Floating languorously through 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 some distantly related species of longwings are so similar they inspired one Victorian naturalist to theorize that harmless species could mimic deadly ones to avoid predators.
In the age of genomic sequencing, biologists have found other oddities in longwings. In a paper published last week in the Proceedings of the National Academy of Sciences, researchers report that female zebra longwings can see colors that males cannot, thanks to a gene on their sex chromosome. Understanding how it got there might shed light on how differences between sexes can evolve.
Like primates, butterflies have a handful of proteins that are 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 check the species’ genome for a well-known color vision gene. The gene, known as UVRh1, codes for a protein that is sensitive to ultraviolet light. To her surprise, it was nowhere to be found.
Digging deeper, and drawing on genomic data from additional zebra longwings, Dr. Briscoe and her colleagues discovered that UVRh1 was there, but only in females. With lab experiments, they confirmed that females could see markings males couldn’t. They eventually pinpointed the gene in an unexpected place: the butterfly’s tiny sex chromosome.
Sex chromosomes in butterflies are unstable, often shedding genes that are picked up by other chromosomes, or lost entirely, Dr. Briscoe said. That makes them a somewhat 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, perhaps having to do with females spotting males for mating. In those species, seeing some colors would be a waste of resources for the males. But for those insects, the differing vision seems 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 UVRh1 get to where it is today? Did it start out on the sex chromosome? Or did it move 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 the butterflies see, the researchers suggest. It touches on a riddle of evolutionary biology: The most successful female of a species and the most successful male may have very different, even contradictory, traits. What kind of genetic shenanigans are required to create this division?
If UVRh1 originated on the sex chromosome, then it deftly sidesteps a situation in which a trait that is optimal for females hampers the other sex, the researchers write.
Should that be the case, “it suggests that UV color vision started its life already limited to females, forgoing even the possibility of being a burden in males,” said J.J. Emerson, a professor at the University of California, Irvine, and also an author of the new paper. “This jump would be a neat trick if UVRh1 were harmful to males.”
It should be possible to deduce what happened in zebra longwings with further research, Dr. Briscoe said. There are some longwing species whose genomes have yet to be adequately studied. One in particular, the Aoede longwing, which lives in remote forests in the Amazon Basin, promises a potential answer.
If it too has females that see more than males, then it suggests that UVRh1 indeed started out on the sex chromosome, Dr. Briscoe said.
That would be another data point in a growing body of work examining the differences between sexes in butterflies.
“In the old days, nobody would study the difference between the sexes” in butterflies, Dr. Briscoe said. “There was no attempt to figure out if they were doing anything differently.”
She suspects that evolution may have wrought other intriguing differences in longwings that scientists are just at the beginning of uncovering.