Anything We can Do Better : Female of this species sees the world in colors that her male partner can't comprehend.

Floating languorously through forests and jungles of the Americas, longwing butterflies have many, many secrets.

The 30-odd species in this group include many mimics, with wing markings on some distantly related species so similar that one Victorian naturalist theorized that harmless species mimicked deadly ones to avoid predators.

In the age of genomic sequencing, biologists studying longwings have found other oddities.

In a new paper researchers report that a female zebra longwing can see colors that a male cannot, thanks to a gene on the female's 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 ; working together the proteins produce the ability to distinguish colors.

Curious about the zebra longwing's vision, Adriana Briscoe, a professor of 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, UVRh1, whose codes for a protein sensitive to ultraviolet light.

The student couldn't find it.

YET it was there, Dr. Briscoe and her colleagues discovered - 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 a butterfly are unstable, often shedding genes that are picked up by other chromosomes, or lost entirely.

That makes them a somewhat unusual place to keep something as important as a gene for color vision.

There are other butterflies whose males and females have different color vision abilities, perhaps enabling females to spot males for mating. In those species, seeing some colors would be a waste of resources for the males.

But for those insects, the variance in vision seems related to the way genes are regulated, not to their placement on the sex chromosome. Somehow, in the zebra longwing, it is different.

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 touches on a riddle of evolutionary biology :

The main successful female of a species and the most successful male may have different 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 wrote. [ Veronique Greenwood ]


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