Hybrids — and a marker pen — offer insights into a butterfly family tree.
The various species of Heliconius butterfly found across Central and South America have long intrigued evolutionary biologists. Between them, the 60 or so species display a dazzling array of specific colour patterns — a diversity that is very hard to explain by genetic mutation alone. This led to the suggestion that some of the species arose as hybrids.
On page 868, Jesús Mavárez and his colleagues present evidence confirming that Heliconius heurippa is a hybrid of H. cydno and H. melpomene. They also show that H. heurippa uses wing markings as a guide to select other hybrids as mates and so propagate its species. All three butterflies have black wings, but H. melpomene sports red markings, H. cydno has yellow markings, and the hybrid H. heurippa features both red and yellow marks.
“We decided to attack the problem using three different strategies. That is what made our study so strong,” says Mavárez, who is a postdoc at the Smithsonian Tropical Research Institute in Panama. For the first part, the researchers sought out genetic markers that would allow them to distinguish between H. cydno and H. melpomene — no mean feat as the butterflies' genomes are almost identical. Further analysis revealed that H. heurippa shares some genetic markers with the other two species, as well as having some unique markers of its own. This strongly suggests that H. heurippa is indeed a hybrid.
The team next put this prediction to the test by exploiting the butterfly-handling expertise of their collaborators at the University of the Andes in Bogota, Colombia. By crossing H. melpomene and H. cydno, they recreated H. heurippa in the lab. “We thought it would be hard to do, but in three generations of crosses we were able to create a phenotype indistinguishable from wild-type H. heurippa,” says Mavárez.
But the team still needed to show that these hybrids could survive and propagate. If the hybrids chose either of the parental species as mates in preference to other hybrids, the new species would be doomed. So the researchers exposed H. heurippa males to females from all three species. Invariably, the males went for mates of their own kind. When the scientists modified the wing patterns of H. heurippa females — by ‘erasing’ either the yellow or red bands using a black marker — the males snubbed them.
“Somehow when these butterflies inherited both colour patterns, they also inherited the preference for both markings,” says Mavárez, who now plans to investigate the genetics behind this behaviour.
The trickiest part of the experiment, says Mavárez, was maintaining the stock of butterflies long enough to do the experiments. Two students made this part of the project possible with very few resources, says Mavárez. Gathering the butterflies was also a challenging and sometimes dangerous task. “We were lucky that H. heurippa and H. melpomene are found near Bogota, which is a relatively safe area of Colombia,” he says. But H. cydno comes from a more dangerous part of the country, so the team had to go to Venezuela to get specimens.
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Jesús Mavárez. Nature 441, xi (2006). https://doi.org/10.1038/7095xia