Little bluish butterflies high in the Sierra Nevada mountains have an unusual history. Researchers report that these insects belong to one of the few animal species known to have arisen from crossbreeding of two other species.
Crossbreeding of animal species isn’t unusual in itself, explains Zachariah Gompert of Texas State University in San Marcos. But the descendants of most hybrid offspring meld back into the parent species or don’t compete successfully against the parental lines.
The not-yet-named butterflies in the Lycaeides genus, however, flourish in the harsh zone above the timberline, where the parent species can’t cope, Gompert and his colleagues say. When they began studying the high-elevation butterflies, team members already suspected that two neighboring species had played some role in the high-living population’s history.
The upper-alpine species shared some DNA variations with Lycaeides idas from wet meadows on the western slopes of the mountains, and it has wing patterns like those of Lycaeides melissa from the drier eastern slopes. All these butterflies belong to the genus studied by 20th-century novelist Vladimir Nabokov.
Gompert and his colleagues did extensive genetic work on the three Sierra Nevada species. The pattern of markers and sequenced genes best fit the scenario of the high-alpine lineage arising from the other two species and later following its own evolutionary path, the researchers say in the Dec. 22 Science.
Some new hybrid species arise with an increase in the number of chromosomes. However, the butterfly hybrid has the same number of chromosomes as its parents do, so some other barrier must prevent it from interbreeding with those species.
Not only does the hybrid survive in a different habitat, says Gompert, but the butterflies strongly prefer as host plants a specialized high-altitude Astragalus in the pea family. The parent species rely on other plants, such as alfalfa and lupines.
Moreover, the hybrid females have the unusual quirk of laying eggs on their host plant without glue, although the parent species use glue on their eggs. The eggs without glue quickly tumble off the plant, and that tumble protects the next generation. In winter, alpine gales blast away dried-up plants and any hitchhikers. However, eggs that fell to the ground hatch in spring near emerging shoots.
Botanists have already accepted the idea of this kind of hybrid species, such as some native sunflowers, says insect evolutionary ecologist Mark Scriber of Michigan State University in East Lansing. As zoologists, “we’d been brainwashed into thinking hybrids are dead ends,” he says.
Yet recent research has revealed animal species that seem to have come from hybrids, including the unusual tiger swallowtail butterflies that Scriber studies in the Appalachian Mountains.
Biologists have also recently proposed hybrid speciation to explain a new Rhagoletis fruit-eating fly that has appeared on an invasive honeysuckle shrub. Other teams report that they have re-created a type of Heliconius butterfly by crossbreeding two related species in the lab (SN: 6/17/06, p. 371: Available to subscribers at
Hybrid species with the same chromosome number as their parent species “may be more common than we thought possible,” Scriber says.