Caught in the act

Breaking up may actually not be hard to do, say scientists who’ve found a population of butterflies that may be on its way to a split into two distinct species. What’s the cause of this particular breakup? A shift in wing color and mate preference.

In a paper published in the journal Science, the researchers at Harvard University, the University of Texas, and the University of Hawaii describe the relationship between diverging color patterns in Heliconius butterflies and the long-term divergence of populations into new and distinct species.

“Our paper provides a unique glimpse into the earliest stage of ecological speciation, where natural selection to fit the environment causes the same trait in the same population to be pushed in two different directions,” said Marcus Kronforst, a co-author of the study and a Bauer Fellow in the Center for Systems Biology at Harvard University. “If this trait is also involved in reproduction, this process can have a side effect of causing the divergent subpopulations to no longer interbreed. This appears to be the process that is just beginning among Heliconius butterflies in Ecuador.”

Heliconius species display incredible color-pattern variation across Central and South America, with closely related species usually sporting different colors. For instance, in Costa Rica, the two most closely related species differ in color: One species is white and the other yellow. In addition, both display a marked preference for others of the same color.

The Ecuadorean population examined by Kronforst and his colleagues shows the same white and yellow variation found in Costa Rica, but this population has not yet reached a level of strong reproductive isolation. The entire population lives in close proximity, and individuals of both colors come in contact with — and mate with — each other.

But by studying the Ecuadorean population in captivity, scientists found that the two colors do not mate randomly. Despite the genetic similarity between the groups — white and yellow varieties differ only at the color-determining gene — yellow Ecuadorean individuals show a preference for those of the same color. White male butterflies, most of which are heterozygous at the gene that controls color, show no color preference.

“This subtle difference in mate preference between the color forms in Ecuador may be the first step in a process that could eventually result in two species, as we see in Costa Rica,” said Kronforst, who began studies of Heliconius color pattern and behavioral genetics as a doctoral student in the laboratory of Lawrence Gilbert, professor in the cection of integrative biology at the University of Texas, Austin.

Previous studies of species formation have focused on the characteristics of well-differentiated species, and the health and viability of their hybrids in particular, in an effort to identify how the species may have emerged and how they stay distinct. Heliconius provides a model for a different kind of study. The researchers considered species emergence from the opposite end, studying populations that have yet to diverge into separate species in order to identify the role of mate choice in the potential emergence of new species.

Having identified color-based mate preference in Heliconius, the researchers used a battery of genetic markers to compare the genomes of the white and yellow varieties, showing that they are genetically identical except for their different colors and preferences. The work suggests that the genes for color and preference are very close to one another in the genome; the two traits could even be caused by the same gene. The researchers’ next step is to identify the gene or genes responsible for the differences in color and mate preference.

“If we can identify this gene or genes, we can say conclusively how they influence both color and mate choice,” said Kronforst. “Subsequent work could elucidate exactly how changes in individual genes can, over long periods of time, lead to novel species.”

”This study shows the great potential of the genus Heliconius as a model system for integrating genetics, development, behavior, ecology, and evolution,” said co-author Gilbert.

Co-authors on the “Science” paper with Kronforst and Gilbert are Nicola L. Chamberlain and Ryan I. Hill, both of Harvard; and Durrell D. Kapan of the University of Hawaii. Their work was funded by the National Science Foundation and the National Institutes of Health.