Eastern tiger swallowtails, as their name suggests, have vivid orange and black stripes. These showy butterflies are so popular in the eastern U.S., they’re the state insect of five states. But their caterpillars, on the other hand, are fierce-looking stinkers. They’re disguised as stubby snakes with a big, fake green head, markings that look like black and yellow eyes, and (the best part) a forked, fleshy orange tongue. That tongue-like structure is technically a defensive, glandular protusion called the osmeterium, and it can turn outwards, expand, and move—all the while emitting a stinky substance that wards off their would-be predators.
And now, a University of Texas Southwestern Medical Center team led by Nick Grishin have sequenced the complete genome of one, wild-caught male eastern tiger swallowtail (Papilio glaucus) from Lake Ray Roberts State Park in Texas. The findings, published in Cell Reports last week, help explain how their young became so awesomely fearsome.
To the right is an eastern tiger swallowtail in Oklahoma.
The genome data uncovered mutations in the proteins that are responsible for their circadian rhythm, the body’s internal clock. That might explain why eastern tiger swallowtails mature into adults quickly and break out of their chrysalises right away— instead of sleeping through the winter the way Canadian tiger swallowtails do. These two species split about 600,000 years ago, though they do still hybridize in the Appalachian Mountains.
As for the stinky substance released by caterpillars, the team found that a gene encoding enzymes for synthesizing terpenes—chemicals produced in the osmeterium that convert into noxious odors—were multiplied in their genome. Rather than just one or two, these butterflies have many diverse proteins that do the job.
“We think the power of comparative genomics can be fully exploited only when there are thousands of genomes available,” Grishin says in a news release. That’s why one of their primary goals was to come up with a way to sequence genomes with high levels of genetic variation at a reasonable cost. "With our protocols, the cost per new genome falls below $4,000," he adds. "Go out, catch a bug, sequence the genome, learn something new."
Images: Edith Smith, Shady Oak Butterfly Farm (top), Bryan Reynolds, the Butterflies of the World Foundation (middle)