If you like your chile peppers hot, thank a fungus.
The spicy fruits developed their kick to ward off invading pathogens bent on destroying chile seeds before they could grow into new plants, according to a study published Tuesday. The bigger the threat from microbial invaders, the more pungent the pepper.
"It is a great example of the power of natural selection," said Joshua Tewksbury, the University of Washington biologist who led the study, presented in the Proceedings of the National Academy of Sciences.
Chiles are native to South America, where heat and humidity nurture the toxic fungus Fusarium semitectum. The fungus enters chiles through puncture holes made by hungry insects.
Chile peppers responded by growing built-in pesticides -- capsaicinoids, the chemical compounds that give chiles their distinctive zesty taste.
Scientists had surmised that capsaicinoids evolved as a defense against invading bugs, be they insects or fungi. The recent discovery of wild chiles with varying amounts of capsaicinoids allowed Tewksbury and his team to test that hypothesis.
The researchers collected samples from seven distinct chile populations in a 618-square-mile area of southeastern Bolivia. They counted the scars on the peppers to gauge the extent of the fungal threat in each group and examined the seeds inside. For any given number of scars, the chiles with more capsaicinoids had fewer infected seeds, according to the study.
Then the team exposed pristine peppers to fungus-carrying bugs. Fungal loads in spicy chiles were 45% to 55% lower than in their milder counterparts, the researchers reported. The team also engineered fake fruit with varying degrees of capsaicinoids in lab dishes and found that the chemicals combated the fungus in a dose-dependent way.
Those antifungal properties may have been the reason why people first domesticated chiles more than 6,000 years ago, said Paul W. Sherman, a professor of neurobiology and behavior at Cornell University who has studied the use of spices. Without refrigeration, ancient cooks would have needed a way to keep microbes from spoiling fresh food.
Chile peppers "make us healthier by cleansing what we will eat of food-borne pathogens and parasites," said Sherman, who wasn't involved in the study. "Humans have borrowed the plants' evolutionary recipes for survival and reproduction."