Properties of ouabain-resistant ATPase from the excretory system of Poekilocerus bufonius

Convergence and Divergence among Herbivorous Insects Specialized on Toxic Plants: Revealing Syndromes among the Cardenolide Feeders across the Insect Tree of Life

AbstractRepeatable macroevolutionary patterns provide hope for rules in biology, especially when we can decipher the underlying mechanisms. Here we synthesize natural history, genetic adaptations, and toxin sequestration in herbivorous insects that specialize on plants with cardiac glycoside defenses. Work on the monarch butterfly provided a model for evolution of the "sequestering specialist syndrome," where specific amino acid substitutions in the insect's Na+/K+-ATPase are associated with (1) high toxin resistance (target site insensitivity [TSI]), (2) sequestration of toxins, and (3) aposematic coloration. We evaluate convergence for these traits within and between Lepidoptera, Coleoptera, Diptera, Hemiptera, Hymenoptera, and Orthoptera, encompassing hundreds of toxin-adapted species. Using new and existing data on ∼28 origins of specialization, we show that the monarch model evolved independently in five taxonomic orders (but not Diptera). An additional syndrome occurs in five orders (all but Hymenoptera): aposematic sequesterers with modest to medium TSI. Indeed, all sequestering species were aposematic, and all but one had at least modest TSI. Additionally, several species were aposematic nonsequesterers (potential Batesian mimics), and this combination evolved in species with a range of TSI levels. Finally, we identified some biases among these strategies within taxonomic orders. Biodiversity in this microcosm of life evolved repeatedly with a high degree of similarity across six taxonomic orders, yet we identified alternative trait combinations as well as lineage-specific outcomes.

Defense of Milkweed Bugs (Heteroptera: Lygaeinae) against Predatory Lacewing Larvae Depends on Structural Differences of Sequestered Cardenolides

Predators and parasitoids regulate insect populations and select defense mechanisms such as the sequestration of plant toxins. Sequestration is common among herbivorous insects, yet how the structural variation of plant toxins affects defenses against predators remains largely unknown. The palearctic milkweed bug Lygaeus equestris (Heteroptera: Lygaeinae) was recently shown to sequester cardenolides from Adonis vernalis (Ranunculaceae), while its relative Horvathiolus superbus also obtains cardenolides but from Digitalis purpurea (Plantaginaceae). Remarkably, toxin sequestration protects both species against insectivorous birds, but only H. superbus gains protection against predatory lacewing larvae. Here, we used a full factorial design to test whether this difference was mediated by the differences in plant chemistry or by the insect species. We raised both species of milkweed bugs on seeds from both species of host plants and carried out predation assays using the larvae of the lacewing Chrysoperla carnea. In addition, we analyzed the toxins sequestered by the bugs via liquid chromatography (HPLC). We found that both insect species gained protection by sequestering cardenolides from D. purpurea but not from A. vernalis. Since the total amount of toxins stored was not different between the plant species in H. superbus and even lower in L. equestris from D. purpurea compared to A. vernalis, the effect is most likely mediated by structural differences of the sequestered toxins. Our findings indicate that predator-prey interactions are highly context-specific and that the host plant choice can affect the levels of protection to various predator types based on structural differences within the same class of chemical compounds.

New ways to acquire resistance: imperfect convergence in insect adaptations to a potent plant toxin

Evolution of insensitivity to the toxic effects of cardiac glycosides has become a model in the study of convergent evolution, as five taxonomic orders of insects use the same few similar amino acid substitutions in the otherwise highly conserved Na,K-ATPase α. We show here that insensitivity in pyrgomorphid grasshoppers evolved along a slightly divergent path. As in other lineages, duplication of the Na,K-ATPase α gene paved the way for subfunctionalization: one copy maintains the ancestral, sensitive state, while the other copy is resistant. Nonetheless, in contrast with all other investigated insects, the grasshoppers' resistant copy shows length variation by two amino acids in the first extracellular loop, the main part of the cardiac glycoside-binding pocket. RT-qPCR analyses confirmed that this copy is predominantly expressed in tissues exposed to the toxins, while the ancestral copy predominates in the nervous tissue. Functional tests with genetically engineered Drosophila Na,K-ATPases bearing the first extracellular loop of the pyrgomorphid genes showed the derived form to be highly resistant, while the ancestral state is sensitive. Thus, we report convergence in gene duplication and in the gene targets for toxin insensitivity; however, the means to the phenotypic end have been novel in pyrgomorphid grasshoppers.

Relative Selectivity of Plant Cardenolides for Na+/K+-ATPases From the Monarch Butterfly and Non-resistant Insects

A major prediction of coevolutionary theory is that plants may target particular herbivores with secondary compounds that are selectively defensive. The highly specialized monarch butterfly (Danaus plexippus) copes well with cardiac glycosides (inhibitors of animal Na+/K+-ATPases) from its milkweed host plants, but selective inhibition of its Na+/K+-ATPase by different compounds has not been previously tested. We applied 17 cardiac glycosides to the D. plexippus-Na+/K+-ATPase and to the more susceptible Na+/K+-ATPases of two non-adapted insects (Euploea core and Schistocerca gregaria). Structural features (e.g., sugar residues) predicted in vitro inhibitory activity and comparison of insect Na+/K+-ATPases revealed that the monarch has evolved a highly resistant enzyme overall. Nonetheless, we found evidence for relative selectivity of individual cardiac glycosides reaching from 4- to 94-fold differences of inhibition between non-adapted Na+/K+-ATPase and D. plexippus-Na+/K+-ATPase. This toxin receptor specificity suggests a mechanism how plants could target herbivores selectively and thus provides a strong basis for pairwise coevolutionary interactions between plants and herbivorous insects.

Na+/K+-ATPase resistance and cardenolide sequestration: basal adaptations to host plant toxins in the milkweed bugs (Hemiptera: Lygaeidae: Lygaeinae)

Despite sequestration of toxins being a common coevolutionary response to plant defence in phytophagous insects, the macroevolution of the traits involved is largely unaddressed. Using a phylogenetic approach comprising species from four continents, we analysed the ability to sequester toxic cardenolides in the hemipteran subfamily Lygaeinae, which is widely associated with cardenolide-producing Apocynaceae. In addition, we analysed cardenolide resistance of their Na(+)/K(+)-ATPases, the molecular target of cardenolides. Our data indicate that cardenolide sequestration and cardenolide-resistant Na(+)/K(+)-ATPase are basal adaptations in the Lygaeinae. In two species that shifted to non-apocynaceous hosts, the ability to sequester was secondarily reduced, yet Na(+)/K(+)-ATPase resistance was maintained. We suggest that both traits evolved together and represent major coevolutionary adaptations responsible for the evolutionary success of lygaeine bugs. Moreover, specialization on cardenolides was not an evolutionary dead end, but enabled this insect lineage to host shift to cardenolide-producing plants from distantly related families.

Physiological screening for target site insensitivity and localization of Na(+)/K(+)-ATPase in cardenolide-adapted Lepidoptera

Cardenolides are toxic plant compounds which specifically inhibit Na(+)/K(+)-ATPase, an animal enzyme which is essential for many physiological processes, such as the generation of action potentials. Several adapted insects feeding on cardenolide-containing plants sequester these toxins for their own defence. Some of these insects were shown to possess Na(+)/K(+)-ATPases with a reduced sensitivity towards cardenolides (target site insensitivity). In the present study we screened five species of arctiid moths feeding on cardenolide-containing plants for target site insensitivity towards cardenolides using an in vitro enzyme assay. The derived dose response curves of the respective Na(+)/K(+)-ATPases were compared to the insensitive Na(+)/K(+)-ATPase of the monarch butterfly (Danaus plexippus). Na(+)/K(+)-ATPases of all arctiid species tested were highly sensitive to ouabain, a water-soluble cardenolide which is most widely used in laboratory studies. Nevertheless, we detected substantial amounts of cardenolides in the haemolymph of two of the arctiid species. In caterpillars of the sequestering arctiid Empyreuma pugione and of D. plexippus we localized Na(+)/K(+)-ATPase by immunohistochemistry and western blot (in D. plexippus). Both techniques revealed strong expression of the enzyme in the nervous tissue and indicated weak expression or even absence in other tissues tested. We conclude that instead of target site insensitivity the investigated arctiid species use a different strategy to tolerate cardenolides. Most plausibly, the perineurium surrounding the nervous tissue functions as a barrier which prevents cardenolides from reaching Na(+)/K(+)-ATPase in the ventral nerve cord.

Coping with toxic plant compounds--the insect's perspective on iridoid glycosides and cardenolides

Specializing on host plants with toxic secondary compounds enforces specific adaptation in insect herbivores. In this review, we focus on two compound classes, iridoid glycosides and cardenolides, which can be found in the food plants of a large number of insect species that display various degrees of adaptation to them. These secondary compounds have very different modes of action: Iridoid glycosides are usually activated in the gut of the herbivores by β-glucosidases that may either stem from the food plant or be present in the gut as standard digestive enzymes. Upon cleaving, the unstable aglycone is released that unspecifically acts by crosslinking proteins and inhibiting enzymes. Cardenolides, on the other hand, are highly specific inhibitors of an essential ion carrier, the sodium pump. In insects exposed to both kinds of toxins, carriers either enabling the safe storage of the compounds away from the activating enzymes or excluding the toxins from sensitive tissues, play an important role that deserves further analysis. To avoid toxicity of iridoid glycosides, repression of activating enzymes emerges as a possible alternative strategy. Cardenolides, on the other hand, may lose their toxicity if their target site is modified and this strategy has evolved multiple times independently in cardenolide-adapted insects.

Absorption of sugars and amino acids by the epidermis of Aphidius ervi larvae

Aphidius ervi Haliday (Hymenoptera, Braconidae) is an endophagous parasitoid of several aphid species of economic importance, widely used in biological control. The definition of a suitable artificial diet for in vitro mass production of this parasitoid is still an unresolved issue that, to be properly addressed, requires a deeper understanding both of its nutritional needs and of the functional properties of the larval epithelia involved in nutrient absorption. The experimental evidence presented in this paper unequivocally demonstrates that the uptake of sugars and amino acids takes place through the body surface of the larval stages of A. ervi. These nutrients are efficiently absorbed by the larval epidermis, but the transport rate progressively declines over time. The epidermis exhibits a cross-reactivity to antibodies raised against the mammalian facilitative glucose transporter GLUT2 and the sodium cotransporter SGLT1. The analysis of sugar transport sensitivity to specific inhibitors indicates the involvement of GLUT2-like transporters, while a role for SGLT1-like transporters is not supported. The peculiar pathways of nutrient absorption in A. ervi larvae further corroborate the general idea that the pre-imaginal stages of endophagous koinobiont Hymenoptera, like Metazoan parasites, show a high degree of physiological integration with their hosts.

Properties of the V-type ATPase from the excretory system of the usherhopper, Poekilocerus bufonius

The bafilomycin A(1) and N-ethylmaleimide (NEM)-sensitive (V-type) ATPase was partially purified from the apical membrane-rich fractions of excretory system (Malpighian tubules and hind gut) of P. bufonius. Enzymatic activity was inhibited by bafilomycin A(1) (IC(50) = 1.3 nM) and NEM (IC(50) = 10.1 microM). The V-type ATPase activity is confined to the apical membrane fraction, while the activity of Na(+)/K(+) -ATPase forms the major part of the basal membrane fraction. The optimal pH required for maximal activity of V-type ATPase was pH 7.5. The effect of 30 mM of various salts on ATPase activity was investigated. NaCl and KCl caused increases of 175% and 184%, respectively. Other chloride salts also caused an increase in activity in the following ascending order: RbCl, LiCI, choline Cl, NaCI, KCl and tris-HCl. The activity of V-type ATPase was stimulated by a variety of different anions and cations, and HCO(3)(-) was found to be the most potent cationic activator of ATPase activity. The present results show that the properties of V-type ATPase of P. bufonius are similar to those reported for other insect tissues.

Effect of high ionic strength and inhibitors of H,K-ATPase on the ouabain-sensitive K-p-nitrophenylphosphatase activity in the sea anemone Stichodactyla helianthus

The ouabain-sensitive, K-stimulated p-nitrophenyl phosphatase (K-pNPPase) activity, an associated activity of the Na,K-ATPase, was assayed in tentacles of the sea anemone Stichodactyla helianthus to investigate the possibility that the sea anemone Na,K-ATPase activity is an associated activity of an H,K-ATPase. Activity was maximal at pH 6.5-7.0, decreasing only slightly in acidic medium but falling abruptly in alkaline medium to 60% of maximum at pH 7.4. The pH of maximum activity was not remarkably altered in high ionic strength medium (560 mM choline chloride), but ouabain-sensitive K-pNPPase activity of both rat and sea anemone was strongly inhibited. Inhibitors of the gastric H,K-ATPase, 100 microM omeprazole and 10 microM SCH 28080, did not inhibit the ouabain-sensitive K-pNPPase activity. Activity of the sea anemone enzyme was inhibited by 10 microM ammonium vanadate, an inhibitor of P-type ATPase, and not by 2.5 mM sodium azide, an inhibitor of both F-type and V-type ATPase. Because the sea anemone K-pNPPase activity was previously found to be more sensitive to ouabain than the Na,K-ATPase activity, K(+)-ouabain antagonism was investigated and found to be relatively muted, whereas K(+)-Na+ competition was stronger than in the rat kidney.

Different ouabain sensitivities of Na+/K(+)-ATPase from Poekilocerus bufonius tissues and a possible physiological cost

1. The properties of Na+/K(+)-transporting ATPase in microsomal preparation from mid-gut of the grasshopper, Poekilocerus bufonius, were investigated and compared with the same enzyme from brain and excretory system. 2. Two components of ATPases activity are present in the three tissues studied. 3. The physiochemical properties of Na+/K(+)-transporting ATPase from mid-gut, brain and excretory system (hind-gut plus Malpighian tubules) are essentially the same. 4. The calculated values of PI50 were 2 (I50 = 1 x 10(-2) M), 3.7 (I50 = 2 x 10(-4) M) and 6.4 (I50 = 3.98 x 10(-7)) for Na+/K(+)-ATPase from mid-gut, excretory system and brain, respectively. The mid-gut contains the most ouabain-resistant Na+/K(+)-ATPase. 5. The results suggest that P. bufonius have developed some tolerance to toxic cardiac glycosides (CGS), but there is a possibility of autotoxicity as indicated by the presence of ouabain-sensitive ATPase from brain tissue. 6. It was concluded that the dissimilarities of Na+/K(+)-ATPases from different tissues of P. bufonius are probably due to tissue-dependent differences in ouabain sensitivity (or isoenzymes pattern) available in the same insect. 7. The atrophy of female flight muscle of P. bufonius suggests the possibility of physiological cost inflicted on insects consuming poisonous plants.