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Switcheroo. By switching to a non-native plant, a hybrid fly may have become isolated enough to form its own species.

A New Hybrid, Animal Style

Is it possible to combine two animal species to make a new one? According to a study of wild flies, it's already happened in North America with the help of a foreign flower.

One of the central questions in biology is how species come to be. New species usually arise when populations become isolated from each other and acquire so many mutations that they can no longer produce viable offspring with members of the other population. Such a process gave rise to the lemurs and other unique creatures of Madagascar after it broke away from Africa.

Another road to speciation is hybridization, in which two species interbreed and form a new one. This has been known to happen in plants but is thought unlikely in animals. One common problem is that animal offspring of two parent species, like mules, are sterile and thus an evolutionary dead end. Another hurdle is that even if the hybrids can reproduce, they must then become isolated from the parent species. Otherwise, interbreeding with the parent populations will keep them from developing into a unique creature.

In 1997, while a graduate student at Pennsylvania State University (PSU), University Park, Dietmar Schwarz found a possible candidate for an animal hybrid species. He noticed that flies that infest Lonicera honeysuckles looked like those of two closely related species, the apple maggot fly and the blueberry maggot fly. In addition, some of the flies' proteins contained patterns seen in both the apple maggot and blueberry maggot species.

Further testing this year has sealed the case. Schwarz, now a postdoc at PSU, and colleagues found that the blueberry and apple flies can mate in the lab to form a hybrid similar to the honeysuckle-infesting species, dubbed the Lonicera fly. The Lonicera, in turn, can mate with the parent species and produce viable offspring, but DNA analysis indicates that this is not happening in the wild.

The researchers argue in the 28 July issue of Nature that the honeysuckle may be the reason why. After Europeans introduced the plant to North America 250 years ago, hybrid flies that switched to it may have become isolated from the parent fly species.

"The paper is convincing," says Loren Rieseberg, a geneticist at Indiana University, Bloomington. But to nail down how exactly the Lonicera fly separated from its parent species, we need to know more about its behavior, he says, such as whether members of the species mate exclusively on honeysuckle flowers.

Related sites
Loren Rieseberg's site
All about speciation