Friday, September 24, 2010
When you think of fruit what do you envision? A sweet, juicy treat? That's what I think of, particularly strawberries. Mmmmmm....
This sweet, juicy goodness, evolutionarily speaking, is a reward to animals for the dispersal of the fruits' seeds. In fact, it is considered to be one of the key innovations in the radiation of angiosperms (flowering plants). However, the plants don't receive just the goodly seed-dispersers, they also end up attracting consumers (vertebrates, invertebrates and microbes) that are detrimental to the plants' fitness (put very very simply, how much it reproduces). These detrimental consumers come in the form of seed predators that reduce the likelihood of seed dispersal and viability. The middle ground or balancing factor comes in the form of the fruits' chemistry. The makeup of the fruit can deter seed predators, reduce microbial attack, and/or attract specific seed dispersers, all without compromising seed viability. Generally, this explains the presence of noxious, bitter, and sometimes toxic chemicals in many ripe fruits.
Now that we know some general fruit stuff, how do we test something like the variance in microbial pathogen pressure as it is related to the variance in the chemistry of wild, ripe fruits? Well, you would need to start with a fruit species in which you know the chemistry pretty well. You would then need to show that the chemistry of the fruit deters microbial pathogens. Start playing with concentrations of both and you've got yourself an experiment.
Hmmm...what is a fruit that deters things from eating it? If you've looked at the picture above then you can probably guess: Chilies. Chilies belong to the genus Capsicum and were one of the first plants to be domesticated in the New World. They contain what are called capsaicinoids, which produce the spicy/hot capsicum (hotter = more pungent fruit), are unique to this genus, are well characterized, and are broadly antimicrobial. Capsaicinoids also increase in their concentrations during fruit ripening and are limited to the fruit itself (rather than to other parts of the plant). The authors of this paper also rediscovered a polymorphism for capsaicinoid production in wild populations of multiple chili species which allowed them to test the variability of these chemicals in the wild.
The researchers did most of their work with Capsicum chacoense Hunz., which is native to the Chaco region of Bolivia, Argentina, and Paraguay. They used a geographic gradient to study the impact of microbial pathogens on fruit chemistry. The results showed that across all populations the only significant cause of fruit and seed damage was microbial infection, primarily caused by the fungus Fusarium semitectum. This particular fungus enters the fruit by way of the piercing proboscises of hemipteran bugs. On closer inspection they found that the fungal infection of seeds increased with the number of foraging scars on the fruit and that fruits with no insect damage had no fungal infection. This pattern was seen in both pungent and nonpungent fruits, but the slope of the relationship was significantly steeper for nonpungent fruits, the infection rates for nonpungent fruits being almost twice as high. Because pungent and nonpungent fruits are indistinguishable in the wild there must be something else going on. When the scientists created an artificial fruit media that mimicked the nutritional composition of the C. chacoense fruit except for the presence/absense and concentration of capsaicinoid chemicals they found that the inhibition of F. semitectum was dose-dependent. The reduction of infection by F. semitectum was completely accounted for by the capsaicinoid chemicals.
The protection that chilies receive from these chemicals shapes the chemistry of the fruits, provides a selection pressure to the fungus, and explains among-population variation of capsaicinoid production. As fungal pressure increases there will be an increase pungent phenotypes in the chilies. So why not just stay hot and spicy all the time? As with most things there is a trade-off. If the plant puts more into chemical production it puts less into seed coats. That means that pungent plants will protect their seeds from fungal infection but those seeds will not be as well protected when they travel through the digestive system of a seed disperser. You get the idea right?
Being firmly in the medium-salsa crowd I must then ask, why do humans love the hot stuff? Some argue that chilies help lower blood pressure, that the antimicrobial effects also benefit us, and that they increase salivation allowing for better digestion. All good notions, in my opinion, but that's still not gonna make me eat a super-hot chili. No way. According to a NY Times article on this topic, Dr. Rozin (who studies human emotions, likes, and dislikes) says he has evidence for what he calls benign masochism. In the article, Dr. Rozin says that in his experiment he tested chili eaters by gradually increasing the pungency of the chili until they said they could go no further. He then asked what level of heat they liked the best, and they chose the highest level of unbearable pain. Crazy? I think so. I'll stick with bell peppers, thank you.
Here is the paper on chili evolution:
Tewksbury, Joshua J., et al. (2008) Evolutionary ecology of pungency in wild chilies. PNAS: 105(33), 11808-11811. (DOI: 10.1073/pnas.0802691105)
And this is the NY Times article:
(image from topnews.in)