Natural Function
Capsaicin is present in large quantities in the placental tissue (which holds the seeds), the internal membranes and, to a lesser extent, the other fleshy parts of the fruits of plants in the genus Capsicum. The seeds themselves do not produce any capsaicin, although the highest concentration of capsaicin can be found in the white pith of the inner wall, where the seeds are attached.
The seeds of Capsicum plants are predominantly dispersed by birds. The TRPV1 channel to which capsaicin binds does not respond to capsaicin and related chemicals in birds (avian vs mammalian TRPV1 show functional diversity and selective sensitivity). Chili pepper seeds consumed by birds pass through the digestive tract and can germinate later, but mammals have molar teeth, which destroy seeds and prevent them from germinating. Thus, natural selection may have led to increasing capsaicin production because it makes the plant less likely to be eaten by animals that do not help it reproduce. In addition, there is evidence that capsaicin evolved as an anti-fungal agent. The fungal pathogen, Fusarium, is known to infect wild chilies which reduces seed viability. Capsaicin deters the fungus, and in doing so limits this form of predispersal seed mortality.
In 2006, it was discovered that the venom of a certain tarantula species activates the same pathway of pain as is activated by capsaicin, the first demonstrated case of such a shared pathway in both plant and animal anti-mammal defense.
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