The European house mouse has invaded almost every corner of the Americas since its introduction by colonizers a few hundred years ago, and now lives virtually anywhere humans store their food.
Yet during this relatively short period – 400 to 600 generations of mice – the populations of the east and west coasts changed their body size and nest-building behavior almost identically to adapt to similar environmental conditions, according to a new study by biologists. at the University of California, Berkeley.
To make these adaptations – at least in the case of body size – mice in the western United States evolved many of the same genetic changes as their eastern cousins, showing that evolution often works. on the same genes in different populations when these populations face similar environmental conditions.
The study represents one of the first times that scientists have traced the genetic changes underlying a complex adaptive trait in mammals, although similar studies have been conducted with laboratory insects, such as fruit flies. , and in fish.
“The big takeaway message from this article is that there is some predictability of evolution, both at the organism level and at the genetic level,” said study leader Michael Nachman, professor. of Integrative Biology at UC Berkeley and Director of the Campus Museum of Vertebrate Zoology. “We have shown that the same genes were recruited independently from two different domains, throughout the very short evolutionary period. This is a good example of rapid evolutionary change over short periods of time for a complex adaptive trait.”
The results also have implications for mammals other than rodent pests.
“The mice we are studying are also a reasonable model to think about human evolution, since humans have been in the Americas for as many generations as house mice,” Nachman said. “We found genes involved in mouse body size that had previously been involved in metabolism and things like obesity in humans, so there are parallels between humans and mice as well.”
Nachman and his colleagues were able to prove that these traits had a genetic cause – they were the result of nature, not education – by breeding mice from different laboratory environments under the same environmental conditions. House mice in New York and Alberta, Canada have always produced larger nests than mice in Florida and Arizona, even when nesting at the same temperature, while mice in the north have always been larger than southern mice, although they were raised under similar conditions.
“We and many others have been studying natural populations for a long time and you can document the differences between natural populations,” he said. “But in this case, we took it a step further and took these animals into the lab and raised them for several generations so that we could ask ourselves if the differences that we see in nature are due to the environment or to the genetic. from wild animals and animals in the laboratory, we can discover the genetic component of the traits we are studying. “
The study appeared today in the journal PLOS Genetics.
Canadian mice vs Arizona mice
Nachman and his team have been studying house mice in North and South America for several years to determine how they have adapted to different environments – physiologically and genetically – since arriving as stowaways on ships from Europe. . His goal was to link changes in physiology and instinctive behavior to changes in specific genes, in order to understand how multiple genes interact to create changes in complicated adaptations.
To date, most studies linking genetic change – genotype – to physiological change – phenotype – have involved relatively simple characteristics, such as changes in fur color with the environment, or the development of resistance to them. insecticides.
House mice – Mus musculus domesticus, the ancestor of all laboratory mice in use today – seemed to be a good subject since they exist in a wide variety of habitats across the Americas, including in the Andes at altitudes above 4,000 meters (13,000 feet). Over the years, Nachman and his colleagues have collected hundreds of individual mice from Tierra del Fuego to the tip of South America to southern Canada and upstate New York, and different altitudes in Ecuador and Bolivia, and have sequenced all the genes in their genomes.
Nachman said he couldn’t help but notice that in the wild, mice in the north – from Edmonton, Alta. And Saratoga Springs, New York – tend to be larger than mice from the south. Tucson, Arizona, and Gainesville, Florida. this was not always clear given that the mice differed in age, diet and health. Greater body size is a well-known adaptation to colder weather, called Bergmann’s Rule after the 19th century German biologist Carl Bergmann.
“Mice at 45 degrees north latitude are about 50% larger than equator mice,” Nachman said. “A mouse from the equator weighs about 12 grams; a mouse from upstate New York weighs about 18 grams. That’s a big difference. If we were talking about humans, it would average a person of 100 pounds against 150 pounds. “
Northern mice also build larger nests – about twice the size of southern mice.
To determine the genetic relationships between these mouse populations, his team collected 10 mice from 5 different sites in the West, roughly equally spaced from Tucson to Edmonton, sequenced their genes, and compared them to the genes of mice that his team had collected earlier along the East. Coast from Gainesville to Saratoga Springs. He found that although the northeast mice and the northwest mice developed larger bodies and built larger nests, they were not closely related. New York mice were more closely related to other eastern mice than to Alberta mice, while Alberta mice were more closely related to other western mice than to New York mice. York.
If the eastern and western mice are two separate populations that have independently adapted to cold weather using the same strategy – larger size and larger nests – have their genes changed as well?
To find out, he and his colleagues established lab colonies with 41 individual mice from the extreme areas sampled: Edmonton (EDM) in the north and Tucson (TUC) in the south. By breeding them under identical indoor conditions, the Nordic mice over five generations retained their larger body size and greater nest-building behavior, showing that behavioral and physiological changes were actually encoded in their genetics. Three years ago, Nachman conducted similar experiments with mice from Saratoga Springs (SAR) and Gainesville (GAI), and found similar results from these eastern populations.
“By introducing them to the common laboratory environment and keeping them at a comfortable room temperature for a few generations, we can conclude that all the differences we see have a genetic basis,” he said.
They then conducted a genome-wide association study to identify which variant or allele of each gene in the genome changed in frequency with the change in body mass. They are still looking for the genetic causes of nest building.
Finding the genetic basis of Bergmann’s rule
Researchers have identified eight mutations in five genes associated with increased body size in Edmonton mice.
“We found that of all these genes that we identified for body size, four of the five genes show strong natural selection signatures in both the East and the West,” he said. “This suggests that we have found genes that contribute to this pattern, known as Bergmann’s rule, which is actually one of the most prevalent geographic patterns in the evolution of warm-blooded animals. We are starting to find the genetic basis for it. study.”
Combining this data with the results of Nachman’s previous study on house mice in New York and Florida, the researchers found a total of 16 genes that showed parallel evolution along the north-south gradient on both coasts. , many of which are involved in regulating body temperature. . For example, the Trpm2 gene, which forces mice to avoid very high temperatures, has shown genetic changes in southern populations.
While eastern and western mice showed evidence of independent evolution in many of the same genes, each region also harbored genetic adaptations not seen in the other region. In the West, for example, the color of the fur varied with the color of the soil: Nordic mice had a darker shade, which corresponded to an attempt to blend in with moister, darker soil. Oriental mice did not show such a variation.
Nachman continues to study variation in house mice and eventually hopes to use CRISPR genome editing to modify genes in his mouse populations to confirm their involvement in traits such as body size.
“We would like to change some of these genes and see if we can take a mouse from Canada and turn it into a mouse that looks like a Tucson mouse, or vice versa,” he said.
The work was funded by the National Institutes of Health (RO1 GM074245, R01 GM127468). The NIH and the Jackson Laboratory are also supporting Nachman’s development of new strains of house mice – SAR, GAI, EDM, TUC and a fifth strain, MAN, from Manaus, Brazil – that researchers can use in place of the mouse. inbred lab – the most popular strain is called C57BL / 6 – if they want more genetic variation in their mouse studies.