Hybrid butterfly

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tiger swallowtail butterfly
A yellow form of the female eastern tiger swallowtail butterfly

Canadian tiger swallowtail
The Canadian tiger swallowtail butterfly occurs in Canada and in the bordering areas of the United States.

Flitting among the cool slopes of the Appalachian Mountains is a tiger swallowtail butterfly that evolved when two other types of swallowtails hybridized long ago, biologists from the University of Texas at Austin and Harvard University have found.

They discovered that the Appalachian tiger swallowtail, Papilio appalachiensis, evolved from mixing between the Eastern tiger swallowtail, P. glaucus, and the Canadian tiger swallowtail, P. canadensis.

The Appalachian tiger swallowtail is a unique mixture of the two in both its outward traits and inward genetic makeup.

The research was published today in the journal PLoS Genetics.

Understanding how new types of organisms come into being is basic to explaining the evolution of life on Earth.

"But we are just beginning to understand the process," says says Sam Scheiner, program director in NSF's Division of Environmental Biology. "These researchers made use of an unusual hybrid to unlock knowledge of the genetic basis of species formation."

"This is a remarkable demonstration of how hybridization can create populations with a new combination of life history and morphological traits, allowing colonization of novel environments by a 'mosaic genome,'" says biologist Larry Gilbert of the University of Texas at Austin, a coauthor of the paper.

These large insects are generally recognized by yellow wings with black stripes and small "tails" on their hind wings. Of the three different types, Eastern tiger swallowtails prefer warmer climes and lower elevations, and the females come in two different forms. They are either striped--yellow and black--or almost entirely black, the latter mimicking a poisonous butterfly called the Pipevine swallowtail, Battus philenor.

Canadian tigers are only striped yellow and black, and found in cooler habitats at higher latitudes and elevations. The Appalachian tiger exhibits a mix of those traits. It shares an affinity for cooler habitats with the Canadian tiger, while sharing the ability to mimic the black Pipevine swallowtail with the Eastern tiger.

Digging into the butterflies' genomes, the scientists found that the Appalachian tiger inherited genes associated with cold habitats from males of the Canadian tiger, and inherited a gene for mimicry from Eastern tiger females.

They also found that the Appalachian tiger's genome has become distinct from the genomes of its two parents, even though the butterflies come into contact with each other in the wild.

The Appalachian tiger's range nudges against the Canadian tiger in the northern Appalachian Mountains, and against the Eastern tiger in the lower elevations surrounding the mountains.

As for identifying these butterflies in the wild, Appalachian tigers are twice the size of Canadian tigers. Kunte says it's more difficult to tell apart the Eastern and Appalachian tigers. The Eastern tiger has more blue on the hind wing and a spotted yellow band on its fore wing underside compared with a solid broad band on the Appalachian tiger.

"Once you train your eyes to tell them apart," says Kunte, "they are relatively easy to distinguish."

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Adapted from materials obtained from the AAAS

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