Two Arctic Fish Are Breaking the Rules of Genetics

 

To survive in the frigid ocean waters around the Arctic and Antarctica, marine life evolved many defenses against the lethal cold. One common adaptation is the ability to make antifreezing proteins (AFPs) that prevent ice crystals from growing in blood, tissues, and cells. It’s a solution that has evolved repeatedly and independently, not just in fish but in plants, fungi, and bacteria.

It isn’t surprising, then, that herrings and smelts, two groups of fish that commonly roam the northernmost reaches of the Atlantic and Pacific Oceans, both make AFPs. But it is very surprising, even weird, that both fish do so with the same AFP gene—particularly because their ancestors diverged more than 250 million years ago and the gene is absent from all the other fish species related to them.

A March 2021 paper in Trends in Genetics holds the unorthodox explanation: The gene became part of the smelt genome through a direct horizontal transfer from a herring. It wasn’t through hybridization, because herring and smelt can’t crossbreed, as many failed attempts have shown. The herring gene made its way into the smelt genome outside the normal sexual channels.

Laurie Graham, a molecular biologist at Queen’s University, in Ontario, and the lead author on the paper, knows she’s making a bold claim in arguing for the direct transfer of a gene from one fish to another. That kind of horizontal DNA movement once wasn’t imagined to happen in any animals, let alone vertebrates. Still, the more she and her colleagues study the smelt, the clearer the evidence becomes.

Nor are the smelt unique. Recent studies of a range of animals—other fish, reptiles, birds, and mammals—point to a similar conclusion: The lateral inheritance of DNA, once thought to be exclusive to microbes, occurs on branches throughout the tree of life.

Sarah Schaack, an evolutionary genomicist at Reed College, in Portland, Oregon, believes these cases of horizontal transfer still have “a pretty big wow factor” even among scientists, “because the conventional wisdom for so long was that it was less likely or impossible in eukaryotes.” But the smelt discovery and other recent examples all point to horizontal transfers playing an influential role in evolution.