Scientists say they’ve found the genetic origin of stripes in chipmunks and other mice. They published their findings today in the journal Nature.
Cute though they may be, rodents’ stripes are hardly ornamental. Like a jaguar’s rosettes or a peppered moth’s sooty wings, stripes evolved to allow their bearers to vanish into their surroundings. On a large scale, we understand how these patterns came about: animals with camouflage markings survived and bred, while those without died out. On a smaller scale, we’ve still got a lot to learn.
To zoom in on the specific genetics of mammal stripes, an international team of scientists decided to take a very close look at the four-striped grass mouse (Rhabdomys pumilio), a resilient little rodent that spends its days munching seeds in southern Africa.
J. F. Broekhuis
The scientists first examined the individual hairs that form each mouse’s stripes. They found three distinct types: light hairs, with black bases and unpigmented hair shafts; black hairs, which were dark from base to tip; and banded hairs, with black bases and yellow shafts. All three hair types were found in both dark and light stripes, albeit in different proportions: dark stripes simply had a lot more black hairs, while light stripes were mostly light hairs.
Next, they bred baby grass mice in the lab, tracking the appearance of their skin and fur as they grew from embryos to pups. They found that just 19 days after fertilization, the length of the rodents’ fur began to vary over the areas that would one day be striped. Three days later, the embryos’ skin started to lighten in the same places that light-striped fur would later appear. At birth, the mouse pups’ coats showed variation in both hair length and skin color. Two days after that, their characteristic stripes were clearly visible.
To understand what was causing these shifts, the researchers scanned the rodents’ genomes at all four points in development. They found that, as early as day 19 of embryonic development, a gene called ALX3 was showing up on the embryos’ backs in the same sites where the light stripes would one day appear.
The researchers learned that ALX3 was kind of a bully to a pigment cell–producing protein called microphthalmia-associated transcription factor (MITF). Wherever ALX3 appeared, pigment production was repressed, leading to very pale cells, which in turn led to light stripes.
Furthermore, the team found that the same mechanism—ALX3 smothering MITF activity—appears in similarly striped Eastern chipmunks (Tamias striatus). While mice and chipmunks are both members of the rodent family, their last common ancestor lived around 70 million years ago. The fact that two such distinct species share a similar stripe backstory suggests to the researchers that this useful genetic trick may have evolved a number of times across the mammal family tree—a phenomenon known as convergent evolution.