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A Darwin Day delve into how the chipmunk got its stripes

Chipmunk.2This Sunday is Darwin Day, the 208th anniversary of Charles Darwin’s birthday. As the great biologist Ernst Mayr wrote, “…no biologist has been responsible for more — and for more drastic — modifications of the average person’s worldview than Charles Darwin.”

To honor Darwin and his theory of evolution by common descent, I reached out to Gregory Barsh, MD, PhD, a Stanford professor of genetics, emeritus, who is also affiliated with the HudsonAlpha Institute of Biotechnology. Barsh has a long-standing interest in the genetics of color patterns in mammals. Last year, he published a paper on dun-colored horses. This year, he looked into chipmunks and striped mice.

“Many people would like to know why zebras have stripes,” Barsh said. “But you can’t really bring zebras into the lab.” So he and a group of other researchers — including lead author Ricardo Mallarino, PhD, at Harvard — recently published a paper in Nature on how stripes develop in the Southern African striped mouse.

What the researchers found was that a gene that normally causes pale belly fur has been co-opted to help create a pattern of light and dark stripes on the backs of both striped mice and on the distantly-related chipmunks. The mice generally have tan fur. The color of each hair is controlled by melanocytes, the cells that make pigment.

Now, researchers have found that one set of gene regulatory mechanisms creates a black stripe on each side of the mouse's back. Then, during the embryonic development of the mouse, a second kind of gene regulator makes a white stripe down the middle of each black stripe, creating a striking black-white-black stripe. And chipmunks, it turns out, make stripes the same way. The result, shown on the chipmunk above, is a black stripe (whose color is controlled by one set of genes) bisected by a pale stripe whose lack of color results from another gene.

Why do chipmunks, and some mice, have these stripes? Biologists tend to assume they are adaptive and possibly help camouflage the small animals. But it was not until very recently that we even could guess why zebras have stripes, so the function of chipmunks' stripes may have to wait for additional research.

Amazingly, the striped mice and chipmunks are separated by tens of million years of evolution. But Barsh said that whether each group evolved its method of making stripes separately or just re-used the same set of molecular pathways they had carried forward generation after generation is still unknown. Another interesting tidbit: in humans, a gene analog of one of the genes involved in making chipmunk and mouse stripes is associated with diseases of the bones and other medical conditions.

Previously: Happy Birthday, Charles Darwin: Stanford researchers reflect on evolutionNew technique offers glimpse at human evolution in actionAn evolutionary look at cancerEvidence for “mother’s curse” — a quirk of evolution — found in flies
Photo by Gilles Gonthier

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