Parallel adaptation: The house mice of the east and west coasts of North America independently developed similar traits, research has shown. Accordingly, both of them changed their body size and their nest building behavior in order to adapt to the cold. The special thing about this is that the same mutations developed independently of one another. With regard to other environmental conditions, however, unique genome changes in the mice developed.
The house mouse (Mus musculus) spread as a roommate of humans around 15,000 years ago in the Levant. Around 6,500 years ago a subspecies made the leap to Eastern Europe and around 2,000 years later another subspecies of the house mouse (Mus musculus domesticus) was also found in Western Europe. This came across the Atlantic with the conquest of America by the Europeans. When they first settled in North America, the mouse populations encountered significantly colder temperatures in some regions than in their European homeland.
How did they adapt?
How did the house mouse populations adapt to the cold? Scientists working with Kathleen Ferris from the University of California at Berkeley have now investigated the question. They examined and compared the physical adaptations and genetic changes of the mice in the northern areas of the west and east coast of North America with those of populations from more southern climes.
To do this, the researchers first caught wild house mice from five populations along the west coast of North America from Arizona to Alberta and sequenced the genomes of 50 mice. They continued to breed around 40 of the captured mice in the laboratory to observe their physical development. The team compared the results with data from 50 mice from the east coast, which were also exposed to colder environmental conditions.
Mice bigger in the north
First it was shown that the house mice in the two northern coastal regions of America tended to grow larger and stronger than the control animals from the south. “Mice from the 45th parallel north are about 50 percent larger than mice from the equator,” says Ferris colleague Michael Nachman. “A mouse from the equator weighs about twelve grams, a mouse from Upstate New York about 18 grams. That’s a big difference.” The researchers also found that the “northern mice” also built nests twice as large as their southern relatives .
According to Ferris and her colleagues, these differences in body size and nest construction are adaptations to the colder climate. Because according to the climate rule of the biologist Carl Bergmann, individuals in populations from cold areas are larger than their fellows in warm areas. The reason: A large body loses less heat than a small one through its relatively small surface area in relation to its volume.
Genetic parallels, but also differences
Interesting, however: The genetic analyzes showed that the mice from the northeast and the northwest, although both larger and built larger nests, were not closely related to one another. Instead, for example, the house mice from New York were more closely related to other eastern mice than to the northern Alberta mice. The latter, however, showed a closer relationship with other mice from the west coast than with New York mice.
In the next step, Ferris and her team investigated the genetic changes underlying these adaptations. They identified eight mutations in five genes that were linked to the increased body size of the mice from Alberta. In the house mice on the east coast of New York, they found 16 genes that were altered compared to mice from Florida. Many of these genes were involved in temperature regulation and independently changed in the same way in both northern populations.
In addition to the gene changes that occurred in parallel on the east and west coast, the research team also identified specific genetic adaptations in each of the two northern populations. The scientists attribute this to different regional conditions in the west and east, such as precipitation or soil color. For example, the western “northern mice” had a darker coat color than the eastern ones as an adaptation to the moister and darker soils.
Predictable evolution
Thus, not only the parallel physical and genetic evolution of the two house mouse populations to adapt to the temperature could be demonstrated. But also that the adaptations in the east and west still led to unique genetic changes. “Overall, our results show that a mixture of parallel and unique changes at both the phenotypic and genetic level can be expected when closely related populations adapt to parallel latitude gradients,” the team concluded.
“The most important result of this work is that there is a certain predictability of evolution, both on the physical and on the genetic level,” adds Nachman. In the future, Ferris and her colleagues plan to genetically modify the genes in mouse populations to confirm their involvement in traits such as body size. (PLOS Genetics, 2021, doi: 10.1371/journal.pgen.1009495)
Quelle: University of California – Berkeley
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