The Big Picture
GGI BLOG
Brian E. Sedio, STRI
Tropical Forests: Pharmacopeias of Chemical Diversity
Tropical forests are remarkably diverse. A quarter of a square mile of forest in Ecuador can contain over 1,000 tree species, or roughly as many as the 1.6 million square miles of temperate forests in North America, Europe, and Asia combined. Understanding how so many species of tree coexist in tropical forests remains one of the greatest challenges in ecology. It is thought that species must differ in some important way in order to coexist, such that each species exploits a distinct “niche” and avoids competing with other species for common resources. Animals, for example, can exploit distinct food resources. Where pumas and jaguars overlap, the lighter pumas tend to focus on fleet-footed deer, whereas the more powerful jaguars focus on larger or tougher game like tapirs and peccaries. Plants, however, all require a small number of common resources, such as light, water, carbon dioxide, and a small number of nutrients. Given these similarities, are there really 1,000 ways to be a tree in a small plot of forest in Ecuador?
I am Brian Sedio, and I am a postdoctoral researcher at the Smithsonian Tropical Research Institute (STRI) in Panama. I have a Bachelor’s Degree in Biochemistry and Genetics and a Ph.D. in Ecology and Evolutionary Biology. My work seeks to identify the characteristics, the niches, that allow tree species to coexist in tropical forests, and I think they have less to do with soil and water than with the astonishing diversity of chemical compounds that distinguish the trees of the rainforest. To test these ideas, the Smithsonian Institution Global Genome Initiative (GGI) has supported my recent study of the relationships among species of plants in the genus Psychotria (in the coffee family, Rubiaceae) in Panama, and of the genes responsible for the astonishing chemical diversity found in these plants.
Why are there so many species of tree in the rainforest? A not altogether unrelated problem in ecology is that known as the Green World problem: Why is the world green? Plants are immobile organisms, sitting ducks for hungry herbivores on the prowl. Why don’t the many beetles, caterpillars, monkeys, and deer simply increase in abundance until the world around them is denuded of vegetation? The renowned ecologist Dan Janzen proposed this hypothesis: “To an herbivore, the world isn’t green. It’s colored L-dopa, cocaine, and caffeine.”
Brian E. Sedio, STRI
Tropical Forests: Pharmacopeias of Chemical Diversity
Tropical forests are remarkably diverse. A quarter of a square mile of forest in Ecuador can contain over 1,000 tree species, or roughly as many as the 1.6 million square miles of temperate forests in North America, Europe, and Asia combined. Understanding how so many species of tree coexist in tropical forests remains one of the greatest challenges in ecology. It is thought that species must differ in some important way in order to coexist, such that each species exploits a distinct “niche” and avoids competing with other species for common resources. Animals, for example, can exploit distinct food resources. Where pumas and jaguars overlap, the lighter pumas tend to focus on fleet-footed deer, whereas the more powerful jaguars focus on larger or tougher game like tapirs and peccaries. Plants, however, all require a small number of common resources, such as light, water, carbon dioxide, and a small number of nutrients. Given these similarities, are there really 1,000 ways to be a tree in a small plot of forest in Ecuador?
I am Brian Sedio, and I am a postdoctoral researcher at the Smithsonian Tropical Research Institute (STRI) in Panama. I have a Bachelor’s Degree in Biochemistry and Genetics and a Ph.D. in Ecology and Evolutionary Biology. My work seeks to identify the characteristics, the niches, that allow tree species to coexist in tropical forests, and I think they have less to do with soil and water than with the astonishing diversity of chemical compounds that distinguish the trees of the rainforest. To test these ideas, the Smithsonian Institution Global Genome Initiative (GGI) has supported my recent study of the relationships among species of plants in the genus Psychotria (in the coffee family, Rubiaceae) in Panama, and of the genes responsible for the astonishing chemical diversity found in these plants.
Why are there so many species of tree in the rainforest? A not altogether unrelated problem in ecology is that known as the Green World problem: Why is the world green? Plants are immobile organisms, sitting ducks for hungry herbivores on the prowl. Why don’t the many beetles, caterpillars, monkeys, and deer simply increase in abundance until the world around them is denuded of vegetation? The renowned ecologist Dan Janzen proposed this hypothesis: “To an herbivore, the world isn’t green. It’s colored L-dopa, cocaine, and caffeine.”
The Green World: Tropical moist forest along the Panama Canal. Tropical forests and other plant communities sustain a lot of standing biomass. Why don’t herbivores eat it all up? The answer most likely lies in the astonishing diversity of chemical defenses with which plants defend themselves.
That is to say, to the human eye, a tropical forest appears as a sea of green. Yet the forest is likely able to sustain such an impressive standing biomass of vegetation precisely because of its diversity, and that diversity manifests not only in the number of tree species, but in the astonishing diversity of chemical compounds with which plants defend themselves from an endless onslaught of insect herbivores and microbial pathogens.
The chemical diversity of plants has long stymied biologists studying ecological communities or broad-scale evolution. Any individual plant can be characterized by hundreds of small molecule metabolites. A forest can contain hundreds of thousands of unique molecules. Conversely, any given compound can be found in a small number of very distantly related plants, but few species in a given community. Caffeine, for example, is found in coffee (Coffea arabica and Coffea canephora, native to Ethiopia), tea (Camellia sinensis, native to Asia), yerba mate (Ilex paraguaiensis, native to Argentina and southern Brazil), kola nut (Cola acuminata, native to West Africa) and chocolate (Theobroma cacao, native to Amazonia). None of these species occur together naturally, hence caffeine may be wholly unique to a cacao within an Amazonian tree community. But just how different chemically is Theobroma cacao from its neighbors in the eyes of a beetle? Enough to turn away a hungry bug?
Fortunately, our ability to study the chemical differences among plant species was given a significant boost by innovations in mass spectrometry by analytical chemist Pieter Dorrestein and collaborators at the University of California San Diego. Dorrestein, Mingxun Wang, Nuno Bandeira, and others developed a method for comparing the structures of unknown molecules, and using those similarities to build “molecular networks”, the links of which indicate structural similarities among molecules. Their method works because two molecules that are similar in structure will, when broken, break into many of the same pieces. Hence, one can compare the pattern of pieces generated when different molecules are shattered as a means of measuring their similarity, even without knowing the true structure of the molecules in question. This gives ecologists and evolutionary biologists a tool for assessing the chemical similarity of species and for identifying potentially important compounds even in chemically diverse, species-rich, and understudied plant communities such as tropical rainforests.
The chemical diversity of plants has long stymied biologists studying ecological communities or broad-scale evolution. Any individual plant can be characterized by hundreds of small molecule metabolites. A forest can contain hundreds of thousands of unique molecules. Conversely, any given compound can be found in a small number of very distantly related plants, but few species in a given community. Caffeine, for example, is found in coffee (Coffea arabica and Coffea canephora, native to Ethiopia), tea (Camellia sinensis, native to Asia), yerba mate (Ilex paraguaiensis, native to Argentina and southern Brazil), kola nut (Cola acuminata, native to West Africa) and chocolate (Theobroma cacao, native to Amazonia). None of these species occur together naturally, hence caffeine may be wholly unique to a cacao within an Amazonian tree community. But just how different chemically is Theobroma cacao from its neighbors in the eyes of a beetle? Enough to turn away a hungry bug?
Fortunately, our ability to study the chemical differences among plant species was given a significant boost by innovations in mass spectrometry by analytical chemist Pieter Dorrestein and collaborators at the University of California San Diego. Dorrestein, Mingxun Wang, Nuno Bandeira, and others developed a method for comparing the structures of unknown molecules, and using those similarities to build “molecular networks”, the links of which indicate structural similarities among molecules. Their method works because two molecules that are similar in structure will, when broken, break into many of the same pieces. Hence, one can compare the pattern of pieces generated when different molecules are shattered as a means of measuring their similarity, even without knowing the true structure of the molecules in question. This gives ecologists and evolutionary biologists a tool for assessing the chemical similarity of species and for identifying potentially important compounds even in chemically diverse, species-rich, and understudied plant communities such as tropical rainforests.
In 1964, Paul Ehrlich and Peter Raven imagined that the ecological importance of chemical differences among plants might be so powerful as to drive the evolution of new species of plants and the insects that feed on them, and ultimately generate much of the world’s biodiversity. With regards to plants, Ehrlich and Raven envisioned a mutation resulting in a novel defense, followed by ecological success as the plant population grows unchecked by herbivores. This microevolution of a novel defense in a population could be followed by speciation, as the new, incipient species benefitted from distinguishing itself from less chemically novel populations. Eventually, insects and microbes would themselves adapt to the new defense and colonize the new plant species. Over time, this evolutionary arms race would generate many species of both plants and herbivores. Most interestingly, if some chemical or genetic traits were more evolvable than others, the rate of speciation would accelerate in plant lineages with highly evolvable chemical defenses. Such chemical defenses, or classes of chemical defenses, would therefore be key innovations that distinguish a diverse adaptive radiation of related species from a sister lineage with less evolutionarily labile chemistry.
An herbivorous weevil from Barro Colorado Island, Panama. Photo: Yonatan Munk.
|
The late botanist Alwyn Gentry noticed that a small number of exceptionally species-rich tree genera contribute a large fraction of the species in many tropical forests. Gentry called genera such as Eugenia (the myrtle family, Myrtaceae), Inga (the bean family, Fabaceae-Mimosoideae), Miconia (the melastome family, Melasomataceae), Piper (the black pepper family, Piperaceae) Pouteria (the sapote family, Sapotaceae), and Psychotria (the coffee family, Rubiaceae) “species swarms” because of the astonishing number of ecologically similar, closely related species that can be encountered within a fraction of an acre. These genera can comprise between a quarter and a third of the tree species in forests from Mexico to Brazil. My recent work indicates that they comprise an even greater part of the chemical diversity of these forests, and I and others have found that closely related species in these genera tend to be very different chemically. Are Gentry’s species swarms adaptive radiation in chemical defense?
One of the most interesting of Gentry’s species swarms is Psychotria, so named because some species contain psychotropic, or hallucinogenic, compounds. Psychotria is perhaps the fourth most species-rich plant genus on the planet, with over 2,000 species found in subtropical and tropical forest environments in the Americas, Africa, southern Asia, and even Pacific islands such as Hawaii. Psychotria is also diverse at local spatial scales, with perhaps 160 species in Panama, or to give another example, over 60 species in a quarter-square-mile forest plot in Ecuador. Much of this diversity may be a result of the chemical differentiation of species, as Ehrlich and Raven predicted.
Psychotria are known for their alkaloids, small organic molecules that contain nitrogen, often in a ring structure. Alkaloids can be very biologically active. The caffeine that makes coffee, tea, and chocolate stimulating is an alkaloid. So are the nicotine in tobacco (Nicotiana tabacum) and the narcotic cocaine (derived from several species of Erythroxylum). The most well-known alkaloids found in Psychotria are emetine and cephaeline, found in a Central American species known as ipecacuanha, or more commonly as ipecac. Syrup of ipecac was used historically to induce vomiting if poison had been ingested (physicians no longer recommend this practice prior to diagnosis). These alkaloids also exhibit antiprotozoal activity.
Another known alkaloid of Psychotria is dimethyltryptamine, or DMT, derived from the Amazonian species Psychotria viridis, or chacruna. DMT is found naturally in many plants and animals, including humans, though Psychotria viridis produces the alkaloid in high concentrations. DMT is structurally similar to serotonin, a neurotransmitter associated with feelings of euphoria.
Interestingly, if one were to consume a leaf of chacruna, nothing would happen, as an enzyme that occurs in human saliva would metabolize the DMT prior to passage to the small intestine. However, indigenous peoples of the western Amazon long ago discovered that the roots or bark of an unrelated vine, Banisteriopsis caapi or ayahuasca, can be used to liberate the effects of DMT on the body. This is because ayahuasca contains a monoamine oxidase inhibitor (MOI) that prevents the enzyme in one’s saliva from altering DMT. In traditional societies in western Amazonia, ayahuasca root or bark is mixed with chacruna leaves to yield a psychotropic tea. I must confess that my interest in these practices is more intellectual than experiential, as the first effects of ayahuasca consumption include several hours of projectile vomiting and diarrhea. Afterwards, the participant typically experiences a vision in which they are visited by a powerful animal, such as a jaguar, which imparts wisdom. Or so I am told. It’s hard for me to imagine any wisdom a jaguar could give me that would be worth several hours of vomiting.
Let me point out here that it is easy to view the practice of consuming ayahuasca through the lens of Western, recreational drug use. However, both Psychotria viridis and Banisteriopsis caapi contain compounds that function as antihelmintic drugs. That is, the consumption of ayahuasca is a treatment for parasitic worms, which I imagine is a widespread malady in traditional Amazonian societies. It turns out the jaguar spirit vision may just be a pleasant side-effect that makes an otherwise unpleasant medical treatment more bearable.
Emetine and DMT are only two of thousands of compounds produce by species of Psychotria with no clear function in primary metabolism. Many of these alkaloids, terpenoids, flavonoids, and other secondary metabolites may serve to defend Psychotria from insects and pathogens. And the vast diversity of Psychotria species may be a result of the ease which with Psychotria evolve new chemical compounds, allowing them to temporarily escape pests and pathogens and generate new species of Psychotria in the process. In fact, in Panama, the most closely related species of Psychotria are often remarkably different chemically. This suggests that natural selection imposed by herbivores results in chemical divergence among closely related species, and may result i speciation and hence the radiation of the genus.
To understand how closely related species of Psychotria can differ so dramatically in their chemistry, we need to understand the relationships among species of Psychotria and to identify the genes responsible for the alkaloids and other metabolites. To that end, the Smithsonian Institution Global Genome Initiative (GGI) supported our collecting expedition and genetics research on the Psychotria of Barro Colorado Island, Panama.
Psychotria are known for their alkaloids, small organic molecules that contain nitrogen, often in a ring structure. Alkaloids can be very biologically active. The caffeine that makes coffee, tea, and chocolate stimulating is an alkaloid. So are the nicotine in tobacco (Nicotiana tabacum) and the narcotic cocaine (derived from several species of Erythroxylum). The most well-known alkaloids found in Psychotria are emetine and cephaeline, found in a Central American species known as ipecacuanha, or more commonly as ipecac. Syrup of ipecac was used historically to induce vomiting if poison had been ingested (physicians no longer recommend this practice prior to diagnosis). These alkaloids also exhibit antiprotozoal activity.
Another known alkaloid of Psychotria is dimethyltryptamine, or DMT, derived from the Amazonian species Psychotria viridis, or chacruna. DMT is found naturally in many plants and animals, including humans, though Psychotria viridis produces the alkaloid in high concentrations. DMT is structurally similar to serotonin, a neurotransmitter associated with feelings of euphoria.
Interestingly, if one were to consume a leaf of chacruna, nothing would happen, as an enzyme that occurs in human saliva would metabolize the DMT prior to passage to the small intestine. However, indigenous peoples of the western Amazon long ago discovered that the roots or bark of an unrelated vine, Banisteriopsis caapi or ayahuasca, can be used to liberate the effects of DMT on the body. This is because ayahuasca contains a monoamine oxidase inhibitor (MOI) that prevents the enzyme in one’s saliva from altering DMT. In traditional societies in western Amazonia, ayahuasca root or bark is mixed with chacruna leaves to yield a psychotropic tea. I must confess that my interest in these practices is more intellectual than experiential, as the first effects of ayahuasca consumption include several hours of projectile vomiting and diarrhea. Afterwards, the participant typically experiences a vision in which they are visited by a powerful animal, such as a jaguar, which imparts wisdom. Or so I am told. It’s hard for me to imagine any wisdom a jaguar could give me that would be worth several hours of vomiting.
Let me point out here that it is easy to view the practice of consuming ayahuasca through the lens of Western, recreational drug use. However, both Psychotria viridis and Banisteriopsis caapi contain compounds that function as antihelmintic drugs. That is, the consumption of ayahuasca is a treatment for parasitic worms, which I imagine is a widespread malady in traditional Amazonian societies. It turns out the jaguar spirit vision may just be a pleasant side-effect that makes an otherwise unpleasant medical treatment more bearable.
Emetine and DMT are only two of thousands of compounds produce by species of Psychotria with no clear function in primary metabolism. Many of these alkaloids, terpenoids, flavonoids, and other secondary metabolites may serve to defend Psychotria from insects and pathogens. And the vast diversity of Psychotria species may be a result of the ease which with Psychotria evolve new chemical compounds, allowing them to temporarily escape pests and pathogens and generate new species of Psychotria in the process. In fact, in Panama, the most closely related species of Psychotria are often remarkably different chemically. This suggests that natural selection imposed by herbivores results in chemical divergence among closely related species, and may result i speciation and hence the radiation of the genus.
To understand how closely related species of Psychotria can differ so dramatically in their chemistry, we need to understand the relationships among species of Psychotria and to identify the genes responsible for the alkaloids and other metabolites. To that end, the Smithsonian Institution Global Genome Initiative (GGI) supported our collecting expedition and genetics research on the Psychotria of Barro Colorado Island, Panama.
Back in the lab at the Smithsonian Tropical Research Institute (STRI), Christian isolated RNA from frozen leaf tissue. Christian and STRI research technician Marta Vargas then reverse-transcribed the RNA into the original DNA code in which the RNA sequences are stored in the cells chromosomes. We then sequenced the DNA to provide us with DNA sequence for hundreds of genes that are actively expressed in developing leaf tissue. With the collaboration of Owen McMillan at STRI and Paul Frandsen and Rebecca Dikow at the Smithsonian Office of Research Information Services (ORIS), we will now use this DNA sequence to identify genes that are shared among all Psychotria, the variation in which we can use to determine the relationships among hundreds or thousands of plant species in the genus. We will also identify genes that differ, either in their DNA sequence or their expression levels, between very closely related species of Psychotria. This will allow us to identify candidate genes underpinning the defensive chemistry that may define the most important ecological niches of Psychotria species and to understand why Psychotria is so diverse, both in terms of species and chemistry.
For now, with support from GGI, we seek to understand the chemistry, ecology, relationships, and genetic architecture of Psychotria. Yet the tools we are developing through this research will make it easier in the future to apply these same methods to other potential adaptive radiations in defensive chemistry—Gentry’s species swarms—to understand what makes tropical forests so diverse, and so green. I have a strong hunch that Janzen, Ehrlich, and Raven were right in their predictions that chemistry drives much of plant ecology and evolution.
For now, with support from GGI, we seek to understand the chemistry, ecology, relationships, and genetic architecture of Psychotria. Yet the tools we are developing through this research will make it easier in the future to apply these same methods to other potential adaptive radiations in defensive chemistry—Gentry’s species swarms—to understand what makes tropical forests so diverse, and so green. I have a strong hunch that Janzen, Ehrlich, and Raven were right in their predictions that chemistry drives much of plant ecology and evolution.
