Butterfly sex genes fool predators


A doublesex gene in some butterflies manages pattern development in wings, making harmless species look like toxic relatives. By adopting colors and wing patterns similar to those of toxic insects, butterflies can often avoid predation.  

This process, known as mimicry, was first described in the 1860's by naturalist Alfred Russel Wallace, after he made a trip to southeast Asia. 

Researchers were surprised that such a dramatic effect could be produced by a single gene. Most believed the process was directed by a "supergene" - a group of tightly-linked genes, with each coding proteins to carry out specific activites to produce the coloration. 

"Conventional wisdom says that it should be multiple genes working together to control the whole wing pattern of a butterfly. But in this case, it's just this one. This single gene that controls sexual differentiation has been co-opted to do a totally new job," Marcus Kronforst, of the University of Chicago and senior author of the study, said

Investigators studied a variety of butterfly called Papilio polytes, a type of swallowtail butterfly, native to Asia. This flying insect is often known as a common Mormon butterfly. They allowed specimens  exhibiting differing wing patterns to mate, producing around 500 offspring. Researchers then sequenced the genomes of 30 of the insects, comparing their genetic code to wing patterns. They noticed a clear correlation between the doublesex gene and wing patterns on the insects. 

"We've illustrated the genetic basis of female-limited mimicry in these butterflies. But this is just the first step. How doublesex became involved in this process is still uncertain, and requires further study," Wei Zhang, from the University of Chicago and co-author of the study, said. 

Doublesex genes travel throughout the body of the insect, telling cells whether to produce male or female characteristics. Female Papilio palytes butterflies have four distinct forms, three of which make them look like poisonous cousins to the species. This is known as Batesian mimicry. 

Research into how the doublesex gene in Papilio palytes affects wing patterns could lead to new discoveries concerning causes of mimicry in other butterfly species and animals. Another example of similar behavior in other animals is the milk snake. Although harmless, the species looks much like highly-poisonous coral snakes. 

Strangely, this effect only happens in females of the species, leaving males vulnerable to attack from predators. Naturalists are perplexed why this evolutionary advantage is not also employed by both genders of the species. 

Study of the regulation of wing patterns by the doublesex gene was profiled in the journal Nature on 5 March.

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