Why Some Butterflies Have Shimmering, Colourful Wings And How They Get It

The researchers observed two species of North American butterflies, namely the orange sulphur (Colias eurytheme) and the clouded sulphur (Colias philodice).

(Clockwise from left) Male Orange Sulphur Butterfly; Orange Sulphur Butterfly glowing against a black background; Clouded Sulphur Butterfly On A Flower, Close-up view of a Clouded Sulphur Butterfly (Photos: Getty)

New Delhi: Butterflies, one of the most beautiful creatures in the world, are admired for their attractive wings. However, not all butterfly wings shimmer.

Now, a question arises. How do butterflies get bright, shimmering wings?  

Researchers at George Washington University in Washington, United States, recently identified a gene that determines whether ultraviolet iridescence can be seen in the wings of butterflies.

Iridescence is the glowing, shifting, colourful quality of a rainbow, also seen in a butterfly wing. 

The study was recently published in the journal, Proceedings of the National Academy of Sciences.

The researchers showed that removing a gene in butterflies whose wings are devoid of UV colouration results in bright patches of UV iridescence in their wings. 

What Role Does The Gene Play?

The gene plays an important role in the evolutionary process by which species become distinct from one another, the researchers noted in the study. 

Arnaud Martin, the lead author of the paper, said the researchers, as evolutionary biologists, are interested in identifying and understanding the genes driving physical differences between species, according to a statement issued by George Washington University.

Two Separate Butterfly Species Studied

The researchers conducted the study in two separate butterfly species. 

Martin said the geographic ranges of the two species overlap today. Therefore, the species must be visually distinct to be able to find a mate.

The researchers observed two species of North American butterflies, namely the orange sulphur (Colias eurytheme) and the clouded sulphur (Colias philodice). 

UV light is reflected by the wings of the male orange sulphur butterfly. The naked eye cannot detect this light. 

UV colouration is not displayed by the wings of the female orange sulphur butterfly and the male and female clouded sulphur butterflies, the study said. 

Animals are able to recognise their own species and track potential mates with the help of visual cues such as patterns and colouration, including UV iridescence, according to the researchers. 

When a butterfly mates with the wrong species, its offspring may be infertile, and can cost the environment.

Female orange sulphur butterflies are able to recognise the male butterflies of their own species with the help of their luminescent wings, according to the study.

How Did The Two Species Develop Distinct Traits?

Ecological isolation before the modern era caused the two species to develop distinct traits. 

When farmers in the United States increased the cultivation of alfalfa, which is a favourite food source for sulphur butterflies, the two species swarmed the alfalfa fields. 

This increased the chances of mating and hence, genes were swapped. 

Earlier, the orange sulphur bacteria was restricted to the western half of North America. Due to gene swapping, it has now colonised the eastern part of North America, and merged with populations of clouded sulphur bacteria, the study said.

In order to understand which genes merged and which stayed distinct during the past century of hybridisation, the researchers scanned the genomes of orange sulphurs and clouded sulphurs from an eastern population, in the lab.

They found that the two species had clearly swapped and shared genes. Also, their respective chromosomes appeared very similar.

Which Chromosome Was An Exception?

The sex chromosome was the only exception, and remained intact between the two species, the study said.

This indicates that the sex chromosome hosts an important gene which keeps the two species somewhat distinct. UV colouration is one of the distinctive features.

The Bric-A-Brac Gene

The researchers identified the part of the sex chromosome that causes UV iridescence, and found a gene called the bric-a-brac gene. 

When this gene is expressed in cells, it gives rise to the individual microscopic scales comprising a butterfly's wings.

A few cells, however, did not express the gene, the researchers observed. 

The study found that these were the cells which give rise to the UV-iridescent, or the luminescent scales in wings.

The researchers could also see how the presence or absence of the gene shaped the nanostructure of the scales, and their ability to amplify UV light, using a high-powered electron microscope. 

They used the genome editing technique CRISPR (Clustered regularly interspaced short palindromic repeats) to switch off the gene in the non-iridescent butterflies. 

How The Gene Determines The Butterfly's Outward Appearance

Switching off the gene changed not only the nanoscale structure of the scales, but also the outward appearance of the butterflies, the study said.

Vincent Ficarrotta, co-first author of the study, said that large portions of the butterflies' bodies were covered in UV-reflecting scales, including the clouded sulphur bacteria.

She said scales that normally would only be yellow or orange now reflected UV light.

Martin noted that studying the two butterfly species offered a one-of-a-kind opportunity to see evolution in action.

He called the pair of species a 'goldilocks system' for studying speciation because the species are not too identical, not too distinct, and the hybridisation is intense but is less than a century old.