Researchers have identified a gene that determines whether ultraviolet iridescence shows up in the wings of butterflies. The team showed that removing the gene in butterflies whose wings lack UV coloration leads to bright patches of UV iridescence in their wings. According to the researchers, the gene plays a critical role in the evolutionary process by which species become distinct from one another.
«As evolutionary biologists, we’re interested in identifying and understanding the genes that drive physical differences between species,» Arnaud Martin, an assistant professor of biology at GW and lead author on the paper, said. «Here we showed how a single gene determines whether ultraviolet coloration is switched on or off in two separate butterfly species. Because the geographic ranges of these two species overlap today, that visual distinction is all the more important when it comes time to find a mate.»
In the study, the researchers looked at two species of North American butterflies: the orange sulphur (Colias eurytheme) and the clouded sulphur (Colias philodice). The wings of the male orange sulphur butterfly reflect UV light, which is invisible to the naked eye, whereas the wings of the female orange sulphur butterfly and the male and female clouded sulphur butterflies do not display UV coloration.
According to the researchers, visual cues such as patterns and coloration, including UV iridescence, help animals recognize their own species and locate potential mates. Mating with the wrong species can be evolutionarily costly and lead to infertile offspring. The male orange sulphur butterfly’s UV-iridescent wings help females of the same species recognize the males as their own.
The two species were ecologically isolated from one another prior to the modern era and developed distinct traits. But, as farmers in the United States intensified their cultivation of alfalfa — a favorite food source for sulphur butterflies — the two species swarmed alfalfa fields, increasing the chances that some would mate with one another and swap genes. The orange sulphur butterfly in particular, once restricted to the western half of North America, has now colonized the eastern part of the continent and merged with populations of the clouded sulphur species.
In the lab, the researchers scanned the genomes of orange sulphurs and clouded sulphurs from an eastern population to see which genes merged and which stayed distinct during the past century of hybridization. Their analysis showed that the two species had clearly swapped and shared genes, and their respective chromosomes looked very similar. The one exception, however, was the sex chromosome, which remained distinct between the two species. This suggested that the sex chromosome hosts key genes that keeps the two species somewhat distinct, including the UV coloration.
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