Variation in cardiac glycoside content of monarch butterflies from natural populations in eastern North America

Do Invasive Jorō Spiders (Trichonephila clavata) from Asia Avoid Eating Unpalatable Monarch Butterflies (Danaus plexippus) in North America?

An invasive spider from East Asia has established in the U.S. southeast (the "jorō spider," Trichonephila clavata) and is rapidly expanding its range. Studies assessing the impact of this species are needed, including how expansive its diet is. An open question is whether monarch butterflies, Danaus plexippus, are a potential prey item for this spider, given that jorō spiders do not coexist with monarchs in their native range. Since monarch larvae feed on milkweed, they sequester cardiac glycosides into their adult tissues, rendering them unpalatable to many predators. At sites within northeast Georgia, we staged a series of trials (n = 61) where we tossed monarchs into jorō spider webs and, for comparison, performed similar trials with another aposematic species, gulf fritillary (Agraulis vanilla), and a palatable species, tiger swallowtail (Papilio glaucus). We recorded the outcome of the trials, which included whether the spider attacked or did not attack the prey. We also conducted a visual survey during the same fall season to look for evidence of jorō spiders consuming monarchs naturally. Our findings revealed that jorō spiders avoided eating monarchs; spiders only attacked monarchs 20% of the time, which was significantly less than the attack rates of similarly sized or larger butterflies: 86% for gulf fritillaries and 58% for tiger swallowtails. Some jorō spiders even removed monarchs from their webs. From our visual surveys of the surrounding area, we found no evidence of natural monarch consumption and, in general, butterflies made up only a fraction of the jorō spider diet. We conclude that jorō spiders appear to recognize monarch butterflies as being unpalatable, even without having a prior history with the species. This invokes questions about how these spiders can immediately recognize their unpalatability without touching the butterflies.

Compound-Specific Behavioral and Enzymatic Resistance to Toxic Milkweed Cardenolides in a Generalist Bumblebee Pollinator

Plant secondary metabolites that defend leaves from herbivores also occur in floral nectar. While specialist herbivores often have adaptations providing resistance to these compounds in leaves, many social insect pollinators are generalists, and therefore are not expected to be as resistant to such compounds. The milkweeds, Asclepias spp., contain toxic cardenolides in all tissues including floral nectar. We compared the concentrations and identities of cardenolides between tissues of the North American common milkweed Asclepias syriaca, and then studied the effect of the predominant cardenolide in nectar, glycosylated aspecioside, on an abundant pollinator. We show that a generalist bumblebee, Bombus impatiens, a common pollinator in eastern North America, consumes less nectar with experimental addition of ouabain (a standard cardenolide derived from Apocynacid plants native to east Africa) but not with addition of glycosylated aspecioside from milkweeds. At a concentration matching that of the maximum in the natural range, both cardenolides reduced activity levels of bees after four days of consumption, demonstrating toxicity despite variation in behavioral deterrence (i.e., consumption). In vitro enzymatic assays of Na+/K+-ATPase, the target site of cardenolides, showed lower toxicity of the milkweed cardenolide than ouabain for B. impatiens, indicating that the lower deterrence may be due to greater tolerance to glycosylated aspecioside. In contrast, there was no difference between the two cardenolides in toxicity to the Na+/K+-ATPase from a control insect, the fruit fly Drosophila melanogaster. Accordingly, this work reveals that even generalist pollinators such as B. impatiens may have adaptations to reduce the toxicity of specific plant secondary metabolites that occur in nectar, despite visiting flowers from a wide variety of plants over the colony's lifespan.

The price of defence: toxins, visual signals and oxidative state in an aposematic butterfly

In a variety of aposematic species, the conspicuousness of an individual's warning signal and the quantity of its chemical defence are positively correlated. This apparent honest signalling is predicted by resource competition models which assume that the production and maintenance of aposematic defences compete for access to antioxidant molecules that have dual functions as pigments and in protecting against oxidative damage. To test for such trade-offs, we raised monarch butterflies (Danaus plexippus) on different species of their milkweed host plants (Apocynaceae) that vary in quantities of cardenolides to test whether (i) the sequestration of cardenolides as a secondary defence is associated with costs in the form of oxidative lipid damage and reduced antioxidant defences; and (ii) lower oxidative state is associated with a reduced capacity to produce aposematic displays. In male monarchs conspicuousness was explained by an interaction between oxidative damage and sequestration: males with high levels of oxidative damage became less conspicuous with increased sequestration of cardenolides, whereas those with low oxidative damage became more conspicuous with increased levels of cardenolides. There was no significant effect of oxidative damage or concentration of sequestered cardenolides on female conspicuousness. Our results demonstrate a physiological linkage between the production of coloration and oxidative state, and differential costs of sequestration and signalling in monarch butterflies.

Defence mitigation by predators of chemically defended prey integrated over the predation sequence and across biological levels with a focus on cardiotonic steroids

Predator-prey interactions have long served as models for the investigation of adaptation and fitness in natural environments. Anti-predator defences such as mimicry and camouflage provide some of the best examples of evolution. Predators, in turn, have evolved sensory systems, cognitive abilities and physiological resistance to prey defences. In contrast to prey defences which have been reviewed extensively, the evolution of predator counter-strategies has received less attention. To gain a comprehensive view of how prey defences can influence the evolution of predator counter-strategies, it is essential to investigate how and when selection can operate. In this review we evaluate how predators overcome prey defences during (i) encounter, (ii) detection, (iii) identification, (iv) approach, (v) subjugation, and (vi) consumption. We focus on prey that are protected by cardiotonic steroids (CTS)-defensive compounds that are found in a wide range of taxa, and that have a specific physiological target. In this system, coevolution is well characterized between specialist insect herbivores and their host plants but evidence for coevolution between CTS-defended prey and their predators has received less attention. Using the predation sequence framework, we organize 574 studies reporting predators overcoming CTS defences, integrate these counter-strategies across biological levels of organization, and discuss the costs and benefits of attacking CTS-defended prey. We show that distinct lineages of predators have evolved dissecting behaviour, changes in perception of risk and of taste perception, and target-site insensitivity. We draw attention to biochemical, hormonal and microbiological strategies that have yet to be investigated as predator counter-adaptations to CTS defences. We show that the predation sequence framework will be useful for organizing future studies of chemically mediated systems and coevolution.

Defence mitigation by predators of chemically defended prey integrated over the predation sequence and across biological levels with a focus on cardiotonic steroids

Predator-prey interactions have long served as models for the investigation of adaptation and fitness in natural environments. Anti-predator defences such as mimicry and camouflage provide some of the best examples of evolution. Predators, in turn, have evolved sensory systems, cognitive abilities and physiological resistance to prey defences. In contrast to prey defences which have been reviewed extensively, the evolution of predator counter-strategies has received less attention. To gain a comprehensive view of how prey defences can influence the evolution of predator counter-strategies, it is essential to investigate how and when selection can operate. In this review we evaluate how predators overcome prey defences during (i) encounter, (ii) detection, (iii) identification, (iv) approach, (v) subjugation, and (vi) consumption. We focus on prey that are protected by cardiotonic steroids (CTS)-defensive compounds that are found in a wide range of taxa, and that have a specific physiological target. In this system, coevolution is well characterized between specialist insect herbivores and their host plants but evidence for coevolution between CTS-defended prey and their predators has received less attention. Using the predation sequence framework, we organize 574 studies reporting predators overcoming CTS defences, integrate these counter-strategies across biological levels of organization, and discuss the costs and benefits of attacking CTS-defended prey. We show that distinct lineages of predators have evolved dissecting behaviour, changes in perception of risk and of taste perception, and target-site insensitivity. We draw attention to biochemical, hormonal and microbiological strategies that have yet to be investigated as predator counter-adaptations to CTS defences. We show that the predation sequence framework will be useful for organizing future studies of chemically mediated systems and coevolution.

Repurposing Cardiac Glycosides: Drugs for Heart Failure Surmounting Viruses

Drug repositioning is a successful approach in medicinal research. It significantly simplifies the long-term process of clinical drug evaluation, since the drug being tested has already been approved for another condition. One example of drug repositioning involves cardiac glycosides (CGs), which have, for a long time, been used in heart medicine. Moreover, it has been known for decades that CGs also have great potential in cancer treatment and, thus, many clinical trials now evaluate their anticancer potential. Interestingly, heart failure and cancer are not the only conditions for which CGs could be effectively used. In recent years, the antiviral potential of CGs has been extensively studied, and with the ongoing SARS-CoV-2 pandemic, this interest in CGs has increased even more. Therefore, here, we present CGs as potent and promising antiviral compounds, which can interfere with almost any steps of the viral life cycle, except for the viral attachment to a host cell. In this review article, we summarize the reported data on this hot topic and discuss the mechanisms of antiviral action of CGs, with reference to the particular viral life cycle phase they interfere with.

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.

Variation of chemical compounds in wild Heliconiini reveals ecological factors involved in the evolution of chemical defenses in mimetic butterflies

Evolutionary convergence of color pattern in mimetic species is tightly linked with the evolution of chemical defenses. Yet, the evolutionary forces involved in natural variations of chemical defenses in aposematic species are still understudied. Herein, we focus on the evolution of chemical defenses in the butterfly tribe Heliconiini. These neotropical butterflies contain large concentrations of cyanogenic glucosides, cyanide-releasing compounds acting as predator deterrent. These compounds are either de novo synthesized or sequestered from their Passiflora host plant, so that their concentrations may depend on host plant specialization and host plant availability. We sampled 375 wild Heliconiini butterflies across Central and South America, covering 43% species of this clade, and quantify individual variations in the different CGs using liquid chromatography coupled with tandem mass spectrometry. We detected new compounds and important variations in chemical defenses both within and among species. Based on the most recent and well-studied phylogeny of Heliconiini, we show that ecological factors such as mimetic interactions and host plant specialization have a significant association with chemical profiles, but these effects are largely explained by phylogenetic relationships. Our results therefore suggest that shared ancestries largely contribute to chemical defense variation, pointing out at the interaction between historical and ecological factors in the evolution of Müllerian mimicry.

Analysis of defensive secretion of a milkweed bug Lygaeus equestris by 1D GC-MS and GC×GC-MS: sex differences and host-plant effect

The composition of defensive secretion produced by metathoracic scent glands was analysed in males and females of the milkweed bug Lygaeus equestris (Heteroptera) using gas chromatography with mass spectrometric detection (GC-MS). The bugs were raised either on cardenolide-containing Adonis vernalis or on control sunflower seeds in order to determine whether the possibility to sequester cardenolides from their host plants would affect the composition of defensive scent-gland secretion. Profiles of the composition of defensive secretions of males and females raised on sunflower were closely similar, with predominant presence of (E)-2-octenal, (E)-2-octen-1-ol, decanal and 3-octen-1-ol acetate. The secretion of bugs raised on A. vernalis was more sexually dimorphic, and some chemicals e.g. (E,E)-2,4-hexadienyl acetate and 2-phenylethyl acetate were dominant in males, but absent in females. Compared to bugs from sunflower, the scent-gland secretion of bugs raised on A. vernalis was characterized by lower overall intensity of the peaks obtained for detected chemicals and by absence of some chemicals that have supposedly antipredatory function ((E)-2-hexenal, (E)-4-oxo-hex-2-enal, 2,4-octadienal). The results suggest that there might be a trade-off between the sequestration of defensive chemicals from host plants and their synthesis in metathoracic scent-glands.

A draft genome and transcriptome of common milkweed (Asclepias syriaca) as resources for evolutionary, ecological, and molecular studies in milkweeds and Apocynaceae

Milkweeds (Asclepias) are used in wide-ranging studies including floral development, pollination biology, plant-insect interactions and co-evolution, secondary metabolite chemistry, and rapid diversification. We present a transcriptome and draft nuclear genome assembly of the common milkweed, Asclepias syriaca. This reconstruction of the nuclear genome is augmented by linkage group information, adding to existing chloroplast and mitochondrial genomic resources for this member of the Apocynaceae subfamily Asclepiadoideae. The genome was sequenced to 80.4× depth and the draft assembly contains 54,266 scaffolds ≥1 kbp, with N50 = 3,415 bp, representing 37% (156.6 Mbp) of the estimated 420 Mbp genome. A total of 14,474 protein-coding genes were identified based on transcript evidence, closely related proteins, and ab initio models, and 95% of genes were annotated. A large proportion of gene space is represented in the assembly, with 96.7% of Asclepias transcripts, 88.4% of transcripts from the related genus Calotropis, and 90.6% of proteins from Coffea mapping to the assembly. Scaffolds covering 75 Mbp of the Asclepias assembly formed 11 linkage groups. Comparisons of these groups with pseudochromosomes in Coffea found that six chromosomes show consistent stability in gene content, while one may have a long history of fragmentation and rearrangement. The progesterone 5β-reductase gene family, a key component of cardenolide production, is likely reduced in Asclepias relative to other Apocynaceae. The genome and transcriptome of common milkweed provide a rich resource for future studies of the ecology and evolution of a charismatic plant family.

The specialization continuum: Decision-making in butterflies with different diet requirements

Differences in diet requirements may be reflected in how floral visitors make decisions when probing nectar sources that differ in chemical composition. We examined decision-making in butterflies that form a specialization continuum involving pyrrolizidine alkaloids (PAs) when interacting with PA and non-PA plants: Agraulis vanillae (non-specialist), Danaus erippus (low demanding PA-specialist) and D. gilippus (high demanding PA-specialist). In addition, we assessed whether experience affected decision-making. Butterflies were tested on either Tridax procumbens (absence of PAs in nectar) or Ageratum conyzoides flowers (presence of PAs in nectar). Agraulis vanillae showed more acceptance for T. procumbens and more rejection for A. conyzoides; no differences were recorded for both Danaus species. Agraulis vanillae fed less on A. conyzoides than both Danaus species, which did not differ in this regard. In all butterfly species, experience on PA flowers did not affect feeding time. In the field, butterflies rarely visited PA flowers, regardless of the specialization degree. Our findings reveal that the specialization continuum seen in butterflies explains, at least in part, decision-making processes related to feeding. Additional factors as local adaptation mediated by the use of alternative nectar sources can affect flower visitation by specialist butterflies.

Cardenolide Intake, Sequestration, and Excretion by the Monarch Butterfly along Gradients of Plant Toxicity and Larval Ontogeny

Monarch butterflies, Danaus plexippus, migrate long distances over which they encounter host plants that vary broadly in toxic cardenolides. Remarkably little is understood about the mechanisms of sequestration in Lepidoptera that lay eggs on host plants ranging in such toxins. Using closely-related milkweed host plants that differ more than ten-fold in cardenolide concentrations, we mechanistically address the intake, sequestration, and excretion of cardenolides by monarchs. We show that on high cardenolide plant species, adult butterflies saturate in cardenolides, resulting in lower concentrations than in leaves, while on low cardenolide plants, butterflies concentrate toxins. Butterflies appear to focus their sequestration on particular compounds, as the diversity of cardenolides is highest in plant leaves, lower in frass, and least in adult butterflies. Among the variety of cardenolides produced by the plant, sequestered compounds may be less toxic to the butterflies themselves, as they are more polar on average than those in leaves. In accordance with this, results from an in vitro assay based on inhibition of Na⁺/K⁺ ATPase (the physiological target of cardenolides) showed that on two milkweed species, including the high cardenolide A. perennis, extracts from butterflies have lower inhibitory effects than leaves when standardized by cardenolide concentration, indicating selective sequestration of less toxic compounds from these host plants. To understand how ontogeny shapes sequestration, we examined cardenolide concentrations in caterpillar body tissues and hemolymph over the course of development. Caterpillars sequestered higher concentrations of cardenolides as early instars than as late instars, but within the fifth instar, concentration increased with body mass. Although it appears that large amounts of sequestration occurs in early instars, a host switching experiment revealed that caterpillars can compensate for feeding on low cardenolide host plants with substantial sequestration in the fifth instar. We highlight commonalities and striking differences in the mechanisms of sequestration depending on host plant chemistry and developmental stage, which have important implications for monarch defense.

The Effects of Milkweed Induced Defense on Parasite Resistance in Monarch Butterflies, Danaus plexippus

Many plants express induced defenses against herbivores through increasing the production of toxic secondary chemicals following damage. Phytochemical induction can directly or indirectly affect other organisms within the community. In tri-trophic systems, increased concentrations of plant toxins could be detrimental to plants if herbivores can sequester these toxins as protective chemicals for themselves. Thus, through trophic interactions, induction can lead to either positive or negative effects on plant fitness. We examined the effects of milkweed (Asclepias spp.) induced defenses on the resistance of monarch caterpillars (Danaus plexippus) to a protozoan parasite (Ophryocystis elektroscirrha). Milkweeds contain toxic secondary chemicals called cardenolides, higher concentrations of which are associated with reduced parasite growth. Previous work showed that declines in foliar cardenolides caused by aphid attack render monarch caterpillars more susceptible to infection. Here, we ask whether cardenolide induction by monarchs increases monarch resistance to disease. We subjected the high-cardenolide milkweed A. curassavica and the low-cardenolide A. syriaca to caterpillar grazing, and reared infected and uninfected caterpillars on these plants. As expected, monarchs suffered less parasite growth and disease when reared on A. curassavica than on A. syriaca. We also found that herbivory increased cardenolide concentrations in A. curassavica, but not A. syriaca. However, cardenolide induction in A. curassavica was insufficient to influence monarch resistance to the parasite. Our results suggest that interspecific variation in cardenolide concentration is a more important driver of parasite defense than plasticity via induced defenses in this tri-trophic system.

REVISING A CLASSIC BUTTERFLY MIMICRY SCENARIO: DEMONSTRATION OF MÜLLERIAN MIMICRY BETWEEN FLORIDA VICEROYS (LIMENITIS ARCHIPPUS FLORIDENSIS) AND QUEENS (DANAUS GILIPPUS BERENICE)

Batesian and Müllerian mimicry relationships differ greatly in terms of selective pressures affecting the participants; hence, accurately characterizing a mimetic interaction is a crucial prerequisite to understanding the selective milieux of model, mimic, and predator. Florida viceroy butterflies (Limenitis archippus floridensis) are conventionally characterized as palatable Batesian mimics of distasteful Florida queens (Danaus gilippus berenice). However, recent experiments indicate that both butterflies are moderately distasteful, suggesting they may be Müllerian comimics. To directly test whether the butterflies exemplify Müllerian mimicry, I performed two reciprocal experiments using red-winged blackbird predators. In Experiment 1, each of eight birds was exposed to a series of eight queens as "models," then offered four choice trials involving a viceroy (the putative "mimic") versus a novel alternative butterfly. If mimicry was effective, viceroys should be attacked less than alternatives. I also compared the birds' reactions to solo viceroy "mimics" offered before and after queen models, hypothesizing that attack rate on the viceroy would decrease after birds had been exposed to queen models. In Experiment 2, 12 birds were tested with viceroys as models and queens as putative mimics. The experiments revealed that (1) viceroys and queens offered as models were both moderately unpalatable (only 16% entirely eaten), (2) some birds apparently developed conditioned aversions to viceroy or queen models after only eight exposures, (3) in the subsequent choice trials, viceroy and queen "mimics" were attacked significantly less than alternatives, and (4) solo postmodel mimics were attacked significantly less than solo premodel mimics. Therefore, under these experimental conditions, sampled Florida viceroys and queens are comimics and exemplify Müllerian, not Batesian, mimicry. This compels a reassessment of selective forces affecting the butterflies and their predators, and sets the stage for a broader empirical investigation of the ecological and evolutionary dynamics of mimicry.

Different rates of defense evolution and niche preferences in clonal and nonclonal milkweeds (Asclepias spp.)

Given the dual role of many plant traits to tolerate both herbivore attack and abiotic stress, the climatic niche of a species should be integrated into the study of plant defense strategies. Here we investigate the impact of plant reproductive strategy and components of species' climatic niche on the rate of chemical defense evolution in the milkweeds using a common garden experiment of 49 species. We found that across Asclepias species, clonal reproduction repeatedly evolved in lower temperature conditions, in species generally producing low concentrations of a toxic defense (cardenolides). Additionally, we found that rates of cardenolide evolution were lower for clonal than for nonclonal species. We thus conclude that because the clonal strategy is based on survival, long generation times, and is associated with tolerance of herbivory, it may be an alternative to toxicity in colder ecosystems. Taken together, these results indicate that the rate of chemical defense evolution is influenced by the intersection of life-history strategy and climatic niches into which plants radiate.

Plant-determined variation in the cardenolide content, thin-layer chromatography profiles, and emetic potency of monarch butterflies,Danaus plexippus L. Reared on milkweed plants in California: 2.Asclepias speciosa

The pattern of variation in gross cardenolide concentration of 111Asclepias speciosa plants collected in six different areas of California is a positively skewed distribution which ranges from 19 to 344 μg of cardenolide per 0.1 g dry weight with a mean of 90 μg per 0.1 g. Butterflies reared individually on these plants in their native habitats ranged from 41 to 547 μg of cardenolide per 0.1 g dry weight with a mean of 179 μg. Total cardenolide per butterfly ranged from 54 to 1279 μg with a mean of 319 μg. Differences in concentrations and total cardenolide contents in the butterflies from the six geographic areas appeared minor, and there were no differences between the males and the females, although the males did weigh significantly more than females. The uptake of cardenolide by the butterflies was found to be a logarithmic function of the plant concentration. This results in regulation: larvae which feed on low-concentration plants produce butterflies with increased cardenolide concentrations relative to those of the plants, and those which feed on high-concentration plants produce butterflies with decreased concentrations. No evidence was adduced that high concentrations of cardenolides in the plants affected the fitness of the butterflies. The mean emetic potencies of the powdered plant and butterfly material were 5.62 and 5.25 blue jay emetic dose fifty units per milligram of cardenolide and the number of ED50 units per butterfly ranged from 0.28 to 6.7 with a mean of 1.67. Monarchs reared onA. speciosa, on average, are only about one tenth as emetic as those reared onA. eriocarpa. UnlikeA. eriocarpa which is limited to California,A. speciosa ranges from California to the Great Plains and is replaced eastwards byA. syriaca L. These two latter milkweed species appear to have a similar array of chemically identical cardenolides, and therefore both must produce butterflies of relatively low emetic potency to birds, with important ecological implications. About 80% of the lower emetic potency of monarchs reared on A. speciosa compared to those reared onA. eriocarpa appears attributable to the higher polarity of the cardenolides inA. speciosa. Thin-layer Chromatographie separation of the cardenolides in two different solvent systems showed that there are 23 cardenolides in theA. speciosa plants of which 20 are stored by the butterflies. There were no differences in the cardenolide spot patterns due either to geographic origin or the sex of the butterflies. As when reared onA. eriocarpa, the butterflies did not store the plant cardenolides withR f values greater than digitoxigenin. However, metabolic transformation of the cardenolides by the larvae appeared minor in comparison to when they were reared onA. eriocarpa. AlthoughA. eriocarpa andA. speciosa contain similar numbers of cardenolides and both contain desglucosyrioside, the cardenolides ofA. speciosa overall are more polar. ThusA. speciosa has no or only small amounts of the nonpolar labriformin and labriformidin, whereas both occur in high concentrations inA. eriocarpa. A. speciosa plants and butterflies also contain uzarigen, syriogenin, and possibly other polar cardenolides withR f values lower than digitoxin. The cardenolide concentration in the leaves is not only considerably less than inA. eriocarpa, but the latex has little to immeasurable cardenolide, whereas that ofA. eriocarpa has very high concentrations of several cardenolides. Quantitative analysis ofR f values of the cardenolide spots, their intensities, and their probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint pattern very different from that previously established for monarchs reared onA. eriocarpa. This dispels recently published skepticism about the predictibility of chemical fingerprints based upon ingested secondary plant chemicals.

Cardenolide content and thin-layer chromatography profiles of monarch butterflies,danaus plexippus L., and their larval host-plant milkweed,asclepias viridis walt., in northwestern louisiana

This paper is the first in a series on cardenolide fingerprinting of monarch butterflies and their host-plant milkweeds in the eastern United States. Spectrophotometric determinations of the gross cardenolide content of 60Asclepias viridis plants in northwestern Louisiana indicate a positively skewed variation ranging from 95 to 432 υg/0.1 g dry weight with a mean of 245 υg/0.1 g. Butterflies reared individually on these plants contained a normal cardenolide distribution ranging from 73 to 591 υg/0.1 g dry weight with a mean of 337 υg/0.1 g. The uptake of cardenolide by the butterflies best fit a logarithmic function of the plant concentration. Female monarchs (385 υg/0.l g) contained significantly greater mean cardenolide concentrations than did males (287 υg/0.1 g). No indications of a metabolic cost for either cardenolide ingestion or storage were adduced from size or dry weight data. Thin-layer chromatograms of 24 individual plant-butterfly pairs developed in two solvent systems resolved 21 individual spots in the plants and 15 in the butterflies.A. viridis plants appear to contain several relatively nonpolar cardenolides of the calotropagenin series which are metabolized to the more polar 3'-hydroxy derivatives calactin and calotropin as well as to calotropagenin in the butterflies. The epoxy cardenolides labriformin and labriformidin were absent, although desglucosyrioside (a 3'-hydroxy derivative) appeared present in both plants and butterflies. Quantitative evaluation of theR f values, spot intensities, and probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint clearly distinct from those previously established for monarchs reared on otherAsclepias species, supporting the use of fingerprints to make ecological predictions concerning larval host-plant utilization.A. viridis is the predominant early spring milkweed throughout most of the south central United States and may be important in providing chemical protection to spring and early summer generation monarchs in the eastern United States.

Cardenolide content and thin-layer chromatography profiles of monarch butterflies,Danaus plexippus L., and their larval host-plant milkweed,Asclepias asperula subsp.Capricornu (woods.) woods., in north central Texas

This paper is the second in a series on cardenolide fingerprinting of monarch butterflies and their host-plant milkweeds in the eastern United States. Spectrophotometric determinations of the gross cardenolide content ofAsclepias asperula plants in north central Texas indicated wide variation ranging from 341 to 1616 μg/0.1 g dry weight. The mean plant cardenolide concentration (886 μg/0.1 g) is the highest for any milkweed species on which monarch cardenolide profiles have been produced. Forty-one butterflies reared individually on these plants contained a skewed distribution of cardenolide concentrations ranging from 231 to 515 μg/0. 1 g dry weight with a mean of 363μg/0.1 g. The uptake of cardenolide by the butterflies was independent of plant concentration, suggesting that saturation occurs in cardenolide sequestration by monarchs when feeding on cardenolide-rich host-plants. Female monarchs contained significantly greater mean cardenolide concentrations (339 μg/0.1 g) than did males (320 μg/0.1 g). The mean dry weight of the male butterflies (0.211 g) was significantly greater than the female mean (0.191) so that the mean total cardenolide contents of males (675 fig) and females (754 μg) were not significantly different. Butterfly size was not significantly correlated to butterfly cardenolide concentration when differences due to sex and individual host-plant concentration were removed. Thin-layer chrornatograms of 24 individual plant-butterfly pairs developed in two solvent systems resolved 22 individual spots in the plants and 15 in the butterflies.A. asperula plants appear to contain several relatively nonpolar cardenolides of the calotropagenin series which are metabolized to more polar derivatives in the butterflies. Quantitative evaluation of theR f values, spot intensities, and probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint clearly distinct from those previously established for monarchs reared on otherAsclepias species. Our data support the use of fingerprints to make ecological predictions concerning larval host-plant utilization.A. asperula subsp.capricornu andA. viridis Walt, are the predominant early spring milkweeds throughout most of the south central United States. Cardenolide-rich monarchs reared on these two species may be instrumental in establishing and reinforcing visual avoidance of adults by naive predators throughout their spring and summer breeding cycle in eastern North America.

Palatability of aposematic queen butterflies (Danaus gilippus) feeding onSarcostemma clausum (Asclepiadaceae) in Florida

Queen butterflies (Danaus gilippus) are generally considered unpalatable to predators because they sequester and store toxic cardenolides from their larval food plants. However, a major queen food plant in Florida, the asclepiadaceous vineSarcostemma clausum, is shown here to be a very poor cardenolide source, and queens reared on this plant contain no detectable cardenolide. A direct evaluation of queen palatability using red-winged blackbirds indicates thatS. clausum-reared butterflies are essentially palatable to these predators (85% of abdomens entirely eaten), indicating little protection from either cardenolides, other sequestered phytochemicals, or de novo defensive compounds. Wild-caught queens that presumably fed as larvae uponS. clausum and also had access to adult-obtained chemicals, such as pyrrolizidine alkaloids (PAs), were relatively palatable as well (77% of abdomens eaten); they did not differ significantly in palatability from the labreared butterflies. Together, these findings suggest that; (1)S. clausumfed queens are poorly defended against some avian predators, and (2) for the particular queen sample examined, PAs do not contribute substantially to unpalatability. The discovery thatS. clausum-feeding queens are essentially palatable is of additional significance because it compels a reassessment of the classic mimicry relationship between queen and viceroy butterflies. Viceroys have been shown recently to be moderately unpalatable; therefore, the established roles of model and mimic may be reversed in some cases.

Mediation of cardiac glycoside insensitivity in the monarch butterfly (Danaus plexippus): Role of an amino acid substitution in the ouabain binding site of Na(+),K (+)-ATPase

The Monarch butterfly (Danaus plexippus) sequesters cardiac glycosides (CG) for its chemical defense against predators. Larvae and adults of this butterfly are insensitive towards dietary cardiac glycosides, whereas other Lepidoptera are sensitive and intoxicated by ouabain. Ouabain inhibits Na(+),K(+)-ATPase by binding to its α-subunit. We have amplified and cloned the DNA-sequence encoding the respective ouabain binding site. Instead of the amino acid asparagine at position 122 in ouabain-sensitive insects, the Monarch has a histidine in the putative ouabain binding site, which consists of 12 amino acids. Starting with the CG-sensitive Na(+),K(+)-ATPase gene fromDrosophila, we converted pos. 122 to a histidine residue as inDanaus plexippus by site-directed mutagenesis. Human embryonic kidney cells (HEK) (which are sensitive to ouabain) were transfected with the mutated Na(+),K(+)-ATPase gene in a pSVDF-expression vector and showed a transient expression of the mutatedDrosophila Na(+),K(+)-ATPase. When treated with ouabain, the transfected cells tolerated ouabain at a concentration of 50 mM, whereas untransformed controls or controls transfected with the unmutatedDrosophila gene, showed a substantial mortality. This result implies that the asparagine to histidine exchange contributes to ouabain insensitivity in the Monarch. In two other CG-sequestering insects, e.g.,Danaus gilippus andSyntomeida epilais, the pattern of amino acid substitution differed, indicating that the Monarch has acquired this mutation independently during evolution.

Spectroscopic sensitivity of real-time, rapidly induced phytochemical change in response to damage

An ecological consequence of plant-herbivore interactions is the phytochemical induction of defenses in response to insect damage. Here, we used reflectance spectroscopy to characterize the foliar induction profile of cardenolides in Asclepias syriaca in response to damage, tracked in vivo changes and examined the influence of multiple plant traits on cardenolide concentrations. Foliar cardenolide concentrations were measured at specific time points following damage to capture their induction profile. Partial least-squares regression (PLSR) modeling was employed to calibrate cardenolide concentrations to reflectance spectroscopy. In addition, subsets of plants were either repeatedly sampled to track in vivo changes or modified to reduce latex flow to damaged areas. Cardenolide concentrations and the induction profile of A. syriaca were well predicted using models derived from reflectance spectroscopy, and this held true for repeatedly sampled plants. Correlations between cardenolides and other foliar-related variables were weak or not significant. Plant modification for latex reduction inhibited an induced cardenolide response. Our findings show that reflectance spectroscopy can characterize rapid phytochemical changes in vivo. We used reflectance spectroscopy to identify the mechanisms behind the production of plant secondary metabolites, simultaneously characterizing multiple foliar constituents. In this case, cardenolide induction appears to be largely driven by enhanced latex delivery to leaves following damage.

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na⁺/K⁺-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.

The evolution of cardenolide-resistant forms of Na⁺,K⁺ -ATPase in Danainae butterflies

Cardenolides are a class of plant secondary compounds that inhibit the proper functioning of the Na(+) , K(+) -ATPase enzyme in susceptible animals. Nonetheless, many insect species are able to sequester cardenolides for their own defence. These include butterflies in the subfamily Danainae (Family: Nymphalidae) such as the monarch (Danaus plexippus). Previous studies demonstrated that monarchs harbour an asparagine (N) to histidine (H) substitution (N122H) in the α subunit of Na(+) , K(+) -ATPase (ATPα) that reduces this enzyme's sensitivity to cardenolides. More recently, it has been suggested that at ATPα position 111, monarchs may also harbour a leucine (L)/glutamine (Q) polymorphism. This later amino acid could also contribute to cardenolide insensitivity. However, here we find that incorrect annotation of the initially reported DNA sequence for ATPα has led to several erroneous conclusions. Using a population genetic and phylogenetic analysis of monarchs and their close relatives, we show that an ancient Q111L substitution occurred prior to the radiation of all Danainae, followed by a second substitution at the same site to valine (V), which arose before the diversification of the Danaus genus. In contrast, N122H appears to be a recent substitution specific to monarchs. Surprisingly, examination of a broader insect phylogeny reveals that the same progression of amino acid substitutions (Q111L → L111V + N122H) has also occurred in Chyrsochus beetles (Family: Chrysomelidae, Subfamily: Eumolpinae) that feed on cardenolide-containing host plants. The parallel pattern of amino acid substitution in these two distantly related lineages is consistent with an adaptive role for these substitutions in reducing cardenolide sensitivity and suggests that their temporal order may be limited by epistatic interactions.

Theoretical investigations of automimicry: multiple trial learning and the palatability spectrum

We previously explored automimicry assuming that a species of prey was so unpalatable as to promote conditioned avoidance for a period of time after a predator encountered a single individual (Case 1). In this paper, we assume that the prey is less noxious and that two encounters are required. Case 2 allows the two encounters with unpalatables to be separated by any number of palatables, while in Case 3 the predator must encounter two unpalatables, consecutively.The general relationships in the three cases are similar, but the automimetic advantage is reduced moderately in Case 2 and greatly in Case 3. To attain the same automimetic advantage as in Case 1 requires an increase in the proportion of unpalatables, or in the induced rejection period, or both. Consequently, selection will tend to increase the unpalatability so that Cases 2 and 3 converge to Case 1.Species that are uniformly and highly unpalatable can afford to be more dispersed than automimetic species. Case-2 and -3 automimetic species will benefit greatly from gregariousness, while in Case-1 automimicry situations this is less important.

Evidence for adaptive radiation from a phylogenetic study of plant defenses

One signature of adaptive radiation is a high level of trait change early during the diversification process and a plateau toward the end of the radiation. Although the study of the tempo of evolution has historically been the domain of paleontologists, recently developed phylogenetic tools allow for the rigorous examination of trait evolution in a tremendous diversity of organisms. Enemy-driven adaptive radiation was a key prediction of Ehrlich and Raven's coevolutionary hypothesis [Ehrlich PR, Raven PH (1964) Evolution 18:586-608], yet has remained largely untested. Here we examine patterns of trait evolution in 51 North American milkweed species (Asclepias), using maximum likelihood methods. We study 7 traits of the milkweeds, ranging from seed size and foliar physiological traits to defense traits (cardenolides, latex, and trichomes) previously shown to impact herbivores, including the monarch butterfly. We compare the fit of simple random-walk models of trait evolution to models that incorporate stabilizing selection (Ornstein-Ulenbeck process), as well as time-varying rates of trait evolution. Early bursts of trait evolution were implicated for 2 traits, while stabilizing selection was implicated for several others. We further modeled the relationship between trait change and species diversification while allowing rates of trait evolution to vary during the radiation. Species-rich lineages underwent a proportionately greater decline in latex and cardenolides relative to species-poor lineages, and the rate of trait change was most rapid early in the radiation. An interpretation of this result is that reduced investment in defensive traits accelerated diversification, and disproportionately so, early in the adaptive radiation of milkweeds.

PLANT DEFENSE SYNDROMES

Given that a plant's defensive strategy against herbivory is never likely to be a single trait, we develop the concept of plant defense syndromes, where association with specific ecological interactions can result in convergence on suites of covarying defensive traits. Defense syndromes can be studied within communities of diverse plant species as well as within clades of closely related species. In either case, theory predicts that plant defense traits can consistently covary across species, due to shared evolutionary ancestry or due to adaptive convergence. We examined potential defense syndromes in 24 species of milkweeds (Asclepias spp.) in a field experiment. Employing phylogenetically independent contrasts, we found few correlations between seven defensive traits, no bivariate trade-offs, and notable positive correlations between trichome density and latex production, and between C:N ratio and leaf toughness. We then used a hierarchical cluster analysis to produce a phenogram of defense trait similarity among the 24 species. This analysis revealed three distinct clusters of species. The defense syndromes of these species clusters are associated with either low nutritional quality or a balance of higher nutritional quality coupled with physical or chemical defenses. The phenogram based on defense traits was not congruent, however, with a molecular phylogeny of the group, suggesting convergence on defense syndromes. Finally, we examined the performance of monarch butterfly caterpillars on the 24 milkweed species in the field; monarch growth and survival did not differ on plants in the three syndromes, although multiple regression revealed that leaf trichomes and toughness significantly reduced caterpillar growth. The discovery of convergent plant defense syndromes can be used as a framework to ask questions about how abiotic environments, communities of herbivores, and biogeography are associated with particular defense strategies of plants.

Automimicry destabilizes aposematism: predator sample-and-reject behaviour may provide a solution

Aposematism, the use of conspicuous colours to advertise unpalatability to predators, is perhaps the most studied signalling system in nature. However, its evolutionary stability remains paradoxical. The paradox is illustrated by the problem of automimicry. Automimics are palatable individuals within a population of unpalatable aposematics. Automimics benefit from predators avoiding warning coloration without carrying the models' cost of unpalatability, and should increase in the population, destabilizing the signalling system, unless selected against in some way. Cautious sampling, instead of avoidance, by predators may offer a solution to this problem. Here, we investigate the effect of automimic frequency on predator sampling behaviour, and whether predator sampling behaviour may provide a selection pressure against mimics. Domestic chicks (Gallus gallus domesticus) were subjected to the task of discriminating between green (signalling) and untreated brown chick crumbs. Some of the green crumbs were quinine treated and thus unpalatable. The frequency of palatable signalling prey items varied in four treatments; all unpalatable, low automimic frequency, high automimic frequency and all palatable. The results show that predator sampling behaviour is sensitive to automimic frequency and that predators may discriminate between models and mimics through sampling, and thereby benefit unprofitable prey. The results suggest somewhat surprisingly that aposematic signalling is stable only because of the actions of those predators not actually deterred by warning signals.

Response in nematocyst uptake by the nudibranch Flabellina verrucosa to the presence of various predators in the Southern Gulf of Maine

Aeolid nudibranchs maintain nematocysts sequestered from their cnidarian prey for protection against predators. Selection for nematocyst incorporation is a function of diet and prey choice, but ratios vary among nudibranchs feeding on a given diet, indicating that other factors may be involved. It is proposed that the presence of predators influences nematocyst incorporation. Nematocyst uptake in the nudibranch Flabellina verrucosa collected from the southern Gulf of Maine was examined in response to various potential predators, including Crossaster papposus, Tautogolabrus adspersus, and Carcinus maenas. Nudibranchs in individual flow-through containers feeding on a diet of the hydroids Tubularia spp. and Obelia geniculata were subjected to tanks containing a predator, then their nematocyst distribution was examined. Although most of the changes over the experimental period were attributable to diet, F. verrucosa responded to both T. adspersus and C. papposus by significantly increasing microbasic mastigophore incorporation. No differential uptake was seen with C. maenas. Response was evident in the nudibranchs both for predators present in the collection area and for those with which they had no previous exposure, indicating that F. verrucosa modulates nematocyst incorporation in response to the presence of predators as well as to diet. A coevolution of nudibranchs and potential predators may govern changes in nematocyst uptake.