Humans relish food spiced with ... fungicide?

Chilies have fascinated plant scientists, anthropologists, and ecologists for years. One nagging question: What purpose does a chili’s heat serve, other than to bring tears to the eyes and beads of perspiration to the foreheads of Thai-food lovers everywhere.

The answer: The chemical responsible for the heat appears to protect seeds from a fatal fungus, ensuring that the fruit will propagate, according to an international team of researchers led by the University of Washington’s Joshua Tewksbury.
The team studied wild populations of Capsicum chacoense, a chili native to Bolivia, Argentina, and Paraguay.

The chilies appeared to be vulnerable to a single species of fungus. They noted that fungus infections were far more prevalent in unspicy specimens of the chili species than in the hot versions in their test area. They found that the chemicals generating the heat reduced seed loss to the fungus. And they noted that the average number of foraging scars on chilies in a particular population served as a good indicator of the proportion of chilies in the group producing the heat-generating chemical defense. Those scars are the fungus’s primary infection route.

This defense against microbes could strengthen the case that humans domesticated chilies initially as food preservatives, the team suggests. The results appear in the current edition of The Proceedings of the National Academy of Sciences.