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Do plant alchemists brew tropical diversity?
STRI Panama

This weevil (Curculionidae) is one of the many thousands of insects collected on Barro Colorado Island by Tupper Fellow Brian Sedio.

As its name suggests, the tropical plant genus Psychotria is laced with mind-bending chemicals. Tens of thousands of chemicals are produced in the leaves, wood and fruit of some 2000 Psychotria species. Caffeine is certainly the best known. There is also dimethyltryptamine, DMT, the active ingredient in the psychedelic ayahuasca brew used by Amazonian indigenous peoples. Most of the rest have never been described or tested for their pharmaceutical potential.

Brian Sedio, a postdoctoral Tupper Fellow at STRI, thinks these mystery chemicals, many of which are toxic and provide defense against plant-devouring insects and microbes, may also explain why there are so many plant species packed into tropical forests—a question that has perplexed ecologists for decades.

The hypothesis arose from earlier work on Panama’s Barro Colorado Island, BCI, where Sedio identified some 8,000 compounds in the leaves of the island’s 22 Psychotria species. He noticed that five or more Psychotria species regularly grow together in the space of a few meters. Theoretically, this is a risky growth strategy. Closely-related plants would likely be targets of the same species of plant-eating insects. By producing different toxic compounds, co-occurring plant species may defend themselves against different insect species, diffusing the risk of a mass attack.

In a gargantuan effort to tease apart the chemical defense systems of BCI’s tree and shrub species, Sedio and colleagues collected and froze 10,000 plant-eating insects. They also gathered leaves from 120 of the 500-plus tree species found on the 15-square-kilometer island.

VISUALIZING CHEMICAL DIVERSITY

Sedio and his team are analyzing this massive sample of insects at STRI’s Naos Island molecular lab and the Panamanian government’s INDICASAT-AIP research center. They use DNA barcoding to identify both the insects and the plant material extracted from their guts.

Just as scientists make family trees to understand evolutionary relationships between animals, Sedio uses a technique called mass-spectral molecular networking to smash plant chemical compounds into their base components and sort them into related groups. This technology has revolutionized Sedio’s field of research by allowing him to visualize the chemical diversity of the forest en masse.

“In the past a chemist would have had to isolate and identify each chemical compound individually,” says Sedio. “If one were to try to catalogue all the compounds that way in a forest like BCI, it would take a lifetime of work.”

Sedio hopes to identify the roles chemical defenses play in promoting tree species coexistence and preventing a handful of species from taking over a forest.

“There are countless ways in which plants use chemistry to create novel combinations of defense compounds,” says Sedio. “The deeper evolutionary question is why so many species arise in the first place.”

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