Floating lazily in the forests and jungles of the Americas, longwing butterflies have many secrets. The 30-odd species in this group include many mimics. The wing markings on several distantly related species of longwings were similar, inspiring a Victorian naturalist to theorize that the harmless species might imitate the deadly ones to avoid predators.
In the age of genomic sequencing, biologists have found other peculiarities in longwings. In a paper published last week in the Proceedings of the National Academy of Sciences, researchers report that female zebra longwings see colors that men don’t, thanks to a gene on their sex chromosome. Understanding how it got there can shed light on how differences between the sexes might evolve.
Like primates, butterflies have a number of proteins sensitive to certain wavelengths of light that, working together, produce the ability to distinguish colors. Curious about the zebra longwing’s vision, Adriana Briscoe, a professor at the University of California, Irvine, and an author of the new paper, asked a student to scan the species’ genome for a known vision gene. of color. The gene, known as UVRh1, codes for a protein that is sensitive to ultraviolet light. To his surprise, it found nothing.
Digging deeper, and obtaining genomic data from additional zebra longwings, Dr. Briscoe and his colleagues found that UVRh1 was present, but only in females. In lab experiments, they confirmed that women can see markings that men can’t. They eventually identified the gene in an unexpected place: the small sex chromosome of the butterfly.
The sex chromosomes in butterflies are unstable, often shedding genes that have been picked up by other chromosomes, or lost entirely, Dr. Briscoe. That makes them a pretty unusual place to keep something as important as a gene for color vision.
Then there are other butterflies whose males and females are known to have different color vision, probably has to do with females discovering males for mating. In those species, seeing certain colors is a waste of resources for males. But for those insects, different vision seems to be related to how genes are regulated, not their placement on the sex chromosome. Somehow, in the zebra longwing, it took a different turn.
How did UVh1 get to where it is today? Did it start with the sex chromosome? Or did it migrate there from the shared chromosomes of the zebra longwing and then — somehow — get edited out of males, for whom maintaining more complex color vision might be more trouble than it’s worth?
The question of what happened and when in the zebra longwing has deeper ramifications than simply understanding how butterflies see, the researchers suggest. This addresses a riddle of evolutionary biology: The most successful female of a species and the most successful male may have very different, even opposite, characteristics. What kind of genetic shenanigans was required to create this division?
If UVRh1 originates on the sex chromosome, then it quickly avoids a situation where a trait that’s best for females inhibits the other sex, the researchers wrote.
If that’s the case, “it suggests that UV color vision began its life limited to females, giving up even the possibility of becoming a burden on males,” said JJ Emerson, a professor at the University of California, Irvine, and an author of the new paper. “This jump would be a neat trick if UVh1 were harmful to men.”
It should be possible to establish what happened to the zebra longwings with further research, Dr. Briscoe said. There are several longwing species whose genomes have not been adequately studied. One in particular, the Aoede longwing, which lives in remote forests in the Amazon Basin, promises a potential answer.
If it also has females seeing more than males, it suggests that UVRh1 actually starts on the sex chromosome, said Dr. Briscoe.
That’s another data point in a growing body of work examining differences between the sexes in butterflies.
“In the old days, nobody would study the difference between the sexes” in butterflies, said Dr. Briscoe. “There was no attempt to find out if they were doing anything else.”
He suspects that evolution may have produced other intriguing differences in longwings that scientists are only just beginning to unravel.