Chilies developed heat to beat fungus, UW research finds

The delicious burn that chili lovers savor is the result of an evolutionary duel between the plants and their predators, according to new research from the University of Washington.

Working in South America, ancestral home of all chilies, scientists showed that the chemical that gives peppers their punch also helps ward 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, lead author of the study published online today in the Proceedings of the National Academy of Sciences.

The research adds to evidence that capsaicinoids (cap-say-sin-oids), the spicy compounds in chilies, can be deadly to a wide range of microbes. Many experts believe that germ-fighting power is the reason ancient people began eating peppers.

"People probably added chilies to their stews because spicy stews were less likely to kill them," Tewksbury said.

Chiles were first cultivated in the New World 6,000 years ago. Traders and explorers, including Christopher Columbus, brought them to Europe. They spread quickly spread around the globe, and are now part of the daily menu for a quarter of the planet's population.

Tewksbury and his colleagues studied a Bolivian chili that comes in different versions, from hot as a jalapeño to bland as a bell pepper.

They found the mild peppers were more than twice as likely to be infected with fungus as the spicy versions.

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

The scientists also tested their hypothesis in the lab, creating a kind of artificial chili soup that contained the same nutrient mix as the real chilies. They added varying amounts of capsaicinoids, then inoculated the cultures with fungus. In mixtures that contained the spicy chemicals, the fungus grew much more slowly.

Tewksbury's earlier studies in Arizona found that producing spicy fruits also helps protect chili plants from being eaten by packrats, mice and other mammals, which can't stomach the heat. But birds are oblivious to spice. Chile seeds pass unscathed through their digestive tracts, and are dispersed across the landscape.

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