Meet the tiny tardigrades, the world's biggest DNA thieves

Tardigrades are animals that live in extreme conditions. Also known as water bears, these microscopic aquatic invertebrates can survive the cold and extreme heat, as well as the harsh conditions of space. If they are dehydrated, they can be revived only with water, even if they have spent decades like this. They are found on every continent, including Antarctica, and live in environments ranging from the ocean trenches to deserts to the top of the Himalayas.

Now a team of scientists has discovered that these beings have another unique condition: their genome contains the most “grafted” DNA among all known animal species. Researchers found that instead of inheriting all of their genes from their ancestors, tardigrades obtained one-sixth of their genetic makeup from plants, bacteria, fungi and archaea. This whole mix revealed that species can form in much less linear ways than we imagined.

"When people think of the diversity of life and the flow of genetic information, they imagine a tree with large branches generating smaller ones, but without any connection between them, " says Thomas Boothby. He is a postdoctoral fellow at the Foundation for Life Scientific Research at the University of North Carolina at Chapel Hill. "We are beginning to realize that instead of the tree of life, it would be more appropriate to think about the web of life, " he continues.

Boothby began studying the genome of the tardigrades in the hope of discovering the most basic foundations of creature extreme survival strategies. To catalog each gene, he and his colleagues first extracted and sequenced several short pieces of DNA from thousands of these beings. They then used a computer program to “sew” all the pieces together and reveal the code in its entirety.

"When we did that, we initially saw that there were a lot of genes that didn't seem to come from animals, " says the scientist. "Our instinctive reaction was to think that we made a mistake or that something should have contaminated our sample, " he said. To recheck the result, the team tested the polymerase chain reaction, a method that amplifies target regions of genetic material only if they match some specific "triggers".

In this case, they wanted to see if they could amplify animal and bacterial genes as individual units, which would only be possible if they were physically linked within the same genome. "We did it for over 100 genes, with 98% success, " said Boothby.

Convinced that the genome readings were correct, the team then reconstructed the evolutionary ancestry of specific gene sequences, which confirmed that what appeared to be foreign genes was just that, rather than something similar developed by the tardigrades themselves. "The results unequivocally revealed that genes that look foreign actually came from non-animal life forms, " ​​said the researcher.

The team concluded that 17.5% of water bear genes are made of some foreign material. Most of these external genes have bacterial origins, with thousands of species represented within the genetic composition of a tardigrade. Scientists believe many of these genes are responsible for playing the same roles in stress tolerance as their "original owners."

In some cases, foreign elements replaced those of tardigrades, while in others these animals retained their own structures, but integrated with them single or multiple copies of one or more species of bacteria. Researchers speculate that this was not a one-off event and may still be happening even today. But what has not yet been discovered is how these creatures can mend their own DNA with foreign genetic material.

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