NPR

Of Fish And Flies: The Evolutionary Role Of Genes

During evolution, genes change, and new species emerge. But scientists are finding that it's not just genes that change. Other pieces of DNA that determine when and where genes turn on and off may be just as important.

A good example of this is work by David Kingsley, a biologist at Stanford University. He's been studying how a small fish called a stickleback has evolved.

In particular, he's been studying the past 10,000 years of stickleback evolution, because 10,000 years ago melting glaciers caused new rivers, streams and lakes to appear. He says that provided sticklebacks with new environments, new food sources and new predators.

Developing Spines, Or Not

Today there are many different varieties of stickleback, each well suited to its particular environment. "We'd like to know what it is at the DNA level, at the genetic level, in order to allow these organisms to colonize and then thrive in new environmental conditions."

Kingsley says one thing that's critical for thriving is having the right defenses.

For example, in a lake full of trout that like to dine on stickleback, the smaller fish tends to have a lot of spines. But in regions where insects are prevalent, having fewer spines is better because insects eat sticklebacks by grabbing onto those spines.

Hard-Working Genes

Kingsbury tracked down the gene responsible for building spines in sticklebacks and found it did a lot more things than just that.

"It normally turns on in the brain, and in the jaw, as well as in a spot along the side of the body where this hind fin or pelvic apparatus will develop," says Kingsley.

That's where the pelvic spines are located.

Disabling this gene everywhere would be fatal to the animal. Even a stickleback needs a brain. What Kingsley discovered is that this gene is virtually identical in every type of stickleback he has studied, no matter what its armament. What's changed is where the gene is switched on.

As he reports in the journal Science, it's what's called a regulatory element that has changed.

Color-Changing Flies

Geneticist Sean Carroll sees a similar phenomenon in fruit flies. In this week's Science, he describes how one of these regulatory elements has caused fruit flies living at high altitudes to be darker than flies in the lowlands.

"And that's occurred by a small number of mutations that have occurred fairly recently in fruit flies living in east Africa," says Carroll.

New DNA Dogma

Once upon a time, the dogma was that genes coded for proteins, and proteins made the tissues that made us. But recently, Carroll says there's been increased interest in the segments of DNA that don't actually code for proteins, but determine how those genes that do code for proteins are expressed.

"If we take a big picture of our genome," says Carroll, "only about 1 1/2 percent of the 3 billion letters of our DNA code for proteins. And we think that several more percent is involved in doing just this. Of controlling how those genes that encode proteins are being used."

In fact, these controlling bits of DNA have already helped explain some inherited conditions. Take lactose intolerance. People who can't digest milk as adults have the same protein coding gene as people who can. Kingsley says what's changed is a regulatory element that decides when to switch off that gene.

"You either only express that during the nursing period or you continue to express it into adulthood. And that simple change in how long is the milk-digestion enzyme expressed, is enough to make the difference between somebody who can enjoy ice cream, or get sick when they try to eat it," says Kingsley.

Kingsley expects these regulatory elements will play a role in a variety of human diseases, not to mention a variety of defensive changes for sticklebacks.

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

ROBERT SIEGEL, Host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MELISSA BLOCK, Host:

And I'm Melissa Block.

During evolution, genes change and new species emerge. But scientists are finding that it's not only genes that change. There are other pieces of DNA that determine when and where genes turn on and off, and they may be just as important.

NPR's Joe Palca explains.

JOE PALCA: Ten thousand years ago, things were warming up in some parts of the world; glaciers were melting, causing new rivers, streams and lakes to appear. For fish, this was a big deal - all sorts of new environments to adapt to.

David Kingsley studies how a small fish about three or four inches long, called the stickleback, adapted. Kingsley is a biologist at Stanford University.

DAVID KINGSLEY: We'd like to know what is it that's happened at the DNA level, the genetic level, in order to allow these organisms to colonize and then thrive in new environmental conditions.

PALCA: Turns out, the genetic changes aren't in genes that make things in the body. They're in bits of DNA that turn on and off the genes that makes things in the body. For example, consider how sticklebacks adapted to different predators. In a lake full of hungry trout, the successful stickleback will have lots of spines.

KINGSLEY: The sticklebacks would raise the spines. It's like trying to eat a pin cushion.

PALCA: But in other places, trout aren't the problem.

KINGSLEY: One of the other things that tries to eat sticklebacks are insects.

PALCA: Those must be big insects.

KINGSLEY: There's lots of insects in Canada that are bigger than sticklebacks.

PALCA: Yikes.

KINGSLEY: Insects eat sticklebacks by grabbing onto the dorsal and the pelvic spines.

PALCA: And then burrowing in from the side. So the successful stickleback in an environment with lots of insects will have fewer spines for insects to grab on to. Kingsley looked at the fish with fewer spines and found something interesting: They still had the gene that makes the protein that makes the spines.

KINGSLEY: What's changed is not whether the gene exists, but where and when the gene is expressed.

PALCA: In other words, where and when the gene is switched on. As Kingsley reports in the online version of the journal Science, it's not the gene that's changing as sticklebacks evolve, it's what's called a regulatory element - something that controls the gene that's changing as sticklebacks evolve.

Geneticist Sean Carroll at the University of Wisconsin sees a similar phenomenon in fruit flies. In this week's print version of Science, he describes how one of these regulatory elements has caused fruit flies living at high altitudes to be darker than flies in the lowlands.

SEAN CARROLL: And that's occurred by a small number of mutations that have accumulated fairly recently in fruit flies living in East Africa.

PALCA: Once upon a time, the dogma was genes coded for proteins and proteins made the tissues, and enzymes and hormones that make us. But recently, Carroll says there's been increased interest in these regulatory elements; segments of DNA that don't actually code for proteins, but control the expression of the genes that do.

CARROLL: If we take a big picture of our genome, our DNA, only about one- and-a-half percent of the three billion letters in our DNA code for proteins. And we think that several more percent is involved in doing just this - of controlling how those genes that encode proteins are being used.

PALCA: In fact, those controlling bits of DNA have already helped explain some inherited conditions. Take lactose intolerance. People who can't digest milk as adults have the same protein coding gene as people who can. David Kingsley says what's changed is a regulatory element that decides when to switch off that gene.

KINGSLEY: You either only express that during the nursing period or you continue to express it into adulthood. And that simple change in how long is the milk digestion enzyme expressed is enough to make the difference between somebody who can enjoy ice cream or get sick when they try to eat it.

PALCA: Kingsley expects these regulatory elements will play a role in a variety of human diseases, not to mention a variety of defensive changes for sticklebacks.

Joe Palca, NPR News, Washington. Transcript provided by NPR, Copyright NPR.

Most Popular