Science

Mice on opposite North American coasts evolved the same way

House mice from cold environments (right) have evolved to become bigger than house mice from warm environments (left) in a few hundred years

Katya Mack

On opposite sides of North America, house mice have strikingly similar adaptations to cold climates, and have independently evolved changes to genes that drive their similar behaviours and hardiness.

House mice (Mus musculus domesticus) are native to Western Europe but have spread across the world thanks to their close association with humans. They arrived in the Americas more than 200 years ago and have since expanded into climates quite different from their original temperate home.

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To find out how these mice have adapted to new environmental conditions, Michael Nachman at the University of California, Berkeley, and his team collected 50 mice from locations in western North America ranging from Arizona in the US north to Alberta, Canada. The team analysed DNA from these wild mice and kept some to breed lab populations, comparing the western mice with those from a previous study on mice from eastern North America, ranging from Florida to New York.

The mice were most closely related to those that shared their side of the continent, suggesting that mice in western and eastern North America had moved north independently after an initial spreading along a southern route, says Nachman.

Yet the lab-reared mice from Alberta and New York were physically similar: both were bigger than southern mice and made larger nests, discouraging heat loss and providing insulation, respectively.

Nachman was surprised the nest style was so clearly genetically encoded after just a few hundred generations. “Even at room temperature in a comfy lab with plenty of water and constant temperature and plenty of food, the mice from Canada build a bigger nest than the mice from Arizona,” he says.

The western and eastern mice from cold climates shared changes in 16 genes, many involved with the regulation of body temperature.

“This suggests that there’s some predictability to evolution, that some of the same genes have changed in parallel to give rise to similar traits,” says Nachman.

But the mice also showed regional adaptations – such as fur colour and changes to hydration regulation genes – which may be due to differences in soil colour and rainfall between west and east.

It is quite rare to be able to identify possible environmental features that explain divergent genetic changes like those seen in these mice, says Kalina Davies at Queen Mary University of London.

Going forward, Nachman wants to see if the same physical and genetic patterns occur in mice spanning the length of South America.

Journal reference: PLOS Genetics, DOI: 10.1371/journal.pgen.1009495

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