H OH OH
Formic acid Acetic acid 2 3
Figure 2 Chemical defense compounds of beetles.
invertebrate and vertebrate predators. From Epilachna sp., the azamacrolide epilachnene 6 and other macrocyclic polyamines with variable ring sizes have been characterized as predator-deterring principles. Probably, the 'combinatorial chemistry' of azamacrolides helps the insect to be optimally defended against a variety of predators. Also pupae of the ladybird Subcoccinella vigintiquatuorpunctata produce cyclic macrolides with variable ring sizes, for example, 7, as a defense cocktail that is stored in glandular hairs.
Some beetles produce toxic steroids to deter attackers. Thus, the antifeedant steroid-derived lucibufagin C provides fireflies of the genus Photinus protection, for example, against attack by spiders. Lucibufagin C 8 is structurally related to the cardenolides from plants (see Plant Defense Strategies).
The larvae of leaf beetles (Chrysomelidae) have pairs of eversible glands along their body which they expose if attacked, thus confront the enemy with repelling chemicals such as the aromatic salicylaldehyde 9 or the terpenoid iridoids such as plagiolactone 10 and chry-somelidial 11 (Figure 3).
Interestingly, the chemical compounds used for defense by leaf beetles are variable, with some species relying on de novo synthesis and others on sequestration. For example, salicin, a glucoside of salicylic alcohol, is taken up by the leaf beetle Chrysomela populi from popular leaves and activated in the defensive gland to salicylalde-hyde 9 (see section on sequestration; Figure 4). Iridoids such as chrysomelidial 11 can be synthesized de novo by the insect or are taken up from plants. The dialdehydes readily add to proteins in the course of Michael reactions and destroy their function.
Gyrinidal 12 of the defensive secretion from gyrinid water beetles reacts similarly. The norsesquiterpene contains in addition to three reactive oxo groups a Michael
acceptor system. As a consequence, fish usually reject this water beetle as food source.
The terpenoid-derived tricyclic cantharidin 13 serves Meloidae and Oedemeridae beetles as defense against predators (e.g., ants). The beetles accumulate the toxin in their hemolymph and react with reflex bleeding upon provocation. Cantharidin 13 is a highly efficient inhibitor of protein phosphatases. It massively disturbs cellular regulation, indirectly affecting receptors and ion channels.
The Kenya fly beetles (Pederus sp.) are dreaded since they cause severe skin blisters. The active principal is pederin 14, a complex polyketide amide that blocks the protein biosynthesis. Interestingly, pederin 14 is exclusively found in female beetles. It is produced by symbiotic microorganisms (see Marine and Aquatic Defense Strategies).
A consequent step of many specialists that evolved mechanisms to detoxify toxins is their sequestration. Toxins are stored after uptake with the food and used for the organism's own defense. In order to protect oneself, the sequestering organism stores the ingested toxin as an inactive precursor that is rapidly converted to the poisonous compound when needed.
Sequestration appears to be a very common strategy among both terrestrial and marine organisms (see Defense Strategies of Marine and Aquatic Organisms). Some species have been studied in great detail, for example, leaf beetles and pyrrolizidine alkaloid sequestering lepidopterans. The sequestration process of the leaf beetle larvae Chrysomela populi is outlined above: Salicin is taken up from poplar trees, absorbed from the gut into the hemolymph, and is then transferred from the hemolymph into the reservoir of the defensive gland. Chrysomela populi needs an efficient uptake system from the gut (transporter 1), an uptake system into the reservoir (transporter 2), and enzymes (a glucosidase and an oxidase) that quickly transform the sequestered plant compound into the defensive compound salicylaldehyde 9 in case of attack (Figure 4).
The frequent observation that many species use the same or slightly modified toxins as their prey suggests a sequestration mechanism (see section on amphibians and pumiliotoxin A 25). Sequesters save costs for the production of own defense products and the generation of necessary enzymes for their biosynthesis. Probably this explains why it is a widespread defense principle. As a drawback, sequestration forces adaptation to new environmental conditions, for example, if common food plants are missing as sources of toxins. Broad substrate specificities as well as the potential to produce own toxins de novo are efficient countermeasures against this problem. Prominent examples for the evolution of adaptation to host plants can be followed within leaf beetles where some species were recognized as de novo producers, some as sequesters, and others show both de novo production and sequestration of defensive compounds.
Caterpillars are soft-bodied organisms which suffer fatal wounds easily. Therefore, many caterpillars are chemically protected (Figure 5). A very common defense reaction of lepidopteran larvae is regurgitation of their foregut content that they spread over their bodies. The caterpillar thus becomes slippery which increases its chance to escape. The high amount of biosurfactants such as A?-acyl amino acids, for example, volicitin 15, in the regurgitate of many lepidopterans ideally supports
defense _ compound
Figure 4 Schematic diagram of sequestration.
Senecionine N-oxide 23
Figure 5 Defensive chemicals used by lepidopterans/hymenopterans.
this protection mechanism. Additionally, the gut fluid exhibits a highly distasteful pH of 9—11.
Besides this behavior, some lepidopterans also have specialized defensive organs. For example, the European cabbage butterfly (Pieris rapae) releases deterrent droplets from glandular hair containing long-chain hydroxy fatty acids esterified with long-chain fatty acids, so-called mayolenes such as 16 that have been shown to deter ants. Swallowtails (Papilionidae) defend themselves with a glandular defensive organ on their head (osmeterium). When disturbed, the swallowtail moves this organ against the aggressor and releases a deterrent mix of chemicals comprising of often short-chain organic acids such as isobutyric acid 17.
The thyridid larvae Calindoea trifscialis have specialized arm-like structures that they use to release HCN 20 after provocation. The larvae store the highly toxic gas in form of mandelonitrile 18 similar to other HCN-using arthropods.
Other specialized lepidopteran larvae such as the monarch butterfly (Danauus plexippus) sequester toxic and distasteful cardenolides such as calotropin 21 from their food plant. If a bird tries to consume the monarch butterfly, it regurgitates its victim nearly immediately, and never tries to feed on such an insect again. In the same way, pyrrolizidine alkaloids, for example, senecionine 22, are taken up from the food plant and serve specialist moths like Tyria jacobaeae as protection against predators.
Tyria jacobaeae takes up the pretoxin senecionine A-oxide, for example, 23 from the food. Senecionine A-oxide 23 is converted in the gut to the toxic senecionine 22. Tyria jacobaeae larvae take pyrrolizidine up from the gut into their hemolymph. There the specialist larvae oxidize the toxic pyrrolizidine back to the safe A-oxide and store the pretoxin, for example, 23.
Similar to lepidopteran larvae, hymenopterans use regurgitation in order to render themselves unattractive to enemies. Particularly interesting is the sawfly (Aeodiprion sertifer) that feeds on Scots pine (Pinus sylvestris). Aeodiprion sertifer not only manages to tolerate the sticky tree resin containing a blend of terpenoids but is able to store a portion of these and use them for its own defense.
Eggs of arthropods are prone to be eaten by predators or attacked by parasites and microorganisms. Their defense requires diverse strategies such as mass production, hiding, and chemical defense. Often eggs are hidden in plant material or earth and are covered with hairs, feces, silk, or sticky secretions. Chemical defenses of eggs usually reflect the same toxins as the adults use for their own protection, for example, pederin 14, cantharidin 13, anthraquinones, cyanogenic glycosides, pyrrolizidine alkaloids, histamine, aldehydes, and cardenolides.
Fascinating strategies evolved on how to transfer the toxins to the eggs. In some cases, defensive compounds are donated by the male as gifts before (overt gifts) or during mating (covert gifts) to the female that uses the toxin from the male to protect the eggs. For example, Meloidae beetles have a complex courtship behavior during which the female probes for the cantharidin 13 content offered by the male before the female allows mating. In case of the fly Drosophila melongaster, there is evidence that males produce the antibacterial peptide andropin exclusively in their ejaculatory tract and transfer it to the female during mating.
However, being well protected can also turn out to be a drawback since some predators are attracted by the toxin or even cannibalism might be promoted. For example, eggs of cantharidin 13 containing Meloidae are highly attractive for cantharidiphilic predators.
Besides insect egg defense, defense and protection of eggs in early life stages is particularly important for marine organisms that use analogous strategies - hiding, mass production, and chemical defense.
The life of social insects such as ants is strongly dependent on their queen. Therefore, many ant species protect their queen deep inside their nest, so that she is far from the reach of predators. Additionally, ants coordinate defense by attacking aggressors together. Ants actively bite and spray their defensive secretion formed in a poison gland. The defense compounds (Figure 6) of many ants are surprisingly simple such as formic acid 2, yet efficient to deter other insects or even bigger animals. The fire ants (Solenopsis invicta) use proteins as defense compounds besides piperidine alkaloids such as isosole-nopsin (2-methyl-6-undecylpiperidine 24). Isosolenopsin 24 is a selective inhibitor of the nitric oxide synthase. This may account for the severe systematic toxic effects of the venom as indicated by occasional reports about death of humans stung by these ants. Indolizidine and quinolizidine alkaloids such as pumiliotoxin A 25 occur in myrmicine ants exerting their defense potential by interaction with sodium channels.
Leaf-cutting ants have been found to spray the powerful antimicrobial myrmicacin (d-3-hydroxy-decanoic acid 26) from their metathoracic glands in order to keep their nest and especially the fungus they feed on free from bacteria.
Ancistrodial 27 derived from the defense glands of termites (Ancistrotermes cavithorax) was characterized as a highly reactive a,/3-unsaturated dialdehyde. This dialde-hyde is efficient to deter their major enemy, the Matabele ant Megaponera foetens. Compound 27 shares structural elements - the dialdehyde moieties and the Michael-acceptor system - with many other defensive compounds. The highly reactive dialdehydes inactivate proteins.
Wasps and bees are examples for actively defended animals having a venom gland and a sting apparatus. Wasps such as Philanthus triangulum use amides in which a polyamine is connected to tyrosine as venoms such as philanthotoxin-433 (28) (Figure 7). The venom acts as antagonist of ionotrophic receptors such as the acetylcholine receptor. In contrast to bees, wasps do survive after stinging an aggressor. Bees introduce their barbed sting into the aggressor, where it becomes fixed introducing more and more venom from the poison gland. Bee venom is a complex mixture of peptides, proteins, and histamine. Among the proteins are melittin (26 amino acids), a phospholipase A2, a hyaluronidase, and apamin (18 amino acids). In addition to the toxin, bees also release volatiles, mainly isoamylacetate 29 (Figure 7), that act as an alarm pheromone warning to other bees, which thus become highly aggressive.
Millipede and Centipede Defense
Millipedes and centipedes are well-defended ancient organisms (Figure 8). Millipedes show protective behavior by
Figure 6 Defensive compounds of ants and termites.
Figure 7 Compounds involved in chemical defense of wasps and bees.
2-Hydroxy-3-methyl-1, Buzonamine Glomerin
4-benzoquinone 31 32
Figure 8 Chemical defense of millipedes and centipedes.
typically coiling together and only using their chemical defenses if further provoked. Centipedes generally appear to be more toxic and more flexible than millipedes. As an extreme example, the agile and fast giant Sonoran centipede (Scolopendra heros), reaching sizes of up to 30 cm, bites with two sharp poison fangs. Although the fangs serve mainly for hunting prey, they are also an efficient defense organ. The venom consists of a complex mixture containing proteases and biogenic amines.
Besides many centipedes, also millipedes are chemically protected with pairwise poison glands along their bodies. These end in micro-openings through which sticky fluids together with the highly poisonous HCN 20 can be emitted upon attack. The HCN 20 is stored similarly to other arthropods in form of mandelonitrile 18. Enzymatic cleavage results in release of HCN 20 similar to defense reactions of cyanogenic plants (see Plant Defense Strategies). Alternatively, deterrents such as 1,4-benzoquinones, for example, 2-hydroxy-3-methyl-1,4-benzoquinone 30 - which are also common defensive secretions of many insects - are released by millipedes. The polyzoniid millipede produces structurally unique alkaloids, for example, buzonamine 31, that are efficient deterrents against attacking ants. Quinazolinones such as glomerin 32 are used by Glomeris marginata millipedes to paralyze potential attackers such as spiders.
The venoms of spiders are usually tailored to immobilize insects - their main food. Although spiders use their poison predominantly in order to catch their prey, it can serve them also for defense purposes. For example, the green lynx spider (Peucetia viridans) sprays its venom upon provocation. Spider toxins usually consist of complex blends of proteins together with small molecules often targeting ion channels. The powerful potential of the toxins of some spider species against attack is demonstrated by unintentional encounters of humans with spider toxins.
Besides their chitin shield and strong pinchers, scorpions are actively defended with their sting apparatus and venom glands at the end of their tail. Clearly, scorpions use their venoms mainly to catch prey and apply it only as ultima ratio for defense. This is indicated by the defensive behavior of scorpions that show their sting and do not attack immediately. The poison such as in spiders consists of a mixture of peptides, proteins, and low molecular weight compounds that interfere with ion channel signaling. Interestingly, scorpions use a prevenom for the first sting. It causes pain and paralysis and differs from the venom injected afterward. The prevenom blocks ion channels and at the same time contains high amounts of potassium ions which are the basis for the pain of the first sting.
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