Scientists: Washington's State Fish Has A Remarkable Evolutionary Past

Apr 23, 2014

It turns out the Washington state fish is a piece of evolutionary wonder. An international group of scientists sequenced the genome of the rainbow trout and found some surprises. 

About 100 million years ago, something odd happened to the ancestor of salmon and rainbow trout. Instead of inheriting two copies of chromosome sets — one from mom and one from dad, they managed to inherit four copies. In evolutionary terms, this was a recent and dramatic event.

That's A Lot Of Chromosomes!

Flowering plants get doubled genomes all the time. It's rarer for vertebrates to pull that off. In humans, having more than two sets of chromosomes results in severe birth defects and death. In many organisms, having an odd number of chromosome sets is a direct route to infertility. But "whole-genome duplication," as it's called, is not impossible.

All Onchorhynchus mykiss are born in freshwater streams. Some go west and take to the seas, where they get big and silvery. Those are called steelhead. Others stay in freshwater lakes and streams, developing a red stripe across the body. Those are called rainbow or redband trout.
All Onchorhynchus mykiss are born in freshwater streams. Some go west and take to the seas, where they get big and silvery. Those are called steelhead. Others stay in freshwater lakes and streams, developing a red stripe across the body. Those are called rainbow or redband trout.
Credit Don Harrison / Flickr

"It's thought to have happened in an ancestor of all vertebrate animals hundreds of millions of years ago," said Gary Thorgaard, a biology professor at Washington State University who raised the line of fish used in the study. 

But that was so deep in evolutionary history that it's really hard to figure out how all those extra genes get sorted and tossed off over the course of millennia. 

"That's why the trout are especially interesting," said Thorgaard. "We can look at a more recent example of that process in an animal, and just follow what's happened after that."

What Does A Fish Do With All Those Extra Genes?

So what happens to all those extra genes? 

"Imagine that the genome is like a recipe book," said Thorgaard. "It's providing instructions for preparing something."

A recipe produces a dish. A genome produces a growing, living body.

"What if your mother doubled her recipe book, kept one of the copies of the recipe book intact, and then started to gradually tinker with the other copy?" said Thorgaard.  "Most of the time, those copies would end up with something really bad; the recipes might not work. But every now and then, you might substitute something and having something good come out of it."

What results might be a better eyeball, for example, or, to look into our own very distant history, the ability to carry oxygen in the blood

As opposed to plodding along with the usual measly pair of chromosomes, hitting a jackpot of new genetic material can give adaptation a fresh jumpstart.

A Surprising Development

Usually, after whole-genome duplication, a lot of the extra gene copies get lost and reorganized as generations pass. But some hang on. In the rainbow trout, the researchers found, some of the clingers are genes that control specific functions, like eye development and visual perception. 

What's more, the process seemed to happen really slowly. Today's rainbow trout still have half of those extra genes that got passed on millions of years ago, which means that the surprisingly slow process of paring down, or gene fractionation, is still going on.

Scientists are interested to find out how and why. In the meantime, anglers kicking off trout-fishing season can rest assured it won’t change their ability to reel in a good catch.