Thieves, Deceivers, and Killers:
Tales of Chemistry in Nature
William AgostaChapter 1
FROM PROTOS AND LEPTS TO NATURE'S SPECIAL CHEMICALS
The Protos' warfare on their Lept neighbors depended heavily on chemicals, but ants are by no means unique in making extensive use of chemicals for communication and warfare. From one-celled organisms to complex plants and animals, many living creatures do the same. As species develop over evolutionary time, it is relatively easy for them to adapt their cellular machinery to producing chemicals for communication, warfare, and other purposes. These chemicals facilitate the way of life of organisms spread all across the biological spectrum.
Like the signals of the Proto scouts, one large group of chemicals carries messages that pass between members of the same species, messages that humans can express in words. Such signals are conveyed by substances called pheromones, which can transmit different kinds of information. Some pheromones are attractants, bringing male and female together for mating, or perhaps assembling a group of creatures for feeding or defense. Others carry such varied messages as "Danger! Flee!" "This is my territory," or "I am pregnant."
Other types of chemical signals that occur may pass between members of different species. These interspecific signals, as they are called, may benefit one or both of the species involved. The Protos' chemical warfare weapon, for example, can be viewed as an interspecific signal that benefits only the sender as it reduces the recipient Lepts to fighting among themselves. On the other hand, sometimes the receiver is the sole beneficiary of a message, as when a predator locates its prey by following the distinctive odor the victim haplessly communicates to its enemy. Other signals between species serve both sender and receiver, as when a flower's delightful fragrance entices a foraging insect to linger and explore. In an exchange profitable to both organisms, the insect pollinates the flower and receives a drop of nectar in return.
Chemicals may also supply more general information about a creature's environment. Salmon return from the sea to their native stream to spawn, guided by the distinctive odor of their birthplace. Thirsty animals follow their noses to locate life-saving water. Other chemicals are closely associated with organisms' characteristic ways of life, such as the silk spun into a silkworm's protective cocoon or the pearl an oyster fashions around a grain of sand in response to the gritty irritation.
Organisms, wherever they are, interact with one another to live, and chemicals mediate many of these contacts. Some interactions may be optional and occasional, whereas others are absolutely necessary to sustain the organism's way of life. Protos can survive only by keeping Lepts as slaves. Various species of slave-making ants differ in their self-sufficiency, but Protos have lost the ability to care for themselves. Deprived of its slaves, a Proto colony soon deteriorates.
Before continuing with the role of chemicals in the living world, we should clarify certain words and concepts that we have been using. The chemicals we have been talking about are chemical compounds made of molecules that are three-dimensional arrangements of atoms joined together by chemical bonds. A molecule is the smallest existing unit of a chemical compound, just as an atom is the smallest existing unit of a chemical element. The specific arrangement of the atoms in a moleculehow the atoms are joined togetheris called its molecular structure. It is this structure that makes a molecule what it is. Molecules come in a multitude of sizes, from as few as two atoms to many thousands. The atoms themselves are of many types, but the biologically active molecules that interest us here are usually limited to atoms of carbon, hydrogen, oxygen, perhaps nitrogen, and, less frequently, a bit of sulfur or phosphorus. Many of these biomolecules are medium sized, with twenty to eighty or ninety atoms. This size range also includes the molecules of such familiar natural compounds as cocaine, penicillin, and cholesterol.
Chemists can make, or synthesize, many biologically active chemical compounds in the laboratory. (Chemical, chemical compound, and compound are interchangeable terms.) One starts with readily available simple molecules and proceeds to build more complex structures, one step at a time, through chemical reactions. This procedure is called stepwise synthesis, and chemists have used it to prepare thousands of compounds that are found in nature, as well as many thousands more that have never been found in living systems. The chemist's repertory comprises many different synthetic reactions that can be carried out in different combinations, so that numerous ways are often available for preparing a specific compound. Living organisms, too, make molecules by stepwise syntheses, employing their enzymes (natural proteins that speed up, or catalyze, chemical reactions) and biochemical machinery rather than the chemist's laboratory reactions. Typically, nothing about an ordinary biomolecule reveals its history or origin, so a particular compound synthesized in the laboratory is indistinguishable from the same one obtained from nature.
Chemists can also study a novel compound from nature, find the specific connections between its atoms, and thus determine its structure. For complex molecules this can be a difficult research problem, particularly if very little of the compound is available. Over the past twenty-five years, however, much structure determination has become routine. Chemists have identified the molecular structures of some of the chemical compounds we shall encounter, but the structural details need not be part of our story. Nonetheless, it is important to keep in mind that molecules are real physical objects; depending on structural details, they may be large, small, globular, flat, floppy, or rigid, but all have mass and occupy space. It is also worth emphasizing that a compound's molecular structure is the basis of its unique physical, chemical, and biological properties. The chemical compounds of living organisms often have remarkable characteristics that proceed directly from the compounds' structures, sometimes in ways that we do not yet fully comprehend. Learning the relationship between molecular structures and the properties they imply remains one of the most intriguing scientific problems.
The chemical compounds that will interest us generally are significantly different from the three or four dozen essential compounds required to sustain life. Some of these essential compounds are building blocks for creating the proteins and nucleic acids (such as DNA) that are necessary for life as we know it. Whether proteins come from blue mussels, fir trees, or hippopotamuses, they consist of the same twenty amino acids strung together, one after the other, in different sequences. Similarly, each organism's DNA incorporates four units, known as nucleotides, that are used over and over again but are strung together in variable order. These amino acids and nucleotides, along with approximately two dozen other compounds, are indispensable components of life on earth.
The compounds that we will consider here, in contrast, serve quite specific needs of various organisms, and for this reason we will call them special chemical compounds. Different species may employ the same special compound, in some cases for the same purpose and in other cases for different ends. For example, the carbon dioxide arising from the respiration of a crowd of ants is an aggregation signal that invites solitary ants to join their nestmates. Corn rootworms, however, use the carbon dioxide that living corn roots emit into the soil as a signal, leading them to their food. Different species may also employ diverse compounds for essentially the same purpose. Various ant species mark their food trails with different chemicals to keep their food sources secret from one another.
As far as we know, one species, or a few closely related ones, often have a monopoly on a particular compound. The tobacco plant (Nicotiana tabacum), for example, synthesizes nicotine to defend itself from attack by herbivores (animals that feed on plants) and as a convenient way of storing nitrogen temporarily. Neither roses, elephants, nor starfish contain nicotine. In fact, apart from a few of tobacco's close relatives, we know of no other living organism that makes nicotine. In the same way, other plants have devised their own defensive compounds for protection from herbivores.
One may wonder why chemical defenses differ from plant to plant, or why carbon dioxide attracts certain creatures but not others. How does a special chemical and the interactions associated with it come to have a place in the life of a particular species? Although we do not have detailed histories of most special chemicals, we can answer in general terms that, like other biological adaptations and developments, they have arisen through the process of biological evolution. Because this topic comes up more than once in our story, it may be useful to say something about it here. Evolution is based on straightforward concepts, but their application can be tricky and is often misunderstood.
Evolution is a two-step process that leads to changes over time in the genes (all organisms' hereditary units, composed of nucleic acids) of a population of a species. After describing the two steps and their consequences, we can discuss examples that should make the process clearer. Step one is genetic variation in every generation throughout the living world. This variation arises mainly through chance mutation of genes, which biologists can now understand on a molecular level. It is this variation that, for example, may result in an insect population that is more resistant to pesticides than its relatives are or individuals with a genetic propensity for a disease such as cystic fibrosis. Genetic variation is natural and unavoidable; without it the population of an organism could not adapt to changing environmental conditions over time.
With genetic variation, adaptation to change is possible by way of an operation called natural selection, which is step two. Owing to genetic variation, individual organisms vary in ability to deal with their environment. As a consequence, some have a greater chance of surviving and reproducing than do others. In most species only a small fraction of individuals survive. Such creatures as goldfish and oysters produce millions of offspring, nearly all of which meet an early death. Survival is mostly a matter of luck, but offspring with genetic makeups best suited to their environment have some advantage. In the long run, such genetically favored organisms are the ones most likely to reproduce, and thus pass their genes on to their progeny. As a result, over time the genes favoring survival and reproduction can spread through a population of organisms, altering the population's characteristics. In this way, the population evolves.
These abstract ideas can be concretely illustrated. The English peppered moth (Biston betularia) is a classic example that is frequently cited in discussions of the concept of evolution. The peppered moth's color is under the primary control of a single gene, and thus is inherited. Genetic variation (step one) has produced two forms of this gene, which cause an individual moth to be either light-colored or dark. Although the two color forms exhibit no other differences, the numbers of light and dark moths are not equal. In the early nineteenth century, nearly all the moths were the light variety and naturalists rarely collected a dark moth. The natural bias against dark moths has a simple explanation. The moths are active at night and spend the daylight hours at rest. Their resting sites were relatively light in color, rendering light-colored resting moths nearly invisible and dark ones quite prominent. As a result, birds found and ate nearly all of the dark moths, but few of the light ones. The birds are agents of natural selection (step two), which in this case disfavors dark moths.
By 1898, however, nearly all the peppered moths in the cities of the north of England were the dark variety. Decades of uncontrolled industrialization had filled the air with soot and pollutants. The ever present soot had darkened the entire countryside, and thus resting moths now lay against a dark background. Once again, hungry birds were agents of natural selection, but now they could more readily spot and consume the light moths. Under these new circumstances, the dark form prospered.
The further passage of time brought still another shift. In the mid-twentieth century, as environmental laws controlling industrial emissions came into effect, soot and pollutants disappeared from the air. Tree trunks and the moths' other resting sites took on their original lighter hue. Again disfavored, dark-colored peppered moths have now begun to disappear from the industrialized areas where they had predominated. The light form, once again better camouflaged and protected from predation, is returning to dominance.
The reversals in the moths' dominant color are not only splendid examples of evolution, but they also illustrate important attributes of the process. One is that natural selection operates on individuals, not genes. The birds devour particular moths; they know nothing about genes. Also, it is not the individuals that evolve over time, but the population. A single moth undergoes no evolutionary change. It is born, lives, and dies either as a dark moth or as a light one. Most significant, the fluctuating fate of light and dark moths demonstrates a general characteristic of evolution: It is not "progress" in any broad sense. The favored color of moth is better strictly with reference to the prevailing local conditions. Neither color form is an absolute improvement over the other. Twice, when conditions changed, a shift occurred in the favored color. Expressed in another way, natural selection is not a guided force; it is simply an effect. Birds eat the moths they can find, and they apparently find nearly all the more visible ones, whatever their color. Comparatively few of these survive to reproduce.
For the peppered moth, genetic variation provided light- and dark-colored forms. Genetic variation is also apparent in a familiar biological development going on around us all the time. Over the past fifty years we have used antibiotics to treat infectious diseases, to poison and destroy the pathogenic bacteria that cause these afflictions. It is well known that pathogens frequently develop resistance to an antibiotic over time and, in fact, some pathogens are now resistant to most available antibiotics. This resistance begins with natural genetic variation (step one). Owing to their genetic differences, individual bacteria vary in their response to a particular antibiotic. Some tolerate it better than others of their species. The more tolerant ones have a better chance of surviving, reproducing, and passing on to their offspring the genes responsible for their greater tolerance. Here the antibiotic is the agent of natural selection (step two). Over generations of such selection, the genes conferring resistance spread through the population of a particular species. Continuing exposure to the antibiotic leads to continuing selection for more resistant individuals. Because the time between successive generations of bacteria is measured in minutes or hours, this process can be relatively rapid. In this way, an entire population of the pathogen may eventually become totally resistant, and the antibiotic will have become useless against it. Once again, it is the population that evolves, not the individuals.
Resistance to antibiotics has a close parallel that has become equally familiar over the past fifty years. On repeated exposure, agricultural pests typically become tolerant of a pesticide that is used to control them. Farmers know that the amount of pesticide required to achieve control increases from year to year. Insect pests develop resistance through evolution in the same way that bacteria do. One or two serious pests are now essentially immune to all available pesticides, and many others are moving in that direction.
These specific examples of evolution are relatively straightforward and interpretable. However, the evolution of special chemicals and their biological interactions is more complex and may take place over millions of years rather than only a few decades. Such developments are rarely understood in detail, but despite the greater complexity evolution here also proceeds by way of repeated genetic variation and natural selection. Some striking examples appear in the chapters that follow.
Before delving into ways the living world uses its special chemicals, we should note that these compounds touch our own lives in important ways. For millennia, humans have been borrowing natural chemicals for their own purposes, most often as drugs. Our oldest medicine is opium, which we prepare from the opium poppy (Papaver somniferum) today much as Mediterranean peoples did four thousand years ago. Just as we do, these early communities valued opium for its ability to kill pain and impart a sense of well-being. The principal constituent responsible for these effects is a chemical compound called morphine, which remains unsurpassed in its ability to control severe pain. In poppies, morphine's toxicity and bitterness presumably repel herbivores looking for a tasty meal.
In terms of monetary value, pharmaceuticals are our most important products obtained from nature, but natural chemicals have long filled other needs as well. Closely allied to drugs are compounds that we call recreational chemicals, most of which come from plants and find general use as mild stimulants. Caffeine from coffee and tea and nicotine from tobacco are the most popular ones. In addition, there are natural products unrelated to drugs. Probably the most ancient of these is a Chinese discovery from almost five thousand years ago. The early Chinese mastered the art of converting a protein from silkworm cocoons into the splendid textile we know as silk. For several millennia, silk manufacturing has not changed fundamentally from the Chinese process originated long ago. Other ancient developments include textile dyes and hunting poisons, as well as the perfumes and incense that have enriched human society and religion since Egyptian times. For centuries, perfumers have created alluring scents by blending dozens of natural odorants, such as musky animal sex attractants and sweet-smelling floral oils.
Another natural product that deserves special mention is the naturally occurring pesticide. The possibility of controlling pests with natural, so-called biorational, preparations is a long-standing dream that probably goes back to classical times. In his herbal, Theatrum Botanicum, published in 1640, John Parkinson noted that books written in ink containing wormwood (Artemisia absinthium) are protected from the ravages of hungry mice. He credited this observation to the second-century Greek physician Galen. Parkinson also recommended common speedwell (Veronica officinalis) and several other plants as useful repellents for clothes moths.
Modern interest in natural pest control rests on the high price we pay for our dependence on chemical pesticides. These synthetic agents have phenomenally improved crop yields in the Western world over the past fifty years, but the agricultural miracle has cost us dearly. For decades, indiscriminate application of pesticides has led to environmental disruption on a vast scale. The pollution of rivers, lakes, and coastal waters, as well as the wholesale destruction of beneficial organisms, are now worldwide problems. Nonetheless, chemical pesticides remain a twenty-five-billion-dollar world market. According to a 1997 federal report, there has been an encouraging decline in coastal pollution (from pesticides and other causes), and environmental improvement is under way. Despite this progress, pesticide-related damage in the United States runs to hundreds of millions of dollars each year.
Natural pesticides show promise for alleviating the pollution problem. Plants make some of these pesticides for their own protection, and bacteria synthesize others for purposes that are poorly understood. In spite of their promise, however, products based on natural pesticides have been on the market for years without much success, their major drawbacks being high cost and a reputation for unreliability. The present hope is that continuing research can transform these natural agents into attractive, dependable alternatives to traditional chemical pesticides.
Another approach to replacing chemical pesticides centers on pheromones. For over twenty-five years, agricultural scientists have explored ways of controlling insect pests by means of chemical signals. Early efforts focused on using a pest's own sex attractant to draw insects to traps or to interrupt their mating. A more recent innovation employs the attractant pheromones of beneficial species that prey on pests. The idea is to lure useful insects to the garden or field, where they can devour invading pests before the pests destroy crops.
Instead of converting compounds from plants into medicines, we can often benefit simply by eating the plants themselves. In some cases, consuming a plant as food has a demonstrably better effect on health than using a single plant constituent as a medicine. We are far from understanding fully the connections between diet and health, but nutritionists now urge us to eat at least five servings of fruits and vegetables each day to remain healthy. At the same time, cancer experts estimate that 30 percent of all types of cancer are linked to diet. Typically, we do not know just what natural compounds in foods are medically important, although new information appears all the time. In early 1997, University of Illinois scientists reported that a common dietary constituent, a compound called resveratrol, helps prevent cancer. Mulberries, peanuts, and grapes are particularly good sources of resveratrol, and the compound passes from grapes into wine. This leads to the attractive possibility that wine may someday emerge as an effective anticancer agent. At about the same time as the report from Illinois, a Harvard group found that a diet rich in red tomatoes reduces the incidence of prostate cancer up to 34 percent. A parallel Italian investigation extended the benefit of tomatoes to other types of cancer as well.
The prospect of turning natural compounds to practical use has fueled much human activity for thousands of years. Where once trial and error guided the search for beneficial chemicals, now biology and chemistry set the course. Agricultural research and the quest for new drugs provide most of the supporting funds, but investigations extend beyond these immediate goals. The result is a lively interdisciplinary undertaking that attracts scientists from several backgrounds, some seeking marketable pharmaceutical or agricultural products and others pursuing more fundamental problems. In one way or another, these scientists are probing chemically mediated interactions between living organisms and their environments. Because relations between organisms and their environments are the subject matter of ecology, the interdisciplinary study of the chemically mediated interactions goes by the name of chemical ecology.
Our interest in the importance of chemicals in organisms' lives then falls into the area of chemical ecology, a subject particularly accessible to curious laypersons. Unlike some other fields of active research, chemical ecology's initial studies are often framed as straightforward and readily comprehensible questions, such as "How do Proto scouts guide a raiding party to a nest of Lepts?" Once you know the story of the Protos' raid on the Lepts, this question is a reasonable one, even if you know nothing about chemistry or biology. Such accessibility permits anyone to appreciate the sorts of questions one kind of research scientist hopes to answer. It is easy to see how the answer to one question can lead to new questions to investigate.
Despite chemical ecology's accessibility, knowledge has come slowly. Scientists are only now beginning to fit this knowledge into the broader structure of biology. Progress was slow for decades because interdisciplinary subjects, such as chemical ecology, were long neglected. Research was partitioned into traditional disciplines, with the result that, for years, chemistry and biology moved forward on separate tracks, leaving poorly explored much that lay between them. This partitioning has now largely disappeared, but the barriers were once substantial.
Progress in chemical ecology also awaited the development of suitable chemical techniques. Some plant products are readily available, but organisms make and use many ecologically important compounds in only tiny amounts, and then often mixed with closely related substances. Until about 1960, chemists simply lacked methods for identifying the components of such mixtures. Once scientists devised the proper tools, chemical ecology (and several related subjects) began to flourish. Over the years, the tools have continued to improve, and investigations have accelerated. For example, the first pheromone to be identified chemically was the sex attractant of the silkworm moth (Bombyx mori). Its identification in 1959 came after twenty years' effort and the dissection of hundreds of thousands of moths. Today, similar research often requires no more than several weeks' work with a few insects, largely as a result of improved tools.
As things now stand, we know something about the special chemicals of a very small fraction of the 1.4 million species of living organisms that scientists have formally described. Several major groups of creatures have received essentially no attention. Our knowledge is much more limited than even this suggests, because the formally described species represent only a very modest fraction of the total number that inhabit the earth. No one knows what this total number is, but biologists now talk seriously of tens of millions of living species, and some authorities suggest as many as one hundred million! We are only beginning to appreciate the extent of chemical interactions in the biological world and to elucidate the details of a small fraction of them.
A number of these interactions form the basis of the accounts in the following chapters. After a few final general comments, we can commence our exploration. Many of the activities we look into are clearly purposive; like the Protos' slave raids, they are directed toward a specific end. Such purposive activities may appear to demand considerable intelligence, but as a rule they are neither rational nor intentional. They result not from an organism's careful planning but represent built-in responses to particular situations and signals from the surroundings. These responses arise through the workings of evolution and render an organism more fit and better equipped to survive. Protos lay chemical trails, raid Lept colonies, and exhibit other extraordinary behaviors, but they do not lay battle plans or discuss strategy among themselves. My opinion is that this in no way detracts from their fascination, nor does it make their story any less engaging. For me, the very existence of Protos and Lepts is in itself astounding, and the same is true of many other creatures and interactions described in the chapters that follow.
In an earlier book on pheromones (Chemical Communication: The Language of Pheromones), I arranged the material biologically from microorganisms to humans, in a sweep of chemical signals across the biological world from simple to complex. In a broader discussion of special chemicals (Bombardier Beetles and Fever Trees), I organized the chemicals by function: pheromones, plant-defense chemicals, lifestyle chemicals, and so forth. In the current work, more than one kind of organism and several different signals appear in some of the accounts. The stories stand on their own and emphasize the accounts themselves rather than fit into a higher organizational structure. Some accounts are not yet complete and stop short of a full picture, although enough is known to make them worth relating now. In practice, a research scientist's investigations are like this more often than not.
Each organism's scientific name, that is, the Latin name of its genus (plural, genera) and species, appears when it is first mentioned, as seen with tobacco, wormwood, and several other organisms referred to above. Larger organizational groupings, such as family or phylum, occasionally appear by name, but I have kept such formalities to a minimum. Following the text, there is a glossary that includes comments on classification of organisms and units of measurement, as well as a list of suggested further reading.
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