A common plant pest may have something important to tell us about how we get new genes. Researchers in Arizona are reporting that if you're a pea aphid and you know if you are - you get new genes by stealing them. It's quite possible that larger animals do that too. Here's NPR's Joe Palca.

JOE PALCA: Forget for a moment that you detest aphids because they destroy the plants in your garden. Try to think of these tiny insects as cute.

Dr. NANCY MORAN (Yale University): They're sort of pear-shaped with sort of big hind ends and smaller heads and long spindly legs and antennae.

PALCA: That's Nancy Moran. She left the University of Arizona for her new home at Yale. She studies pea aphid genetics.

Dr. MORAN: And the interesting thing about them that's related to this work is that they come in two colors - red and green.

PALCA: Many animals, including pea aphids, get their coloring from molecules called carotenoids. You're probably familiar with some carotenoids.

Dr. MORAN: Beta-carotene...

PALCA: The orange in carrots.

Dr. MORAN: Alpha-carotene. Lutein is one that's a bright yellow one.

PALCA: Lycopene's another. It's the carotenoid that makes tomatoes red.

Dr. MORAN: Basically, these little red aphids are just like little tomatoes in a way. They're full of - not lycopene, but they have another red one.

PALCA: Now, there's something important to know here. Animals don't make their own carotenoids. And yes, aphids are animals - small ones, but animals.

Plants make carotenoids, carrots make carotenoids, fungi make carotenoids. Animals don't make carotenoids. They get them by eating plants and carrots and fungi.

But Moran thought, what the heck, I'll just look and see whether aphids are an exception to the rule.

Dr. MORAN: And the big thing that made a difference in the ability to find that out is the genome sequence of the aphid was completed recently.

PALCA: Surely you've heard about the�Aphid Genome Project. Anyway, Moran sat down at her computer and looked through the aphid genome for the genes that are responsible for carotenoids. And holy smokes, there they were, right in the pea aphid genome.

Dr. MORAN: It's totally unexpected. But once the genome sequence is there, it's also extremely easy to look. It's a five-minute job.

PALCA: So the next question was, where did the aphids get the genes? Because as I said before, animals don't make carotenoids. At least not until now. Moran says that took a bit longer to track down, but now, as she reports in the journal�Science,�she believes she's got the answer.

Dr. MORAN: Well, the evidence is pretty clear-cut for this case, that they actually picked them up, believe it or not, from fungi.

PALCA: Fungi include things like yeast and other microbes.

Dr. MORAN: The DNA from a fungus went into the aphids somehow and then stayed there and continued to function.

PALCA: She's not exactly sure how they did it, but the aphids essentially stole the microbe's genes. It's what scientists call lateral gene transfer.

Dr. JOHN WERREN (University of Rochester): The idea that animals picked up DNA from microbes - until recently that was thought to be nonexistent.

PALCA: John Werren is a geneticist at the University of Rochester.

Dr. WERREN: It's only been in the past couple of years that we've become convinced that these kind of exchanges do happen.

PALCA: And the pea aphid study shows that these purloined genes can do something useful, like helping aphids change color to avoid predators. Could humans also be stealing the occasional gene from bacteria or fungi? Julie Dunning Hotopp from the University of Maryland says it doesn't appear so.

Ms. JULIE DUNNING HOTOPP (University of Maryland): The Human Genome Project has shown that there is not lateral gene transfer, at least that's inherited.

PALCA: But Dunning Hotopp's not sure that's the final word. She believes it may just be possible that humans occasionally do pinch a gene or two from bacteria or fungi and she intends to keep looking.

Joe Palca, NPR News, Washington.

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