Around 300 million years ago, as sprawling Carboniferous rainforests receded, dragonflies with two-foot wingspans zipped through the air. Into this world, beetles evolved—and evolve they did. By the numbers, beetles have become a dominant form of life, now representing a quarter of known animal species. Yet all beetles lack one thing: a protein that allows other insects to see blue light.
“It’s funny because beetles expanded and diversified so massively, but they started with a reduced visual system,” says Camilla Sharkey, a postdoctoral researcher who studies the molecular underpinnings of insect vision with Trevor Wardill, an assistant professor in the Department of Ecology, Evolution and Behavior. Together with colleagues, Sharkey and Wardill published findings in Molecular Biology and Evolution that demonstrate how a particular group of beetles—the ofteniridescent jewel beetles—see wavelengths of light their ancestors could not.
Most insects have genes that produce three proteins, or opsins, involved in sensing ultraviolet, blue and green light. Although jewel beetles lack the bluesensitive opsin, behavioral research suggests they still rely on complex color perception. The most notorious jewel beetle in the United States is the emerald ash borer, an invasive species that decimates native ash trees. Foresters know these beetles are attracted to specific shades of purple and green and design traps accordingly. Still, little was known about how this color-sensing occurred.
Sharkey had previously discovered that jewel beetles possess duplicate copies of the two remaining opsin genes. Now, Sharkey and Wardill have linked this duplication to expanded visual sensitivity. They found that, through evolution, one of the duplicated opsin genes has recalibrated for blue light and the other for orange.
The researchers used sophisticated genetic techniques to copy opsin genes from two species of jewel beetles and insert them individually into fruit flies. They also modified the fruit fly genome to deactivate normal visual functions. That way, when the beetle opsins were produced by the flies, the researchers could use electrophysiology to determine which new colors the flies could sense. “We talk about this Drosophila modification as wizardry, but actually, knocking in some of the opsin genes was really difficult,” Wardill says.
This is the first time scientists have directly tested how color-tuning occurs for beetles. “We know very little about beetle opsin function,” Sharkey says, “and in general, compared to what we know about vertebrate opsins, we know so little across insects.” With new connections drawn between opsin genes and function, the enormous diversity of beetles—pests and otherwise—is coming into view.
—JONATHAN DAMERY