PNAS: The gene that makes rainbows appear on butterfly wings

A team of researchers at the University of Washington has discovered a gene that determines whether ultraviolet iridescence occurs on the wings of butterflies
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In a study published in the Proceedings of the National Academy of Sciences, the team showed that removing the gene in butterflies lacking UV color in their wings resulted in bright UV light on the wings rainbow spots

"As evolutionary biologists, we are interested in identifying and understanding the genes that drive physiological differences between species," said Arnaud Martin, assistant professor of biology at George Washington University and lead author of the paper
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"Here, we show how a single gene determines whether UV color is turned on or off in two different butterfly species

For the study, researchers looked at two North American butterflies: the orange sulphur monarch (Colias eurytheme) and the cloudy sulphur monarch (Colias philodice)

According to the researchers, visual cues such as patterns and colors, including ultraviolet iridescence, can help animals identify their own species and find potential mates
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Mating with the wrong species is evolutionarily costly and leads to sterile offspring

Before modern times, the two species were ecologically isolated from each other and developed very different traits
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But as U.
S.

In the lab, the researchers scanned the genomes of orange-sulfur and cloudy-sulfur in eastern populations to see which genes merged and which remained different in crosses over the past century
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Their analysis showed that the two species clearly exchanged and shared genes, and their respective chromosomes looked remarkably similar

The team narrowed down the portion of the sex chromosome that causes the ultraviolet iridescence and identified a gene called the brac gene that, when expressed in cells, produces individual microscopic scales, including butterfly wings
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However, the researchers noticed that some cells did not express the gene

"It's kind of like solving a giant Sudoku puzzle," said Joe Hanly, a postdoctoral scientist in Martin's lab and one of the paper's first authors
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"It was very exciting when we saw the results
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Right before our eyes, we found a gene that makes these species look different from each other
.
"

Using high-power electron microscopy, the team could also see how the presence or absence of genes shaped the nanostructures of the scales, and they suspect, their ability to amplify ultraviolet light
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Using the genome-editing technology CRISPR, they turned off the genes of non-rainbow butterflies
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Not only did the nanoscale structure of the scales change, but so did the appearance of the butterfly
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Vincent Ficarrotta, a doctoral student in Martin's lab and one of the paper's first authors, said: "Much of the butterfly's body is covered with UV-reflecting scales, including Cloud-shaped sulphur butterflies
.
" "The scales, usually only yellow or orange, now reflect UV light
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"

There are still questions to be answered, such as whether there are other genes on the sex chromosomes that are involved in mate selection, and the number and exact nature of trinket gene mutations that make up species differences, Martin said
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However, he noted, studying the two butterfly species offers a unique opportunity to observe evolutionary processes
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"Speciation studies often compare species that are very early in the separation process, or that have been separated for too long and have accumulated too many differences for meaningful genomic analysis," Martin said
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"Here we have a pair that is not quite alike, nor Too different species, hybridization is already strong, but less than a century old
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It's a golden-haired system for studying speciation
.
"

article title

A genetic switch for male UV-iridescence in an incipient species pair of sulphur butterflies