Fish change rapidly through cave life

Photo: Mexican tetra fish living in rivers (top) and caves (bottom)

Fish living in dark caves have rapidly adapted to their environment during evolution. New research shows that some of those changes occur within two years, without mutations to the DNA.

For a long time, scientists thought mutations in cave animals allowed them to survive in a dark environment. New research shows that some modifications occur in a single generation, without the need for mutations in the DNA. Researchers of, amongst others, the University of Maryland discovered that hormones and switching genes on and off in DNA play a role in this. They published their results in the scientific journal eLife.

Transition

The researchers studied the Mexican tetra: a freshwater fish. Approximately two hundred thousand years ago, some animals descended into caves. In the dark cave their sight and pigmentation became useless and eventually they lost both traits. In addition, their digestion slowed down in response to food shortages. Despite these, and more, major differences between the cave and river Mexican tetra, the DNA of the two fish species is almost identical.

The biologists wanted to know how appearance and metabolism in fish can change without mutations in the DNA. Therefore, they imitated the river-to-cave transition by growing river fish in the dark from an early age. They compared them to fish that grew up with a normal day-night rhythm. The fish that grew up in the dark developed a slower metabolism, like that of cave fish, within two years. The researchers also saw that the fish produced more stress hormones.

Dimmer

New environmental factors, such as darkness, influence the activity of genes (pieces of DNA that regulate one hereditary trait). Genes can not only be “on” or “off”, like as a switch, but the dosage is adjustable, like a dimmer. The study found that darkness modifies that dimmer for many different genes, causing fish to develop new traits.

Adjustments based on environmental factors help fish, as well as other animals and plants, survive in a rapidly changing environment. Afterwards evolution takes over: random mutations in the DNA may increase survival chances and are passed on to the next generation.

https://upload.wikimedia.org/wikipedia/commons/6/6b/Texas_blind_salamander.jpg
The Texas blind salamander has no eyes or pigment

Cave animals

Fish are not the only animals that have adapted to life in caves. Many other animals live in caves and are optimally adapted to cave conditions, such as the Texas blind salamander. Like the cave-dwelling form of the Mexican tetra, this animal lost its eyes and pigmentation. The authors of the publication suspect that colonizing a cave has a direct effect on hormone balance and on the “activity” of the genes. Not only in fish, but in all animals. That may explain how cave animals can adapt so quickly to a new environment.

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