By Susan Milius, Science News
MOSCOW, Idaho — Sometimes it’s good to be not so hot.
Capsaicinoid compounds, which give chilies their culinary kick, have the happy effect of discouraging a seed-rotting fungus that grows on plants. But new work has found that protecting seeds has a downside, says David C. Haak of the University of Washington in Seattle. In wild chilies, tests linked pungency with vulnerability to drought and to attacks by ants that devour the seeds, he reported June 14 at the Evolution 09 meeting.
Chili heat may turn out to be an example of populations adapting to particular local circumstances, an important concept in evolutionary theory, Haak says.
And the link between capsaicinoid levels and vulnerability could explain why, even within the same species, not all chilies are hot. Haak says the seed-attacker Fusarium fungus lurks throughout the chilies’ wild range. It’s nasty stuff that ruins about a third of seeds, even in the driest places. Yet he and his colleagues have found plants in dry spots skimp on the protective capsaicinoids. In a dry-zone population, plants yielding mouth-scorching chilies were more rare than in a population in a somewhat wetter place. And the hottest of the dry-zone plants didn’t reach the flamer extremes of the exceptional chilies in more moist zones.
Those hot and not chilies illustrate how “adaptations that are beneficial in one environment may be costly in another — for example, pungency in a dry climate,” says Emily Jacobs-Palmer of Harvard University.
Where chilies grow wild, birds disperse the seeds by eating the fruits and excreting the seeds. To see what difference the degree of heat might make once the seeds arrive at their new home, Haak and his colleagues set out little seed piles from hot, sort-of-hot and not-spicy plants. Video monitoring of the piles showed three kinds of ants, including two little fire ants, settling down and snacking. More pungent seeds were the ants’ favorite, Haak says.
He’s not arguing that ants like hot food. It’s easy food they want. A tough coat of lignin surrounds the rich innards of a pepper seed, and Haak’s microscopic observations have shown that pungent peppers tend to have thinner spots than milder seeds did. The ants can cut into a seed with thin spots, killing it and preventing sprouting.
To investigate another possible cost of heat, Haak and his colleagues used the University of Washington greenhouse to grow seeds collected from various wild populations. The team found that plants that produced hotter chilies didn’t seem to be using water as efficiently as plants from dry zones. For a given watering regime, the pungent plants weren’t setting as much seed.
The profligately pungent plants grow leaves with a denser dotting of breathing pores called stomata, Haak and his colleagues found. Since more stomata can allow more water to escape, botanists take the density of stomata as an indicator of how well, or not, a plant can manage its water.
Haak crossbred plants from his two populations, the hot and not-so-hot, dry-zone plants. The link between chili heat and water use persisted through the next two generations, supporting the team’s conclusion that the two characteristics are truly tied together. Just how the association works isn’t clear, Haak says, but somehow chili heat may mean bad news during a water shortage.
At the Evolution meeting, Jacobs-Palmer also reported a tale about adaptation in different locales. Beach mice have pale fur even though they are a subspecies of the standard dark-coated field mouse. Jacobs-Palmer said she has now linked the evolution of pale coats to the improved ease of scurrying over sand without attracting attention from hawks or other visual predators.