Mixed butterflies: tropical species joins ranks of rare hybrids.

A South American butterfly has a checkered past, say biologists. It's one of the few animal species that seems to have arisen via a supposedly rare path: crossing two older species.

A black butterfly flashing bold stripes, Heliconius heurippa, came from the natural mixing of two other Heliconius species, says Jesus Mavarez of the Smithsonian Tropical Research Institute in Panama.

New experiments with H. heurippa suggest an answer to the difficult question of what keeps the hybrid species from blending back into its parent species. The hybrid's preference for mates of the same stripes keeps its species distinct, Mavarez and his colleagues say in the June 15 Nature.

"In animals, the dogma has been hybridization is a dead end--it's not important for creating species," comments Bruce McPheron of Pennsylvania State University in University Park, who's studied how some flies form species. The butterfly findings suggest that hybridization "can be a much more important source of new species than people have recognized," he says.

Botanists have long noted that many plant species arise from interbreeding, particularly when the hybrids end up with more chromosomes than the parent species. Altered chromosome number in hybrids has seldom been observed among animals.

Biologists are particularly interested in examples of new species in which the chromosome number remains constant. Mavarez and his colleagues focused on H. heurippa as a suspected hybrid butterfly species that has the same chromosome number as its two suspected parent species.

Searching for clues to family history, the researchers found distinctive genetic markers in Heliconius cydno and Heliconius melpomene. H. heurippa, the suspected hybrid, showed markers characteristic of each of the other two species.

More analysis of the genomes suggests that the hybrid split from its parent species at least 300,000 years ago, says Mavarez.

The researchers crossed H. cydno and H. melpomene in the lab and then backcrossed some of the offspring with H. cydno and bred the offspring. The procedure re-created the stripe pattern seen on H. heurippa.

To investigate what might keep the wild hybrids from disappearing back into parental populations, the researchers set up courtship tests at the lab of coauthor Mauricio Linares of the University of the Andes in Bogota, Colombia. Males from the wild H. heurippa species were at least twice as likely to court a member of their own species as to court females of either of the other two species.

When researchers blacked out either one of a female's wing stripes, the males' preference disappeared. Tests with variously colored paper wings yielded the same preferences. Thus, the result didn't depend on the insects' scents or behavior.

The experiments are "very thorough and elegant," says Loren Rieseberg of Indiana University in Bloomington, who has traced hybrid speciation in sunflowers.

Mavarez says that the team hopes to test the mating preferences of the lab-bred hybrids.