Chilies' spicy flavor wards off seed-killing fungus

Chili lovers savor the sensations: The delicious burn that blossoms in your mouth; the beads of sweat; a sharp breath before the next bite.

Hot peppers are part of the daily menu for a quarter of the world's population, but the plants didn't develop their kick just to please the human palate. In fact, many people would no more nibble on jalapeño poppers than shove a flaming torch down their throats.

So why do peppers pack such a punch?

New research from the University of Washington says being spicy helps chilies fight off a seed-killing fungus.

"To me, this answers the fundamental question of why chilies are hot in the first place," said UW biologist Joshua Tewksbury, who has worked for years to unravel the evolutionary rationale behind mouth-burning heat. "I've been able to make a career out of eating chilies," he said, laughing.

Scientists have long suspected spiciness gives chilies an advantage over pests and predators, but the new study, published online Monday in the Proceedings of the National Academy of Sciences, is the first to show it in wild chili populations.

"It is an excellent and important piece of work," said Paul Sherman, professor of behavioral ecology at Cornell University.

Sherman's own research on human cuisine found tropical cultures make much more use of spices, including chilies, than people from northern climes.

That's probably because spices have germ-killing properties that lower the risk of food poisoning in places where stews and stir-frys would spoil quickly without refrigeration, Sherman said.

Capsaicinoids (cap-SAY-sin-oids), the chemical compounds that give chilies their bite, are particularly potent killers of microbes — including fungi, as Tewksbury's team discovered.

So humans learned to embrace hot peppers for the same reason plants produce them, Sherman said: "To make us healthier ... enhancing our own survival and reproduction."

On a hot trail

Tewksbury and his colleagues did their field work in South America, ancestral home of all chilies. The plants were first cultivated in the New World 6,000 years ago. Traders and explorers, including Christopher Columbus, brought them to Europe, and they quickly spread around the globe.

Like dogs, a handful of wild chili species have been coaxed by breeders into a wide assortment of shapes and flavors, from sweet pasillas to New Mexico green chile, and the searing bhut jolokia, an Indian pepper considered the world's hottest.

The key to Tewksbury's most recent study was an unusual Bolivian chili that comes in several varieties, from hot as a habanero to bland as a bell pepper.

Locals call the nonpungent chilies "aji del zorro," a derisive label meaning "peppers for the foxes" because people wouldn't bother to eat them.

But the mild peppers were delectable to the fungus called Fusarium, which can kill up to 70 percent of a plant's seeds. The scientists found that the peppers with no heat were twice as likely to be infected as the spicy versions.

The microscopic fungus slips inside chili pods after their protective skins are pierced by insects with beaklike snouts. In areas where the bugs were plentiful, most of the chili plants produced spicy fruit. In areas with few insects and little danger of fungal infection, most of the chilies were bland.

The project, funded by the National Science Foundation and the National Geographic Society, took the researchers to dry, thorny forests where the nearest town was a three-day drive. The team always carried four spare tires, Tewksbury said.

They nibbled on the small chiles to identify which of the 9-foot-tall plants produced spicy fruit.

"I have a very expensive machine in my lab that will do the same thing," he said, "but our mouth is a very sophisticated heat sensor."

Back in Seattle, the researchers tested the link between heat and fungus by creating a kind of artificial chili soup that contained the same nutrient mix as real chilies. They added varying amounts of capsaicinoids, then inoculated the cultures with fungus. Mixtures with the spicy chemicals inhibited fungal growth.

Plan to expand research

Tewksbury became fascinated with chilies' evolutionary wiles when he was a young activist pushing for creation of a preserve to protect a native chili population in Arizona. He was puzzled that chilies should be so sharp-tasting, when most fruit is sweet — a plant's way of attracting animals to disperse its seeds.

Some of his early research found a partial explanation.

Small mammals, like packrats and mice, stay away from spicy peppers.

That's good from the plant's point of view, because the mammals chew up chili seeds. But birds don't feel the burn. Seeds pass unscathed through their digestive tracts and are often deposited in propitious spots under trees, said Tewksbury, who spent a lot of time poking through bird droppings to prove the point.

"The plant wants the right critter to eat the fruit," he said.

Next, Tewksbury and his colleagues plan to explore the ways fungi are evolving to sidestep the chilies' defenses. And he's intrigued by an observation that chilies might also use their chemical heat to deter hungry cattle.

"Every time we go to a place in Bolivia where there's a fence with cows on one side and not on the other," he said, "you find more plants that produce hot chilies on the cow side."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com