MICHELE NORRIS, host:
And we're rounding the corner now to the world of plant genetics. It turns out that the tomatoes you buy at the supermarket are freaks of nature. And scientists at Cornell University can tell you why, with the little help from NPR's Joe Palca.
JOE PALCA: First of all, let's get one thing straight. A tomato is not a vegetable. It's a fruit. Steve Tanksley is a plant geneticist at Cornell University in Ithaca, New York.
Professor STEVE TANKSLEY (Plant Geneticist, Cornell University): And a tomato, it's a fruit because it holds a seed, just as a, say, a peach is a fruit because it also holds the seed.
PALCA: I see. And so, an eggplant is a fruit?
Mr. TANKSLEY: Mm-hmm. Exactly.
PALCA: But broccoli is not a fruit.
Prof. TANKSLEY: No, broccoli is the actually in mature flowers or fruiting structure before it's even made seeds. So, if you waited long enough and let a broccoli go to seed, you might consider to be a fruit.
PALCA: And zucchini?
Mr. TANKSLEY: Yeah, it's a fruit.
PALCA: All right. Okay. I'm feeling really good about this. Your doing very well.
(Soundbite of laughter)
PALCA: Am I the only one not to know that most of the things I thought were vegetables are really fruits? Anyway.
Tanksley is interested in the generic changes that occurred as a wild plant became a cultivated crop. Tanksley says, once upon a time, tomatoes plants only grew in the wild. And the wild tomato fruit was a little bit of a thing.
TANKSLEY: It would be about the size of the tip of your little finger and weigh only one to two grams. And if you see one of these plants in the wild, the fruit are so inconspicuous that you may not even see them.
PALCA: Even if they tasted good, getting enough to make a meal would be tough. So the first farmers were always in the lookout for larger varieties. Evidently, one day some early Mr. Green Jeans stumbled upon a plant that had undergone a genetic mutation.
Mr. TANKSLEY: This apparently was a new mutation occurred very recently in terms of evolution, and it made a big impact when humans saw it and selected it.
PALCA: The mutation occurs in a gene called faciated. Tanksley says the healthy gene's job is to control the size of the tomato. Here's how it works.
Mr. TANKSLEY: If you were to make a cross section of a tomato fruit, one of the big fruit you see in the supermarket, like a beefstake tomato, you'll see there's a number of compartments in there.
PALCA: These compartments have triangular shapes. There's a little dividing wall, and the soft flesh and seeds are in between. Wild tomatoes only have two to four compartments.
Mr. TANKSLEY: And these large tomatoes have anywhere from five up to 20 of these compartments.
PALCA: So, in these tomatoes, the faciated gene can't do an effective job of keeping a lid on the fruit's size.
Mr. TANKSLEY: When this gene is normally expressed, it's basically controlling and not letting the compartments go above a certain number. It's sort of a stop sign saying, stop here. You already have two or four compartments.
PALCA: So that, in the way, we should be grateful to the faciated gene for preventing the occurrence of giant killer tomatoes.
(Soundbite of laughter)
Mr. TANKSLEY: We should always be grateful to nature.
PALCA: Okay. And speaking of nature, Tanksley's research appears in the journal, Nature Genetics.
Size is just one attribute that can turn a wild plant into one that makes a crop fit for food.
John Doebley is a plant geneticist with the University of Wisconsin in Madison. There are plenty of wild grasses that are now food crops. For them, a key genetic change involved how the plant produces seeds.
Professor JOHN DOEBLEY (Geneticists, University of Wisconsin): The wild grasses, their seeds fall off the plant to the ground, where humans could never harvest them. But in the cultivated grasses - wheat barely, rye, corn, rice -the seeds have to be retained on the plant.
PALCA: Doebley says another important trait control by genes is when a plant form its fruits. In most wild plants, fruits mature throughout the growing seasons.
Prof. DOEBLEY: But if you're a farmer, you don't want to have to go back to the field over several months and harvest the fruits as they mature. You want to go through that field once and get all the mature fruits on one day. So synchrony of flowering has been another trait that farmers have selected for.
PALCA: Farmers are smart about things like that. Now geneticists are helping to explain just how smart.
Joe Palca, NPR News, Washington.
NORRIS: And you're listening to ALL THINGS CONSIDERED from NPR News.