Backward-evolving Lake Washington fish lends clues about genetics

It's got a Dr. Seuss name and a prehistoric appearance, but the threespine stickleback of Lake Washington is opening new frontiers of scientific understanding about how animals adapt to rapid environmental change.

Faced with sudden change in their world — the cleaning up of the once-fetid lake — the homely little fish has faced the fundamental challenge of adapting or dying. And it turns out the three-spine stickleback smokes most any competition.

While evolution is usually expected to creep along over eons, the stickleback has managed to evolve in full-speed reverse to cope with the cleanup of Lake Washington, according to a study led by researchers at Seattle's Fred Hutchinson Cancer Research Center.

The findings, to be published in the May 20 issue of the journal Current Biology, document a rare example of an animal reverting to an earlier evolutionary version to survive a rapid change in its environment, according to the senior author of the paper, Catherine Peichel, a Hutchinson Center researcher. "It was very surprising," said Peichel.

Pollution as safety net

Just five decades ago, Lake Washington was notoriously polluted, full of murky water festering with blue-green algae that thrived on the millions of gallons of raw sewage the cities pumped into it.

Pollution cut visibility in the water to only about 30 inches. And that was great for the stickleback, a rugged looking customer with three sharp spines along its back, because it easily could hide in the murk from its primary predator, cutthroat trout.

Enter a $140 million cleanup of the lake, launched in the 1960s, at the time one of the biggest pollution-control efforts in the nation. Today, Lake Washington is swimmable again, and visibility reaches 25 feet.

Lovely for people — treacherous for the stickleback.

But to their surprise, researchers have discovered that in the space of four decades, the stickleback evolved backward, to an earlier version of the species that had full-body plating.

Marine ancestry

Those ancestors were marine stickleback, a spiny, armor-plated species that invaded freshwater around Puget Sound as the glaciers retreated some 10,000 years ago.

Over time they evolved to shed the bony plating covering their bodies from snout to tail. By the 1960s, only about 6 percent of sticklebacks in Lake Washington were fully plated.

Fast forward to today, though, and about 49 percent of the stickleback sampled for the study were fully plated, the researchers found. And 35 percent were partially armored.

Researchers surmise that is because marine versions of the fish, with armored plates, invaded the lake anew when the fish ladder was put in, mixing their genes with the unplated fish.

So, when the water was cleaned up, the fish were able to tap those genes to zoom backward to their earlier, plated version, and armor themselves against the cut throat trout that suddenly could see — and eat them.

How did the fish adapt so quickly and easily? The researchers say it's because of their rich genetic variation.

That enabled them to cope with a range of environments and the changing conditions of the lake. In fact, they said, it looks like much of the change has happened in less than a decade.

Now it's hoped that the findings will help scientists learn more about the workings of human genetics and adaptation, Peichel said.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com