What Made Chili Peppers So Spicy? New research indicates chili pepper plants may have developed their signature heat as a way to fight off fungal infections caused by insects.

What Made Chili Peppers So Spicy?

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This is Talk of the Nation Science Friday. I'm Ira Flatow. You bite into that cheesy quesadilla slathered in hot salsa and then what happens? Pow! Jalapeno hits your tongue. Pain jolts the nerve cells in your mouth to life. Endorphins kick in and you start to sweat but you love that hot pepper rush, don't you? Well, but what if hot peppers evolved just to make our meals more exciting? I don't think so. I don't think that's how they evolved. They were around here a lot more, a lot longer than we have. Probably not. It turns out that hot Jalapenos or Habaneros or any hot pepper is a defense against disease and it's brought on by bugs that bite the peppers. Interesting story.

Joining me now to talk about it is my guest Joshua Tewksbury, an assistant professor of biology at the University of Washington in Seattle. Thanks for being with us today, Dr. Tewksbury.

Dr. JOSHUA TEWKSBURY (University of Washington): My pleasure.

FLATOW: How does this work now?

Dr. TEWKSBURY: Well, it turns out that chilies are like a lot of other fruits. They've got a problem on their hands. They use pretty basic ways to attract consumers so the fruit on a plant is just there to advertise so that things can eat it and plants like things to eat their fruits because they disperse the seeds. So that's common to all fruits and then the problem is that when plants do that, they attract a lot more than the consumers they want. They attract a lot of things that don't help them at all, things that kill the seeds and these are microbes and fungus and a whole bunch of other things including mammals that might chew seeds.

And so, fruits have to figure out how to attract the good consumers without attracting all the consumers they don't want. And chilies have found out - found a really good way to do this and that's through the heat of the pepper itself. And primarily, what they're doing with that heat is that they are deterring fungus. They're deterring a fungus that gets into the chili and it gets in there because bugs poke holes in the skin and that allows the fungus in and the fungus attacks the chili seeds. And the capsaicin, the things that make your mouth burn, that stops the fungus. In a nutshell, that seems like the most plausible reason we can find why chilies are hot. It's because they're strongly antimicrobial. They stop fungal growth.

FLATOW: Could we use that as an antibiotic then or something or anti-fungal?

Dr. TEWKSBURY: Anti-fungal agent. You probably could. We have a lot of pretty good anti-fungal agents out there and actually, chilies are antimicrobial generally. It's even hypothesized that one of the reasons that chilies were domesticated so early on in the new world was specifically for their antimicrobial properties. Eating food before refrigeration was a dangerous business and anything you could do to preserve that food before it was eaten or it went in your stomach would be a good thing because it kills microbes. Well, chilies kill microbes and so do a lot of the spices we put on foods. Particularly the ones you find around the equator where the humidity and the heat make a microbial attack a real serious problem for human health.

FLATOW: That's why we had those old world ships looking for the new world in the spice trades, I guess.

Dr. TEWKSBURY: Well, in some ways, they were. Probably didn't know that's what they were doing but I think that our predilection or our love for spices is no accident.

FLATOW: Yeah. So, it's not against the bugs. They're not trying to stop the bugs from eating the plants because they want the bugs to eat it. It's to keep the fungus from destroying the plant.

Dr. TEWKSBURY: Well, it's not that they want the bugs to eat them. The bugs actually kill seeds, too, some of them. And most plants don't want insects eating their fruit because insects aren't very good at dispersing seeds and most chemistry in plants is really designed to stop some of those - to stop bugs. Bugs of all sorts, bugs that you can see like insects and then things we sometimes call bugs that are microbes like fungus and microbial consumers, all of which want to eat those same fats and sugars and that's what fruits do, right? They bribe us with their fats and sugars and they say, come eat us. Well the microbes and all the bugs want those fats and sugars, too. And the chemistry in the fruit is a way of trying to keep those bugs out.

FLATOW: How did you stumble on this? You must have been looking for something.

Dr. TEWKSBURY: We were. We started quite a while ago and we started by looking at more - we started over 10, 15 years ago. And our first idea was that chilies may be hot to stop small mammals from eating them because we're the only mammal on earth that willingly eats chilies and so all the other mammals don't eat chilies and that seems like a reasonably good thing for the chili because a lot of mammals like ourselves have molars and a lot of the small mammals, they would eat wild chilies, they'd chew seeds and they'd kill them. And birds can't taste the heat.

They don't have the receptors aligned quite the right way to sense chili capsaicin as heat. It's just not hot to them. And so that's a great, wonderful reason to be hot because you keep all those good consumers, all the birds, and you stop the mammals which might be not such good consumers because they chew the seeds. It's elegant and it does help the chilies but it's probably not the original function for the heat in the chilies. And the reason for that is simply that all birds can't taste the heat and chilies evolved quite a bit later than birds did. And so it's not as if the birds changed in response and wanted to eat chilies and then there was an adaptation to eating chilies which allowed them to not taste the heat.

That's a happy evolutionary accident. It helps chilies. But you have to go back a lot further and say what is the primary problem that fruits have and a lot of the problems that fruits have is keeping the fungus out because fungus and microbes in general, they just never disperse seeds. They're never good things. But they're always attacking fruit and so capsaicin is a wonderfully elegant solution. It stops all the consumers you don't want and it doesn't stop the consumers you want.

FLATOW: But how did you wind up with this?

Dr. TEWKSBURY: Oh, right. Sorry.

FLATOW: That's OK. That's why I'm here for.

Dr. TEWKSBURY: I wound up with it because I'm sort of interested in chilies and I'm interested personally as well as professionally. I think all of us get into sort of studying what we love and in my case, I'm able to actually eat my study subject, that's a lot people can't say and no one gets angry. So it's a happy accident that I get to study chilies and I started by literally wandering around the deserts of Arizona, the northern limit of wild chilies, and helping to establish protected areas for chilies. Essentially protected areas that would highlight the wild relatives of our crops and chilies being a huge crop. I was focusing on those. Also, because I love chilies. And one in every four humans eats chilies everyday.

So it's a big deal to understand why we love the species so much and I was interested in protecting it and in understanding it. And that understanding, it led me into this idea of what is it about chilies that causes them to live in some places and not others, why are them under some shrubs and not others, could that be that they're actually using their heat to determine - to help change where they end up and how successful they are and how would that work? And that sort of led down this road of, maybe it's mammals? So we started looking at that and it turns out it could be mammals. Part of it is mammals but it's not the whole story and we ended up finding this chili that was sometimes hot and sometimes not and it was just deposited in a sea bank in the U.S. in the 1950s from a collection in Bolivia, from a collector who collected 40 years ago, 50 years ago now.

And he put the seeds away and then someone growing it out said, hey, sometimes these plants are hot; and sometimes they're not. So we used those in our tests and then we said, wouldn't it be cool if you could find a species like this in the wild where you could go down and in the national habitat where evolution is taking place, you could go and find plants that were blindingly hot and then, right next to them are plants that don't have heat at all. And that's exactly what this species is. And we got lucky finding it.

FLATOW: What would it count for sometimes hot, sometimes not?

Dr. TEWKSBURY: Well, we call it a polymorphism and there's a lot of polymorphisms and they're really interesting to study. This is simply multiple forms that you have just like you have red eyes - excuse me, green eyes and blue eyes or brown eyes or a lot of other left handedness and right handedness. There are different forms. In this case, it's chemical, right? And you have one with the heat and one without the heat. It turns out it's under genetic control so it's not environmental. A plant once it's not hot, can't be hot.

FLATOW: Mm-hm.

Mr. TEWKSBURY: A plant, once it's a hot plant producing hot chili fruit will always produce hot chili fruit.

FLATOW: Mm-hm.

Mr. TEWKSBURY: And the genetics are, you know, not - they don't appear to be too complicated. A little bit more complicated than Mendel's peas, but not too much more complicated.

FLATOW: Mm-hm.

Mr. TEWKSBURY: And it's not the same genes that determine how hot you are. So that's a separate set of genes. And - but this polymorphism exists throughout at least four species in Bolivia that we've been able to find.


Mr. TEWKSBURY: And so you go into a population, and the only way to tell if they're hot or not is to eat the plant - is to eat the fruit. And this has been sitting in front of our - it's just been out there for, you know, hundreds of years, and scientists have been collecting these plants for a long time, but no one bothered to eat the fruit. And the reason that is, is because, you know, chilies are in a family of plants that have a lot of close relatives that are very deadly if you eat the fruit. So the smart botanists...

FLATOW: What family would that be?

Mr. TEWKSBURY: Solanaceous - it's called the solanaceous plant family...

FLATOW: Mm-hm.

Mr. TEWKSBURY: Solanums. And so these include tomatoes, potatoes, eggplants, chilies...

FLATOW: The nightshade?

Mr. TEWKSBURY: As well as the deadly nightshade.

FLATOW: Nightshades.

Mr. TEWKSBURY: And so, the nightshade family is known for its chemistry. And so the collectors of chilies and other nightshades didn't make a regular habit of eating the fruit. Probably a smart thing.

FLATOW: That's for the grad students or something.

(Soundbite of laughter)

Mr. TEWKSBURY: Well, these are just collect - these are collectors throughout, you know...

FLATOW: Yeah. Yeah. Oh, I see...

Mr. TEWKSBURY: Over a long period of time.


Mr. TEWKSBURY: But the locals in Bolivia and the people we worked with, they know that there are non-hot chilies out there, because the people use these plants. This is a plant that is recognized everywhere you go, everywhere it grows, because it's - all the...


Mr. TEWKSBURY: The native cultures know where the chilies are...


Mr. TEWKSBURY: And they know if they are hot or not.

FLATOW: So where do you go from here? Is there another step to follow?

Mr. TEWKSBURY: There's a lot of steps. One of the interesting ones is that it looks like the fungus isn't taking this lying down. In other words the fungus...

(Soundbite of laughter)

FLATOW: So to speak.

Mr. TEWKSBURY: You know, they want - they're trying to eat that chili. And, you know, by gosh, they're going to figure out a way to do it, even if it is hot. And so the fungus appears to be evolving as well.

So, you know, once you get - when you get fungus out of a hot chili, it's not the same critters if you take it out of a not-hot chili, and that fruit will be called co-evolution, where, you know, there is an arms race going on.


Mr. TEWKSBURY: The fungus figures out how to deal with the heat, and the chili just gets hotter.

FLATOW: So you might theoretically find a really good antifungal? Capsaicin.

Mr. TEWKSBURY: Yeah. I think capsaicin is working...


Mr. TEWKSBURY: Probably like some of our existing antifungal agents. Well, right now, we're trying to track down how does it work.


Mr. TEWKSBURY: How exactly does capsaicin stop fungal growth? And we're pretty close to figuring it out. And it looks like it has a lot to do with how other fungicide stop - other fungicides work.

FLATOW: Mm-hm. We have a quick question from McHaynes(ph) in the Second Life. Why - any connection between chilies harboring the awful E. coli, and funguses, and things like that?

Mr. TEWKSBURY: Well, it's a great question. It's - it turns out that what we've done to chilies, primarily, is make it - is make them milder and bigger. And so the chilies that are harboring E. coli are not actually that hot. Now some of us from northern climates might think they're pretty hot, but in the grand scheme of things, jalapeƱos are not a tremendously hot chili.

And the other issue is that the heat in chilies is concentrated around the seeds. It's there to protect the seeds. And as soon as you make the chili, you know, 10 times bigger, then the concentration of capsaicin is still around the seeds and, as a lot of chili eaters know, you bite the tip of a jalapeƱo and it's not that hot.

So there's plenty of room in those big fruits for things like E. coli and stuff to get started. My hunch is that you have - you probably have less of a problem with infection of that sort with the smaller, spicier chilies. But the direct effects of capsaicin on E. coli specifically, I don't know if that's been nailed down yet.

FLATOW: And there's something for someone else to take on.

Mr. TEWKSBURY: Oh, yeah.

FLATOW: Thank you, Josh.

Mr. TEWKSBURY: All right. Thanks a lot.

FLATOW: Joshua Tewksbury is an assistant professor of biology at the University of Washington in Seattle. We are going to take a short break and when we come back, we are going to be joined by Paul Ehrlich, who is going to talk about what he believes in his new book, "The Dominant Animal: Human Evolution and the Environment."

We may be in a period of extinction and what do we do about that? Well, we'll get his opinions on that. Our number, 1-800-989-8255. Stay with us, we'll be right back.

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FLATOW: I'm Ira Flatow. This is Talk of the Nation: Science Friday from NPR News.

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