Constraints on effectiveness of cyanogenic glycosides in herbivore defense

Cyanogenesis is the process by which hydrogen cyanide is released from endogenous cyanide containing compounds. Many cyanogenic plants release HCN in sufficient quantities to be toxic and, as a result, tend to be avoided by herbivores. However, there are many exceptions with some herbivores either immune to the cyanogenic status of the plant, or in some cases attracted to cyanogenic plants. This has led to a certain degree of scepticism regarding the role of cyanogenic glycosides as defense compounds. In this review, we examine evidence showing that differences in the effectiveness of cyanogenic glycosides in deterring herbivory can usually be reconciled when the morphology, physiology, and behavior of the animals, together with the concentration of cyanogenic glycosides in the host plant, are taken into account. Cyanogenic glycosides are not effective against all herbivores, and not all cyanogenic plants release enough cyanide to be considered toxic. Nevertheless, they do form part of the broad spectrum of toxic and distasteful compounds that herbivores must accommodate if they are to feed on cyanogenic plants.     

Cyanogenesis—a general phenomenon in the lepidoptera?

There are two different pathways known to be used for the detoxification of hydrocyanic acid in insects, viz., rhodanese and -cyano-l-ala-nine synthase. We consider the latter to be indicative for cyanogenesis, while rhodanese might, in general, play a more important role in sulfur transfer for protein synthesis. This paper reports on the distribution of -cyano-l-alanine (BCA) in the Lepidoptera. First reports of cyanogenesis are presented for the following families: Papilionidae, Pieridae, Lycaenidae, Hesperiidae, Lymantriidae, Arctiidae, Notodontidae, Megalopygidae, Limacodidae, Cymatophoridae, Noctuidae, Geometridae, and Yponomeutidae. New and old records for three other families, the Nymphalidae, Zygaenidae, and Heterogynidae, are included to complete the present state of knowledge. Special emphasis has been laid on the Nymphalidae, where BCA has been detected in eight subfamilies. Taxonomic, geographic, and seasonal variation has been found in a number of cases. In all cases observed so far, the source of cyanogenesis in the Lepidoptera is most probably the cyanoglucosides linamarin and lotaustralin, although cyanogenesis based on mustard oil glucosides and cyclopentenoid glucosides might occur as well. BCA has been found in both cryptic and aposematic species, including taxa such as the Pieridae, Danainae, Ithomiinae, and Arctiidae, where the defensive biology is believed to be linked with other compounds, like mustard oil glucosides, cardenolides, or pyrrolizidine alkaloids. The ecological interaction and significance of such secondary compounds is not yet understood.     

Benzoyl cyanide and mandelonitrile benzoate in the defensive secretions of millipedes

Analyses of the defensive secretions of 17 species of polydesmoid millipedes show that other chemicals besides HCN and benzaldehyde are liberated during cyanogenesis. Several members of the families Polydesmidae, Paradoxosomatidae, and Euryuridae are shown to secrete both phenol and guaiacol, with one paradoxosomatid also producing ethyl benzoate and benzoic acid. Also, members of the family Xystodesmidae commonly produce the three following compounds: benozoic acid, mandelonitrile benzoate, and benzoyl cyanide. Benzoyl cyanide has not been found previously as a natural product. The defensive role of these additional natural products as antipredator and antibiotic agents is discussed. For certain predators benzoyl cyanide in particular seems to possess anaesthetic properties. Our studies provide an initial chemotaxonomic basis for distinguishing between various polydesmoid taxa.     

Benzoyl cyanide and mandelonitrile in the cyanogenetic secretion of a centipede

Centipedes of the order Geophilomorpha produce a viscid odorous secretion that is effectively defensive against ants (Formica exsectoides). The secretion is proteinaceous and cyanogenetic. InGeophilus vittatus, the secretion contains two cyanogenetic compounds, mandelonitrile (I) and benzoyl cyanide (II), as well as two products derived from these compounds as a result of hydrogen cyanide production [benzaldehyde (III) and benzoic acid (IV)]. Benzoyl cyanide has not been reported previously from any natural source.This is Paper No. 48 in the seriesDefense Mechanisms of Arthropods.     

Rhodanese in insects

Forty-four species of insects were assayed for the presence of rhodanese, an enzyme generally considered to be responsible for the detoxification of cyanide. Rhodanese was found to be widely distributed in both adults and larvae and was not restricted to those species which encounter exogenous cyanide through feeding on cyanogenic plants. These results indicate that cyanide detoxification is unlikely to be the primary role for rhodanese in insects.     

Preference for Acyanogenic White Clover (Trifolium Repens) in the Vole Arvicola Terrestris. II. Generalization and Further Investigations

Two series of cafeteria tests involving six varieties of Trifolium repens were conducted to confirm the preference of fossorial Arvicola terrestris for acyanogenic white clover (T. repens) and to investigate their responses to repeated exposures to toxic plants. The animals were offered the choice between two bunches of freshly harvested white clover. In 261 simple choice tests, 160 individuals were tested for various combinations of three acyanogenic and three cyanogenic varieties. They showed a clear preference for the acyanogenic morphs, both in comparisons between opposite cyanotypes and in two control situations involving either self-comparisons or comparisons between similar cyanotypes. In 320 repeated choice tests, 40 voles assigned to three groups (comparisons between opposite cyanotypes and control comparisons between similar cyanotypes) were tested eight times for the same alternative. They showed a persistent preference for the acyanogenic morph and significantly reduced their total food consumption when they had a strictly cyanogenic diet. In the latter case, they stored or wasted more plant material than in the other treatments. These voles also ingested high amounts of cyanide which suggests good detoxification abilities and a possible habituation to cyanide.     

Mechanisms of Extraction of Aroma Compounds from Foods,Using Adsorbents. Effect of Various Parameters

Due to their high adsorbing capacity, different solids (carbonaceous adsorbents, zeolites, and polymers) are used to extract and concentrate aroma compounds from foods. Adsorption mechanisms are described in two phases, a kinetic stage that involves the diffusion of the analytes within the adsorbent pores and a thermodynamic stage that can be described by adsorption isotherms. These two phases determine the extraction time and the capacity of the adsorbent for a given aroma compound. Several applications involving adsorbents or sorbents have been developed, including purge and trap, solid-phase microextraction (SPME), and stir bar sorptive extractions (SBSE), headspace sorptive extraction (HSSE), which now are widely used for aroma extractions. Different extraction modes (in the headspace or by immersion in the sample) can be used to recover aroma compounds in foods. The same adsorption mechanisms take place in both cases. Various parameters affecting the extraction kinetics and the capacity of the adsorbent have to be optimized when developing an extraction method. They can be divided into three groups: physicochemical characteristics of the adsorbent, including its porosity and hydrophobia; physicochemical characteristics of the aroma compounds (among them, two parameters play an important role, the sample/adsorbent partition coefficient, which determines the affinity for the adsorbent and the volumetric mass, which influences the diffusion in the adsorbent pores); and the extrinsic parameters that depend on the sampling conditions such as pH, temperature, gas or solvent flows, time, and composition of the sample. Several models used to determine the diffusivity and partition coefficient of aroma between the food sample and the adsorbents have been developed. They are useful to understand the behavior of aroma compounds in regards to various adsorbents and in selecting the adsorbent material with the greatest affinity for the target aroma compounds. Quantitative studies based on adsorbent extraction of volatiles have been carried out. Nevertheless, competition between analytes and the saturation of these materials are limiting parameters. Thus, new strategies (SBSE, HSSE) are being developed to avoid these limiting aspects of adsorbents in order to use them in a quantitative way.     

Cyanogenesis in Canned Stone Fruits

The amount of cyanogenic glycosides in seeds and pulp of stone fruits, and the HCN content in heat processed unstoned fruit have shown that the decisive factors affecting HCN concentration in canned unstoned fruits are the glycoside content of raw fruits and the conditions of heat processing. Enzymatic hydrolysis of glycosides during canning is the main source of HCN in fruit products. HCN contents found in canned fruits (0-4 mg-kg^?1) were not acutely toxic, but are not negligible.     

Effects of Cyanogenesis Polymorphism in Turnera ulmifolia on Euptoieta hegesia and Potential Anolis Predators

We examine the effects of the cyanogenesis polymorphism in Turnera ulmifolia on larvae, pupae, and adults of Euptoieta hegesia, the most damaging herbivore of T. ulmifolia in terms of tissue loss per unit time. We provide evidence that female E. hegesia do not show preference for host plants on the basis of their cyanogenesis level but do prefer T. ulmifolia over equally cyanogenic, closely related secondary host-plant species (Passiflora sp.). Similarly, cyanogenesis in T. ulmifolia has little effect on the food preference, growth, or development of the larvae. The potential host range of E. hegesia is limited, even within the genus Turnera, but this does not appear to be due to host-plant cyanogenesis. Pupae suffer very high mortality levels in the wild that are not associated with host-plant cyanogenesis, although our studies indicate that larvae are capable of sequestering cyanogenic glycosides from their host plants and possibly of synthesizing these or similar compounds. We provide evidence that the presence of sequestered cyanogenic compounds in the larvae protects them from terrestrial-based predators such as Anolis lizards     

Sapindaceae,cyanolipids, and bugs

Scentless plant bugs (Heteroptera: Rhopalidae) are so named because adults of the Serinethinae have vestigial metathoracic scent glands. Serinethines are seed predators of Sapindales, especially Sapindaceae that produce toxic cyanolipids. In two serinethine species whose ranges extend into the southern United States,Jadera haematoloma andJ. sanguinolenta, sequestration of host cyanolipids as glucosides renders these gregarious, aposematic insects unpalatable to a variety of predators. The blood glucoside profile and cyanogenesis ofJadera varies depending on the cyanolipid chemistry of hosts, and adults and larvae fed golden rain tree seeds (Koelreuteria paniculata) excrete the volatile lactone, 4-methyl-2(5H)-furanone, to which they are attracted.Jadera fed balloon vine seeds (Cardiospermum spp.) do not excrete the attractive lactone. Loss of the usual heteropteran defensive glands in serinethines may have coevolved with host specificity on toxic plants, and the orientation ofJadera to a volatile excretory product could be an adaptive response to save time.Mention of a commercial product does not consititute an endorsement by the USDA.