For over 6,000 years, humans have used chili peppers to add a spicy kick to their meals [1]. Not only does chili spice add heat and flavor, it keeps food from spoiling. We’ve all seen mold growing in wet, humid places like bathrooms, and in hot and humid tropical regions this is especially a problem. Native peoples in the Americas have been breeding chilies for their flavor and spice long before the invention of refrigerators [1]. We have now cultivated five very different species of chilies, and even pinpointed the substance responsible for spice, a long compound called capsaicin (cap-SAY-sin). Humans eat capsaicin in abundance, and even synthesize it for topical creams to relieve the symptoms of psoriasis and arthritis.

Capsaicin is very useful to people, but it begs the question: why did chilies start making it in the first place, or, from an evolutionary perspective, what advantage does spice offer the chilies that created it?

Capsaicin As a Targeted Weapon

In 2001, Professor Josh Tewksbury and his team set out to determine whether or not chilies make capsaicin to discourage certain types of animals from eating chili fruits. In Arizona, they used the local spicy chiltepine chilies (Capsicum annuum var. glabrusculum), along with some non-spicy chili varieties from a Bolivian species called Capscium chacoense in feeding trials with birds and rodents. The results showed that cactus mice and packrats avoid spicy fruits, but birds like the curve-billed thrasher eat them like candy[2]. The researchers attributed these findings to the fact that birds lack the taste receptors for capsaicin, but rodents are more like us, with special taste receptor channels called transient receptor potential (TRP) channels. When capsaicin binds to this receptor channel, it triggers calcium ions to enter nearby neurons. When these neurons are agitated by calcium ions, it results in that characteristic burning sensation[3]. Since birds don’t have receptors, they don’t feel pain from eating even the spiciest chilies.

Then the scientists took their question one step further, and asked why the chilies might not benefit from rodents snacking on their fruits. They found that when rodents ate chili seeds, grinding them with their molars, none of the seeds were able to germinate. Consumption by thrashers, on the other hand, “resulted in germination rates similar to those of control seeds” (Tewksbury et al, 2001). This made intuitive sense, because birds are important seed dispersers for many plants. Chilies direct their spice at rodents that grind up their seeds, while encouraging benevolent birds to disperse their seeds far away.

Even with these results, a tantalizing question remained. Why does one of the chilies used in the experiment, Capsicum chacoense, produce so many plants that aren’t spicy? To answer that, Josh Tewksbury ventured to the heart of chili evolution, in Southeastern Bolivia.

A Bug, a Mold, and a Lot of Flat Tires

Bolivia is not an easy country to do field work in. Not only are the towns far apart and the roads ill-maintained, but there are deadly diseases like Dengue Fever and Chaga’s to avoid, and no shortage of ticks or deadly coral snakes. Nonetheless, the team set about finding wild populations of Capsicum chacoense, and discovered a story with far more characters than they were expecting.

Neither the presence of rodents nor birds seemed to influence how many plants in a local population were spicy. The first critter that did correlate with chili spice was a small insect in a related group called the Hemiptera[4]. These bugs use a proboscis, or needle-like tongue, to puncture the fruit and drink the peppery juice (see Photo). Capsaicin probably deters insects much like it deters rodents. The scientists found that within a population of plants, the average number of puncture marks on the fruits correlated with the proportion of plants that made spice. However, the insects weren’t the only critter inflicting damage. As the number of insect foraging holes increased, so did the amount of mold on the seeds. Molds in the genus Fusarium are notorious seed pathogens that affect many agricultural crops worldwide, and the researchers found the mold’s deadliness to be no different for wild chilies. As the amount of visible mold on the seed increased, the chances of the seed’s survival decreased. But just as early American natives discovered, the scientists saw that these fungi are sensitive to capsaicin. Spicy plants exhibited much less fungal damage on their seeds than non-spicy plants. By placing the fungi on Petri dishes with varying amounts of capsaicin, they found that greater amounts of capsaicin resulted in less fungal growth.

Across Bolivia, chili populations that are wetter, with more insects, and where fungi are more prevalent contain a larger proportion of spicy plants than drier places with fewer insects, where fungi do not grow as readily. Weather, insects, and mold all influence just how spicy Capsicum chacoense can be.

Figure 1. An insect, Acroleucus coxalis, uses its proboscis, a specialized tongue, to suck juice from a wild chili pepper, Capsicum chacoense.

A Trade-Off Limits Spiciness

The team had done much to explain why chilies evolved to make capsaicin, but why some plants made no spice was still a mystery. To answer that, they delved into the physiology that governs spice production.

From the chili plant’s metabolic perspective, capsaicin wracks up a very high defense budget. The molecule is relatively large and contains lots of precious nitrogen, which is critical for building proteins and DNA. Furthermore, as a byproduct of making capsaicin, the leaves of spicy plants have more stomata. Stomata are “holes” on plant leaves, guarded by a pair of special cells that monitor how much gas passes through the leaf, and having many stomata means that a plant loses more water when it transpires, a necessary step for performing photosynthesis and making sugars. As a result, when the occasional drought occurs, spicy plants don’t perform as well as non-spicy plants [5]. Non-spicy plants have an advantage over spicy plants during drought, producing more seeds, and thus more progeny, than their spicy brethren. When plants receive enough water, the advantage disappears and the spicy and non-spicy chilies make an equal number of seeds again.

For wild chilies, the dueling selective pressures of fungal pathogens and drought result in a polymorphism, a case where some chilies are spicy and some are not at all. Stay tuned in the next few years as further research emerges on this hot and spicy research system.

Cat Adams is a first year PhD student in the Organismal and Evolutionary Biology program at Harvard University. Visit her blog at www.ScienceIsMetal.com

References

1) Perry, Linda et al. “Starch Fossils and the Domestication and Dispersal of Chili Peppers (Capsicum spp. L.) in the Americas.” Science 315.986 (2007)

2) Tewksbury, Joshua & Gary Nabhan. “Directed Deterrence by Capsaicin in Chilies.” Nature 412 (2001): 403-404.

3) Story, M. Gina & Cruz-Orengo Lillian. “Feel the Burn.” American Scientist 95 (2007): 326-33.

4) Tewksbury, Joshua et al. “Evolutionary Ecology of Pungency in Wild Chilies.” PNAS 105.33 (2008): 11808-11811.

5) Haak, David et al. “Why are not all chilies hot? A trade-off limits pungency.” Proc. R. Soc B. 10.1098 (2011) 1-6.

Further Reading

What’s So Hot About Chili Peppers? http://www.smithsonianmag.com/science-nature/Whats-So-Hot-About-Chili-Peppers.html

What Made Chili Peppers So Spicy? http://m.npr.org/story/93636630