Chemical defense
Chemical defense is a strategy employed by many organisms to avoid consumption by producing toxic or repellent metabolites or chemical warnings which incite defensive behavioral changes. The production of defensive chemicals occurs in plants, fungi, and bacteria, as well as invertebrate and vertebrate animals. The class of chemicals produced by organisms that are considered defensive may be considered in a strict sense to only apply to those aiding an organism in escaping herbivory or predation. However, the distinction between types of chemical interaction is subjective and defensive chemicals may also be considered to protect against reduced fitness by pests, parasites, and competitors. Repellent rather than toxic metabolites are allomones, a sub category signaling metabolites known as semiochemicals. Many chemicals used for defensive purposes are secondary metabolites derived from primary metabolites which serve a physiological purpose in the organism. Secondary metabolites produced by plants are consumed and sequestered by a variety of arthropods and, in turn, toxins found in some amphibians, snakes, and even birds can be traced back to arthropod prey. There are a variety of special cases for considering mammalian antipredatory adaptations as chemical defenses as well.
Prokaryotes and fungi
Bacteria of the genera Chromobacterium, Janthinobacterium, and Pseudoalteromonas produce a toxic secondary metabolite, violacein, to deter protozoan predation. Violacein is released when bacteria are consumed, killing the protozoan. Another bacteria, Pseudomonas aeruginosa, aggregates into quorum sensing biofilms which may aid the coordinated release of toxins to protect against predation by protozoans. Flagellates were allowed to grow and were present in a biofilm of P. aeruginosa grown for three days, but no flagellates were detected after seven days. This suggests that concentrated and coordinated release of extracellular toxins by biofilms has a greater effect than unicellular excretions. Bacterial growth is inhibited not only by bacterial toxins, but also by secondary metabolites produced by fungi as well. The most well-known of these, first discovered and published by Alexander Fleming in 1929, described the antibacterial properties of a "mould juice" isolated from Penicillium notatum. He named the substance penicillin, and it became the world's first broad-spectrum antibiotic. Many fungi are either pathogenic saprophytic, or live within plants without harming them as endophytes, and many of these have been documented to produce chemicals with antagonistic effects against a variety of organisms, including fungi, bacteria, and protozoa. Studies of coprophilous fungi have found antifungal agents which reduce the fitness of competing fungi. In addition, sclerotia of Aspergillus flavus contained a number of previously unknown aflavinines which were much more effective at reducing predation by the fungivorous beetle, Carpophilus hemipterus, than aflatoxins which A. flavus also produced and it has been hypothesized that ergot alkaloids, mycotoxins produced by Claviceps purpurea, may have evolved to discourage herbivory of the host plant.Lichen
Lichens demonstrate chemical defenses similar to those mentioned above. Their defenses act against herbivores and pathogens including bacterial, viral, and fungal varieties. To that end, a variety of chemicals are produced by the lichen's mycobiont via hydrocarbons produced by the lichen's photobiont. However, a single defensive chemical may serve multiple purposes. Usnic acid, for example, is implicated across anti-bacterial, -viral, and -fungal actions. Such defensive chemicals may be stored in various tissue types of the lichen thallus, or they may accumulate on the mycobiont hyphae as extracellular crystals.Mycobiont-produced acids, including but not limited to, evernic, stictic, and squamatic acids exhibit allelopathy, more specifically, lichen defensive chemicals may inhibit a primary metabolic pathway within competing lichens, mosses, microorganisms, and vascular plants. Documented allelopathic targets include jack pine, white spruce, and garden variety tomato, cabbage, lettuce, and pepper plants. Antimicrobial efforts of lichen are also mediated by various mycobiont-produced acids such as lecanoric and gyrophoric. Similar defensive chemicals were found to inhibit herbivores and insects. Some of these lichen defensive compounds show pharmaceutical potential, too.
In 2004 the death of hundreds of elk near Rawlins, Wyoming was linked to consumption of tumbleweed shield lichen . This strangely powerful chemical defense is irregular given that such poisoning is very rare while the consumption of this lichen is fairly regular.
Plants
A wealth of literature exists on the defensive chemistry of secondary metabolites produced by terrestrial plants and their antagonistic effects on pests and pathogens, likely because human society depends upon large-scale agricultural production to sustain global commerce. Since the 1950s, over 200,000 secondary metabolites have been documented in plants. These compounds serve a variety of physiological and allelochemical purposes, and provide a sufficient stock for the evolution of defensive chemicals. Examples of common secondary metabolites used as chemical defenses by plants include alkaloids, phenols, and terpenes. Defensive chemicals used to avoid consumption may be broadly characterized as either toxins or substances reducing the digestive capacity of herbivores. Although toxins are defined in a broad sense as any substance produced by an organism that reduces the fitness of another, in a more specific sense toxins are substances which directly affect and diminish the functioning of certain metabolic pathways. Toxins are minor constituents, active in small concentrations, and more present in flowers and young leaves. On the other hand, indigestible compounds make up to 60% dry weight of tissue and are predominately found in mature, woody species. Many alkaloids, pyrethrins, and phenols are toxins. Tannins are major inhibitors of digestion and are polyphenolic compounds with large molecular weights. Lignin and cellulose are important structural elements in plants and are also usually highly indigestible. Tannins are also toxic against pathogenic fungi at natural concentrations in a variety of woody tissues. Not only useful as deterrents to pathogens or consumers, some of the chemicals produced by plants are effective in inhibiting competitors as well. Two separate shrub communities in the California chaparral were found to produce phenolic compounds and volatile terpenes which accumulated in soil and prevented various herbs from growing near the shrubs. Other plants were only observed to grow when fire removed shrubs, but herbs subsequently died off after shrubs returned. Although the focus has been on broad-scale patterns in terrestrial plants, Paul and Fenical in 1986 demonstrated a variety of secondary metabolites in marine algae which prevented feeding or induced mortality in bacteria, fungi, echinoderms, fishes, and gastropods. In nature, pests are a severe problem to plant communities as well, leading to the co-evolution of plant chemical defenses and herbivore metabolic strategies to detoxify their plant food. A variety of invertebrates consume plants, but insects have received a majority of the attention. Insects are pervasive agricultural pests and sometimes occur in such high densities that they can strip fields of crops.Animals
Terrestrial arthropods
There are many strategies terrestrial arthropods employ in terms of chemical defense. The first of these strategies include the direct use of secondary metabolites. Many insects are distasteful to predators and excrete irritants or secrete poisonous compounds that cause illness or death when ingested. Secondary metabolites obtained from plant food may also be sequestered by insects and used in the production of their own toxins. One of the more well-known examples of this is the monarch butterfly, which sequesters poison obtained from the milkweed plant. Among the most successful insect orders employing this strategy are beetles, grasshoppers, and moths and butterflies. Insects also biosynthesize unique toxins, and while sequestration of toxins from food sources is claimed to be the energetically favorable strategy, this has been contested. Passion-vine associated butterflies in the tribe Heliconiini either sequester or synthesize de novo defensive chemicals, but moths in the genus Zygaena have evolved the ability to either synthesize or sequester their defensive chemicals through convergence. Some coleopterans sequester secondary metabolites to be used as defensive chemicals but most biosynthesize their own de novo. Anatomical structures have developed to store these substances, and some are circulated in the hemolymph and released associated with a behavior called reflex bleeding.The use of chemical alarms and detection is another strategy of chemical defense. Identifying predators and responding swiftly and appropriately is advantageous and leads to higher fitness. These defensive responses can include avoidance and escape responses, safeguarding offspring, aggressive behaviors, and applying "direct defenses". For example, the fruit fly can chemically detect a nearby parasitoid and halt its egg-laying. Delaying oviposition can reduce the risk of predation and falls under the category of protecting offspring. The spider mite can respond to predator volatiles in the environment and will choose to feed in areas without predator cues. Similarly, spider mites are also able to sense damaged body parts of individuals of the same species, or conspecifics, and present the same avoidance behavior as with predator cues. Furthermore, spider mites exhibit a similar behavior with egg-laying as the fruit fly and will elect to move to areas absent of predator cues before oviposition. Spider mites will not avoid areas with other, non-predator volatiles meaning these organisms are able to chemically distinguish threats from non-threats. Parasitic wasps also sense volatiles of their predator, a hyperparasitoid, and fly to new areas devoid of the chemical cues, displaying similar avoidance behaviors as the spider mite.
Alternately, chemical detection of predators or threats can instigate aggressive behaviors in some terrestrial arthropods, rather than escape and avoidance behaviors. Polybia paulista, a vespid wasp, is a social species that forage and defend according to complex social structures. These wasps have evolved to detect pheromones in the venom of members of the same species. Identifying volatiles from the venom of conspecifics allows the vespid wasps to discern a nearby threat. When detected, these pheromones induce an attacking behavior within members of the same species. These wasps will then work together to defeat the threat. Similarly, honeybees release a warning pheromone when threatened. These pheromones intensify the honeybees' defenses by increasing the duration of the stinging behavior in all nearby honeybees.
Aphids, small insects that can be found feeding on the sap of plants, exhibit many strategies in terms of chemical defense. Aphids have structures called cornicles along the posterior side of their abdomen which are used to deliver secretions containing both volatile and nonvolatile compounds. Volatile compounds serve primarily as alarm pheromones. Pheromones are chemicals released from one individual that elicit a response from another. Nonvolatile compounds, such as wax, are used as noxious adhesives that the aphid will smear on their enemies. These smears are used to fatally bind predators' mouthparts, antennas, legs, etc., meaning these compounds are typically used more for physical defense rather than chemical. Pea aphids produce a warning chemical called -β-farnesene which is excreted as a volatile compound in the presence of predators or perceived threats. In many cases, the aphid will respond by leaving the feeding site in search of an area without alarm pheromones. Additionally, pea aphids are highly attune to which predators are in their area as they can chemically identify what is posing as a threat and adjust their response accordingly. For example, pea aphids can identify Adalia bipunctata, the ladybird beetle, by their chemical predator cues. After sensing this predator, pea aphids are known to produce more offspring with wings. The winged offspring are able to better avoid predation; however, winged individuals are less fertile. This trade-off between wings and fertility shows the success of this particular defensive strategy. In "relaxed" conditions, or conditions in which predator cues are absent, more wing-less offspring are produced.
The chemical defense systems of aphids are highly specific. -β-farnesene, the alarm pheromone discussed above, is used by many species of aphids. When released, -β-farnesene will only extend 2-3 centimeters in diameter. This protects farther conspecifics from the alarm chemical so they do not experience any needless pause in feeding or respond unnecessarily. Furthermore, these chemical alarms are detected by structures on the antennae of aphids that utilize specialized binding proteins. Warning chemicals must accumulate to a certain minimum within the binding proteins before a response is produced. These factors are used to highlight the specificity of the chemical defense systems of aphids. Moreover, the chemical warnings used are also highly specific and the method in which the alarm pheromone is distributed can elicit different responses. For example, Ceratovacuna lanigera, the sugarcane wooly aphid, has two methods of distribution of alarm pheromones. When threatened, the alarm pheromones can either be released as a droplet or as a smear. When the alarm is released as a droplet from the aphid's cornicle, the local conspecifics will respond individually and will either avoid or escape the area. However, when alarm pheromones are spread on a predator, other members of the same species will launch a joint attack. As discussed above, waxy cornicle smears are typically used to physically defend an aphid from a predator. In this case, however, the chemical alarms in the wax are eliciting a behavioral change; therefore, this particular strategy can be considered chemical defense.
Other organisms have been able to take advantage of the elaborate chemical defenses of aphids to increase their own fitness. Chemical mimicry is powerful tool in terms of chemical defense. Lysiphlebus fabarum, a parasitoid of aphids, is able to mimic the chemical secretions of specific aphids when infiltrating their colonies. This mimicry serves as a "chemical camouflage" and protects these parasitoids as they go undetected within aphid colonies. Chrysopa glossonae, a lacewing, uses the wax of the woolly alder aphid to chemically disguise itself from formicine ants who have learned to avoid attacking the aphid. This means that nearby formicine ants will ignore the lacewing as it would the wooly alder aphid. This is another instance where waxy secretions are used for chemical defense rather than physical.