldly striped, black and white brush-footed butterfly commonly known as the longwing) are controlled by a cluster of many genes also yoked by a genomic inversion. While Joron noted that the two lineages independently resulted in an extraordinarily similar solution in response to similar pressures for accurate mimicry, he was hungry for more answers – particularly that to the question of what each element of the doublesex gene actually does and why they need to be tethered in order to result in full mimetic patterns.

There is curiosity in the world of these whispery wonders we call butterflies as to whether or not scientists have inaccurately pigeon-holed doublesex as a sex-differentiating gene when in fact it is capable of so much more, as proven by this latest research. Potentially, this gene may be the agent involved in the decision-making process when it comes to other factors in the animal kingdom like the production of deer antlers or even peacock feathers. For now however, the riddles remain riddles, the researchers continue researching, and the butterflies…well, they just stay beautiful.

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