Think of the humble zebrafish. They produce 200 embryos every 7 days, they are cheap and easy to breed, and their young are small and transparent. But their main feature – at least if you’re a researcher – is that they share a high degree of genetic, anatomical and physiological similarities with humans.
Since Hungarian researcher George Streisinger pioneered the use of this small but important fish in 1972, scientists have found ways to use zebrafish to study everything from epilepsy to environmental pollutants. Researchers at the Kavli Institute of Systems Neuroscience are even peering into the brains of genetically modified zebrafish to study the brain’s wiring system.
All those years of domestication — 150 generations of zebrafish, according to a one biologist count — led a team of researchers to realize they had a perfect evolutionary experiment. They wondered what had happened to lab zebrafish over all those generations, when it comes to a trait called plasticity.
“Plasticity allows organisms to adapt” different environmentsto perform consistently over a wide temperature range, for example,” said Rachael Morgan, who recently received her PhD from the Department of Biology at the Norwegian University of Science and Technology. “But plasticity — this ability to modify their physiology — would come at a cost, in which case domestic zebrafish, raised in extremely stable conditions, should lose this plasticity over time.”
So the researchers conducted an experiment with wild and lab zebrafish to see if this was the case. Their findings have been published in the Proceedings of the National Academy of Science†
Stable lab temperatures reduce the need to react
Researchers generally raise their lab zebrafish optimally temperature of about 28 C, which promotes healthy growth and best fertility. Over time, lab zebrafish have adapted to this, as well as living in small aquariums with many other fish, dry food and human handling, the researchers observed in their paper.
So Morgan and her colleagues decided to compare how lab zebrafish cope with different temperature regimes compared to wild zebrafish.
They took 300 juvenile laboratory zebrafish and 300 juvenile wild zebrafish and exposed them to 15 different temperatures for 35 days. The temperatures they selected (10 to 38 C) were based on the temperature range a wild zebrafish might expect.
After the 35-day acclimation period, both wild and laboratory fish were subjected to a series of tests, including swimming activity, maximum swimming speed, metabolism and growth rate.
The results showed that in a series of measures, the lab fish had indeed lost their physiological plasticity, Morgan said.
“What we were investigating is whether there is a cost to plasticity, and if there is a cost, then we would expect plasticity to be selected if it is not needed to preserve it,” she said. “And this is largely what we found. We also show that changes have occurred in many different traits and at different levels of the organism (from genetically to the whole organism), which is quite unique.”
Adapt to the environment they live in
Fredrik Jutfelt, the senior author of the paper, said the study also shows how two populations have adapted through evolution to the environment they are in.
“Lab zebrafish adapted to the narrow range of temperatures they experience in the lab, but have lost their ability to perform as well at temperatures higher or lower than they experience,” he said. “Wild fish experience a wide temperature range and are adapted for this because they can adapt their physiology using physiological plasticity to maintain their function.”
The study also recalls that organisms such as zebrafish, which have been adopted by researchers for a range of research topics and domesticated over the decades, aren’t exactly the same as their wild brethren, the researchers said.
“This study also illustrates how model organisms, such as labo zebrafishmay not be an accurate representation of their wild counterparts,” Jutfelt said. “It shows how quickly changes, in this case loss of thermal plasticity, can occur in an organism.”
Rachael Morgan et al, Decreased physiological plasticity in a fish adapted to stable temperatures, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2201919119
Norwegian University of Science and Technology
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