Color Differences Within And Between Species Have Common Genetic Origin

The body hair difference is even more pronounced between the two species than within our own species.

Do the same genes cause both types of variation? Biologists have puzzled over that question for some time, not just with respect to people, chimps and body hair, but for all sorts of traits that differ within and between species. Now, a study by University of Michigan researchers shows that, at least for body color in fruit flies, the two kinds of variation have a common genetic basis. The research, led by evolutionary biologist Patricia Wittkopp, appears in the Oct. 23 issue of the journal Science.

Wittkopp's group explored the genetic underpinnings of pigmentation differences within and between a pair of closely related fruit fly species: Drosophila americana, which is dark brown, and Drosophila novamexicana, which is light yellow.

"We started by asking which parts of the genome contribute to the pigmentation difference between species," said Wittkopp, an assistant professor of ecology and evolutionary biology. Genetic mapping narrowed the search to two regions that happened to contain genes already known to affect pigmentation. The researchers then focused on one particular gene, known as tan, and used fine-scale genetic mapping to determine that evolutionary changes in that specific gene, not another gene in the same region, have contributed to the pigmentation difference.

To confirm that finding, the team transferred copies of the tan gene from the yellow species into flies of a completely different species, Drosophila melanogaster, and then did the same thing with copies of the tan gene from the brown species. The only difference between the two groups of altered flies was the transferred gene, "and that difference was enough to result in pigmentation differences," Wittkopp said.

Confident that the tan gene was responsible for part of the color difference between species, Wittkopp and coworkers investigated color variation within the brown species. Some flies in that species are noticeably darker than others, and previous experiments have suggested that the basis for the difference is genetic, rather than environmental.

Again using genetic mapping, the researchers found evidence that the tan gene also contributes to color variation among individuals within the brown species. Going a step further, they showed that it is not just the same gene that contributes to color differences within and between species, but also the same genetic changes within this gene. Currently, the team is trying to pinpoint the exact genetic change (or changes) within the tan gene that is responsible for the color shift.

Although this study focused on fruit flies, the work could lead to better understanding of patterns of variation throughout nature, Wittkopp said. "We're using a model system, but when you get down to the basic mechanisms of inheritance---how information is passed from one generation to the next---the process is essentially the same in all living things. While we can't extrapolate about the specific genes involved, it is fair to say that mutations contributing to both variation within species and divergence between species may be a common source of evolutionary change."

Wittkopp's coauthors on the Science paper were undergraduate students Emma Stewart, Laura Shefner, Gabriel Smith-Winberry, Saleh Akhras and Elizabeth Thompson; graduate student Lisa Arnold; and technicians Adam Neidert and Belinda Haerum.

The researchers received funding from the National Science Foundation, the Margaret and Herman Sokol Endowment for Faculty and Graduate Student Research and the University of Michigan.