Mexican cavefish can teach us about the future of our couch potatoes

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With people sheltering in place for weeks at the start of the coronavirus pandemic, many people already had a sedentary lifestyle to attend endless hours attending Zoom meetings, streaming movies and waiting for food delivery.

Yet in the realm of couch potatoes, we are amateurs.

A fish species washed up by floods in black Mexican caves has essentially been sheltering in place for the past 160,000 years, undergoing genetic changes that reset its metabolism, according to a new study in the National Academy of Sciences. For thousands of generations, Mexican cavefish have adapted to life in deep cave-like caves with little light or food; It has learned to swim slowly, starve for long periods of time and get by with less muscle and more fat.

But don’t expect their long-term success in the slow lane to influence your doctor to prescribe a diet of several hours with generous window times in front of the computer and television. A lifetime of inactivity puts people at higher risk of heart disease, obesity, high blood pressure, high cholesterol, stroke, type 2 diabetes and some cancers.

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“I found it interesting that in humans, it’s long been understood that a sedentary lifestyle has many negative health consequences, but that comes over the lifespan,” said Joshua Gross, associate professor of biology at the University of Cincinnati. were not involved in the new research.

“This study provides an idea of ​​how inactivity can play out, not just over the lifespan [in] Long-term evolutionary changes.”

Because inactivity is so bad for human health, scientists consider it unethical to conduct experiments comparing groups of active and inactive people.

Thanks to Cavefish, however, they don’t have to. When a flood carried away some Mexican tetra river fish, Mexican Astyanaxs, in about 30 different caves, other fish of the same species remained on the surface, providing a natural study on contrasting evolutionary paths.

As surprising as it sounds, cavefish are a good model from which to examine the long-term evolutionary changes that may be in store for humans if we can reduce our inactivity over hundreds or thousands of generations.

Both humans and cavefish are vertebrates — animals with backbones — and share about 80 percent of the same genes, said study author Nicholas Rohner, an associate professor at the Stowers Institute for Medical Research, a nonprofit biomedical research organization in Kansas City. Md.

Over the years, scientists have studied blindness and lack of pigmentation in cave fish to better understand these conditions in humans. They found that one of the four genes in humans that can mutate and cause albinism is also important for albinism in cavefish. Furthermore, obesity in both species can be traced to mutations in genes they share.

Some of the same conditions also account for laziness in both species.

“Cavefish move less because they have no predators,” Rohner said, explaining that modern humans enjoy the same luxury. In ponds where cave fish live, there are no currents to push against, meaning that when they swim, they encounter little resistance. As for humans, many have developed car-dependent lifestyles with little need for walking, which means we collectively spend less time putting our feet up or pushing our bodies against strong winds.

In his lab, Rohner reared about five generations of both cave fish and surface fish. He and his colleagues tested wild and lab-reared versions of the fish, comparing everything including swimming speed, body composition, organs, tissues and protein levels.

Scientists found that cave fish swim 3.7 times slower than fish that live outside caves. Because cavefish did not need to use the type of burst-swimming needed to avoid predators, they slowly evolved into continuous swimmers.

The team also discovered the genetic basis of some traits. In cavefish, clusters of genes contributing to the destruction or loss of muscle tissue were formed, meaning they had less muscle than those living outside the cave. However, genes regulating swimming speed and muscle contractility were downregulated.

“This is one of the most intensive studies I’ve ever seen,” said University of Maryland Distinguished Professor of Biology William Jeffery, who has studied cavefish for more than 20 years. “The work done in the lab was then taken out into the field and confirmed—I don’t think I’ve seen anything done so well in any study.”

Jeffery added that the study is the first to show that cavefish experience a trade-off during their development: losing muscle but accumulating fat.

Scientists discovered an interesting difference between the size of muscle fibers in wild and laboratory-raised fish. Generally smaller muscle fibers are associated with less vigorous swimming and muscle atrophy.

“We were surprised to find that lab cave fish have larger muscle fibers than lab surface fish,” paper co-author Luke Olsen, a graduate student in Rohner’s lab, wrote in an email. However, scientists found the exact opposite when examining the wild Fish: Cave fish had smaller muscle fibers than surface fish.

Olsen said they believe that’s because cavefish get more food in the lab than in the wild.

The darkness of the caves does not allow photosynthesis, the process plants use to convert sunlight into energy and produce oxygen. The lack of plant life poses a challenge for cave fish. Some populations share caves with bats and obtain nutrition from bat guano that enters the water. Other cave fish feed on cave crickets and consume microscopic crustaceans carried in by roof drips. In general, however, they face a scarcer food supply than surface-dwelling fish.

In the laboratory, cave fish convert a more generous food supply into sugar and fat, which are stored in muscle fibers, causing them to grow larger.

“It appears that cavefish have rewired the traditional role of muscle fibers in muscle contraction,” Olsen said. Cave fish use their muscle fibers as “storage sites for fuel”. It is this stored extra fat that helps cave fish get through a month or longer of starvation.

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