Predatory fireflies and their toxic firefly prey have evolved distinct toxin resistance strategies

Toxic cardiotonic steroids (CTSs) act as a defense mechanism in many firefly species (Lampyridae) by inhibiting a crucial enzyme called Na+,K+-ATPase (NKA). Although most fireflies produce these toxins internally, species of the genus Photuris acquire them from a surprising source: predation on other fireflies. The contrasting physiology of toxin exposure and sequestration between Photuris and other firefly genera suggests that distinct strategies may be required to prevent self-intoxication. Our study demonstrates that both Photuris and their firefly prey have evolved highly resistant NKAs. Using an evolutionary analysis of the specific target of CTS (ATPα) in fireflies and gene editing in Drosophila, we find that the initial steps toward resistance were shared among Photuris and other firefly lineages. However, the Photuris lineage subsequently underwent multiple rounds of gene duplication and neofunctionalization, resulting in the development of ATPα paralogs that are differentially expressed and exhibit increasing resistance to CTS. By contrast, other firefly species have maintained a single copy. Our results implicate gene duplication as a facilitator in the transition of Photuris to its distinct ecological role as a predator of toxic firefly prey.

Predatory fireflies and their toxic firefly prey have evolved distinct toxin resistance strategies

Toxic cardiotonic steroids (CTS) act as a defense mechanism in many firefly species (Lampyridae) by inhibiting a crucial enzyme called Na+,K+-ATPase (NKA). While most fireflies produce these toxins internally, species of the genus Photuris acquire them from a surprising source: predation on other fireflies. The contrasting physiology of toxin exposure and sequestration between Photuris and other firefly genera suggests that distinct strategies may be required to prevent self-intoxication. Our study demonstrates that both Photuris and their firefly prey have evolved highly-resistant NKAs. Using an evolutionary analysis of the specific target of CTS (ATPα) in fireflies, and gene-editing in Drosophila, we find that the initial steps towards resistance were shared among Photuris and other firefly lineages. However, the Photuris lineage subsequently underwent multiple rounds of gene duplication and neofunctionalization, resulting in the development of ATPα paralogs that are differentially expressed and exhibit increasing resistance to CTS. In contrast, other firefly species have maintained a single copy. Our results implicate gene duplication as a facilitator in the transition of Photuris to its distinct ecological role as predator of toxic firefly prey.

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.

Convergent and divergent evolution of plant chemical defenses

The majority of the several hundred thousand specialized metabolites produced by plants function in defense against insects and other herbivores. Despite this diversity, identical metabolites or structurally distinct metabolites hitting the same targets in herbivorous animals have evolved repeatedly. This convergent evolution may reflect the constraints of plant primary metabolism in providing metabolic precursors, as well as the limited number of readily accessible targets in animals. These restrictions may make it uncommon for plants to develop completely novel toxic and deterrent metabolites, despite the ongoing evolution of resistance mechanisms in insect herbivores. Defensive compounds that are unique to individual genera or species often have long biosynthetic pathways that may complicate the repeated evolution of these metabolites in different plant species.

Knockdown of Na,K-ATPase β-subunits in Oncopeltus fasciatus induces molting problems and alterations in tracheal morphology

The ubiquitously expressed transmembrane enzyme Na,K-ATPase (NKA) is vital in maintaining functionality of cells. The association of α- and β-subunits is believed to be essential for forming a functional enzyme. In the large milkweed bug Oncopeltus fasciatus four α1-paralogs and four β-subunits exist that can associate into NKA complexes. This diversity raises the question of possible tissue-specific distribution and function. While the α1-subunits are known to modulate cardenolide-resistance and ion-transport efficiency, the functional importance of the β-subunits needed further investigation. We here characterize all four different β-subunits at the cellular, tissue, and whole organismal scales. A knockdown of different β-subunits heavily interferes with molting success resulting in strongly hampered phenotypes. The failure of ecdysis might be related to disrupted septate junction (SJ) formation, also reflected in β2-suppression-induced alteration in tracheal morphology. Our data further suggest the existence of isolated β-subunits forming homomeric or β-heteromeric complexes. This possible standalone and structure-specific distribution of the β-subunits predicts further, yet unknown pump-independent functions. The different effects caused by β knockdowns highlight the importance of the various β-subunits to fulfill tissue-specific requirements.

Identification of the NA+/K+-ATPase α-Isoforms in Six Species of Poison Dart Frogs and their Sensitivity to Cardiotonic Steroids

Cardiotonic steroids (CTS) are a group of compounds known to be toxic due to their ability to inhibit the Na⁺/K⁺-ATPase (NKA), which is essential to maintain the balance of ions in animal cells. An evolutionary strategy of molecular adaptation to avoid self-intoxication acquired by CTS defended organisms and their predators is the structural modification of their NKA where specific amino acid substitutions confer resistant phenotypes. Several lineages of poison dart frogs (Dendrobatidae) are well known to sequester a wide variety of lipophilic alkaloids from their arthropod diet, however there is no evidence of CTS-sequestration or dietary exposure. Interestingly this study identified the presence of α-NKA isoforms (α₁ and α₂) with amino acid substitutions indicative of CTS-resistant phenotypes in skeletal muscle transcriptomes obtained from six species of dendrobatids: Phyllobates aurotaenia, Oophaga anchicayensis, Epipedobates boulengeri, Andinobates bombetes, Andinobates minutus, and Leucostethus brachistriatus, collected in the Valle del Cauca (Colombia). P. aurotaenia, A. minutus, and E. boulengeri presented two variants for α₁-NKA, with one of them having these substitutions. In contrast, O. anchicayensis and A. bombetes have only one α₁-NKA isoform with an amino acid sequence indicative of CTS susceptibility and an α₂-NKA with one substitution that could confer a reduced affinity for CTS. The α₁ and α₂ isoforms of L. brachistriatus do not contain substitutions imparting CTS resistance. Our findings indicate that poison dart frogs express α-NKA isoforms with different affinities for CTS and the pattern of this expression might be influenced by factors related to evolutionary, physiological, ecological, and geographical burdens.

Molecular mechanisms of adaptive evolution in wild animals and plants

Wild animals and plants have developed a variety of adaptive traits driven by adaptive evolution, an important strategy for species survival and persistence. Uncovering the molecular mechanisms of adaptive evolution is the key to understanding species diversification, phenotypic convergence, and inter-species interaction. As the genome sequences of more and more non-model organisms are becoming available, the focus of studies on molecular mechanisms of adaptive evolution has shifted from the candidate gene method to genetic mapping based on genome-wide scanning. In this study, we reviewed the latest research advances in wild animals and plants, focusing on adaptive traits, convergent evolution, and coevolution. Firstly, we focused on the adaptive evolution of morphological, behavioral, and physiological traits. Secondly, we reviewed the phenotypic convergences of life history traits and responding to environmental pressures, and the underlying molecular convergence mechanisms. Thirdly, we summarized the advances of coevolution, including the four main types: mutualism, parasitism, predation and competition. Overall, these latest advances greatly increase our understanding of the underlying molecular mechanisms for diverse adaptive traits and species interaction, demonstrating that the development of evolutionary biology has been greatly accelerated by multi-omics technologies. Finally, we highlighted the emerging trends and future prospects around the above three aspects of adaptive evolution.

Larvicidal and repellent potential of Ageratum houstonianum against Culex pipiens

Mosquitoes are unquestionably the most medic arthropod vectors of disease. Culex pipiens, usually defined as a common house mosquito, is a well-known carrier of several virus diseases. Crude ethanol extracts of different organs of Agratum houstonianum are tested with Culex pipiens Linnaeus (Diptera: Culicidae) to determine their larvicidal, antifeedant, and repellency effects. Alongside biochemical analysis, the activity of the AChE, ATPase, CarE, and CYP-450 is detected in the total hemolymph of the C. pipiens larvae to examine the enzymatic action on the way to explain their neurotoxic effect and mode of action. Through HPLC and GC-MS analysis of the phytochemical profile of A. houstonianum aerial parts is identified. The larvicidal activity of aerial parts; flower (AF), leaf (AL), and stem (AS) of A. houstonianum extracts are evaluated against the 3rd instar larvae of C. pipiens at 24-, 48- and 72-post-treatment. A. houstonianium AF, AL, and AS extracts influenced the mortality of larvae with LC50 values 259.79, 266.85, and 306.86 ppm, respectively after 24 h of application. The potency of AF and AL extracts was 1.69- and 1.25-folds than that of AS extract, respectively. A high repellency percentage was obtained by AF extract 89.10% at a dose of 3.60 mg/cm². A. houstonianium AF prevailed inhibition on acetylcholinesterase and decrease in carboxylesterase activity. Moreover, a significant increase in the ATPase levels and a decrease in cytochrome P-450 monooxegenase activity (- 36.60%) are detected. HPLC analysis prevailed chlorogenic and rosmarinic acid as the major phenolic acids in AL and AF, respectively. GC-MS analysis of A. houstonianum results in the identification of phytol as the major makeup. Precocene I and II were detected in AF. Linoleic, linolenic, and oleic acid were detected in comparable amounts in the studied organs. Overall, results suggest that the A. houstonianum flower extract (AF) exhibits significant repellent, antifeedant, and larvicidal activities.

Na,K-ATPase α1 and β-subunits show distinct localizations in the nervous tissue of the large milkweed bug

The Na,K-ATPase (NKA) is an essential ion transporter and signaling molecule in all animal tissues and believed to consist at least one α and one ß-subunit to form a functional enzyme. In the large milkweed bug, Oncopeltus fasciatus, adaptation to dietary cardiac glycosides (CGs), which can fatally block the NKA, has resulted in gene duplications leading to four α1-subunits. These differ in sensitivity to CGs, but resistance trades off against ion pumping activity, thus influencing the α1-subunits’ suitability for specific tissues. Besides, O. fasciatus possesses four different ß-subunits that can alter the NKA's kinetics and should play an essential role in the formation of cellular junctions.

Proteomic analyses revealed the distribution and composition of α1/ß-complexes in the nervous tissue of O. fasciatus. The highly CG-resistant, but less active α1B and the highly active, but less resistant α1C predominated in the nervous tissue and co-occurred with ß2 and ß3, partly forming larger complexes than just heterodimers. Immunohistochemical analyses provided a fine scale resolution of the subunits’ distribution in different morphological structures of the nervous tissue. This may suggest that α1 as well as ß-subunits occur in isolation without the other subunit, which contradicts the present understanding that the two types of subunits have to associate to form functional complexes. An isolated occurrence was especially prominent for ß3 and βx, the enigmatic fourth and N-terminally largely truncated ß-subunit. We hypothesize that dimerization of these ß-subunits plays a role in cell–cell contacts.

Sequestration of Plant Defense Compounds by Insects: From Mechanisms to Insect-Plant Coevolution

Plant defense compounds play a key role in the evolution of insect-plant associations by selecting for behavioral, morphological, and physiological insect adaptations. Sequestration, the ability of herbivorous insects to accumulate plant defense compounds to gain a fitness advantage, represents a complex syndrome of adaptations that has evolved in all major lineages of herbivorous insects and involves various classes of plant defense compounds. In this article, we review progress in understanding how insects selectively accumulate plant defense metabolites and how the evolution of specific resistance mechanisms to these defense compounds enables sequestration. These mechanistic considerations are further integrated into the concept of insect-plant coevolution. Comparative genome and transcriptome analyses, combined with approaches based on analytical chemistry that are centered in phylogenetic frameworks, will help to reveal adaptations underlying the sequestration syndrome, which is essential to understanding the influence of sequestration on insect-plant coevolution.

Every breath you take: assessing metabolic costs of toxin resistance in garter snakes (Thamnophis)

Trait specialization often comes at the expense of original trait function, potentially causing evolutionary tradeoffs that may render specialist populations vulnerable to extinction. However, many specialized adaptations evolve repeatedly, suggesting selection favors specialization in specific environments. Some garter snake (Thamnophis) populations possess specialized mutations in voltage-gated sodium channels that allow them to consume Pacific newts (Taricha) defended by a highly potent neurotoxin (tetrodotoxin). These mutations, however, also decrease protein and muscle function, suggesting garter snakes may suffer evolutionary tradeoffs. We measured a key physiological process, standard metabolic rate (SMR), to investigate whether specialized adaptations in toxin-resistant garter snakes affect baseline energy expenditure. In snakes, skeletal muscles influence metabolism and power ventilation, so inefficiencies of sodium channels in these muscles might impact whole-animal energy expenditure. Further, because sodium channels are membrane-bound proteins, inefficiencies of channel kinetics and performance might be exacerbated at suboptimal temperatures. We measured SMR in 2 species, Thamnophis atratus and Thamnophis sirtalis, that independently evolved tetrodotoxin resistance through unique mutations, providing replicate experiments with distinct underlying genetics and potential physiological costs. Despite our expectations, neither resistance phenotype nor sodium channel genotype affected metabolism and resistant snakes did not perform worse under suboptimal body temperature. Instead, T. atratus and T. sirtalis show nearly identical rates of mass-adjusted energy expenditure at both temperatures, despite differing eco-morphologies, life histories, and distant phylogenetic positions. These findings suggest SMR may be a conserved feature of Thamnophis, and that any organismal tradeoffs may be compensated to retain whole-animal function.

The evolution of coevolution in the study of species interactions

The study of reciprocal adaptation in interacting species has been an active and inspiring area of evolutionary research for nearly 60 years. Perhaps owing to its great natural history and potential consequences spanning population divergence to species diversification, coevolution continues to capture the imagination of biologists. Here we trace developments following Ehrlich and Raven's classic paper, with a particular focus on the modern influence of two studies by Dr. May Berenbaum in the 1980s. This series of classic work presented a compelling example exhibiting the macroevolutionary patterns predicted by Ehrlich and Raven and also formalized a microevolutionary approach to measuring selection, functional traits, and understanding reciprocal adaptation between plants and their herbivores. Following this breakthrough was a wave of research focusing on diversifying macroevolutionary patterns, mechanistic chemical ecology, and natural selection on populations within and across community types. Accordingly, we breakdown coevolutionary theory into specific hypotheses at different scales: reciprocal adaptation between populations within a community, differential coevolution among communities, lineage divergence, and phylogenetic patterns. We highlight progress as well as persistent gaps, especially the link between reciprocal adaptation and diversification.

Concerted evolution reveals co-adapted amino acid substitutions in Na+K+-ATPase of frogs that prey on toxic toads

Although gene duplication is an important source of evolutionary innovation, the functional divergence of duplicates can be opposed by ongoing gene conversion between them. Here, we report on the evolution of a tandem duplication of Na⁺,K⁺-ATPase subunit α1 (ATP1A1) shared by frogs in the genus Leptodactylus, a group of species that feeds on toxic toads. One ATP1A1 paralog evolved resistance to toad toxins although the other retained ancestral susceptibility. Within species, frequent non-allelic gene conversion homogenized most of the sequence between the two copies but was counteracted by strong selection on 12 amino acid substitutions that distinguish the two paralogs. Protein-engineering experiments show that two of these substitutions substantially increase toxin resistance, whereas the additional 10 mitigate their deleterious effects on ATPase activity. Our results reveal how examination of neo-functionalized gene duplicate evolution can help pinpoint key functional substitutions and interactions with the genetic backgrounds on which they arise.

Gene Conversion Facilitates the Adaptive Evolution of Self-Resistance in Highly Toxic Newts

Reconstructing the histories of complex adaptations and identifying the evolutionary mechanisms underlying their origins are two of the primary goals of evolutionary biology. Taricha newts, which contain high concentrations of the deadly toxin tetrodotoxin (TTX) as an antipredator defense, have evolved resistance to self-intoxication, which is a complex adaptation requiring changes in six paralogs of the voltage-gated sodium channel (Na_v) gene family, the physiological target of TTX. Here, we reconstruct the origins of TTX self-resistance by sequencing the entire Na_v gene family in newts and related salamanders. We show that moderate TTX resistance evolved early in the salamander lineage in three of the six Na_v paralogs, preceding the proposed appearance of tetrodotoxic newts by ∼100 My. TTX-bearing newts possess additional unique substitutions across the entire Na_v gene family that provide physiological TTX resistance. These substitutions coincide with signatures of positive selection and relaxed purifying selection, as well as gene conversion events, that together likely facilitated their evolution. We also identify a novel exon duplication within Na_v1.4 encoding an expressed TTX-binding site. Two resistance-conferring changes within newts appear to have spread via nonallelic gene conversion: in one case, one codon was copied between paralogs, and in the second, multiple substitutions were homogenized between the duplicate exons of Na_v1.4. Our results demonstrate that gene conversion can accelerate the coordinated evolution of gene families in response to a common selection pressure.

Avian phenotypic convergence is subject to low genetic constraints based on genomic evidence

Background

Phenotypic convergence between distinct species provides an opportunity to examine the predictability of genetic evolution. Unrelated species sharing genetic underpinnings for phenotypic convergence suggests strong genetic constraints, and thus high predictability of evolution. However, there is no clear big picture of the genomic constraints on convergent evolution. Genome-based phylogenies have confirmed many cases of phenotypic convergence in birds, making them a good system for examining genetic constraints in phenotypic convergence. In this study, we used hierarchical genomic approaches to estimate genetic constraints in three convergent avian traits: nocturnality, raptorial behavior and foot-propelled diving.

Results

Phylogeny-based hypothesis tests and positive selection tests were applied to compare 16 avian genomes, representing 14 orders, and identify genes with strong convergence signals. We found 43 adaptively convergent genes (ACGs) associated with the three phenotypic convergence cases and assessed genetic constraints in all three cases, from (amino acid) site mutations to genetic pathways. We found that the avian orders shared few site mutations in the ACGs that contributed to the convergent phenotypes, and that these ACGs were not enriched in any genetic pathways. In addition, different pairs of orders with convergent foot-propelled diving or raptorial behaviors shared few ACGs. We also found that closely related orders that shared foot-propelled diving behavior did not share more ACGs than did distinct orders, suggesting that convergence among these orders could not be explained by their initial genomic backgrounds.

Conclusions

Our analyses of three avian convergence events suggest low constraints for phenotypic convergence across multiple genetic levels, implying that genetic evolution is unpredictable at the phylogenetic level of avian order. Ours is one of first studies to apply hierarchical genomic examination to multiple avian convergent cases to assess the genetic constraints in life history trait evolution.

ABCB transporters in a leaf beetle respond to sequestered plant toxins

Phytophagous insects can tolerate and detoxify toxic compounds present in their host plants and have evolved intricate adaptations to this end. Some insects even sequester the toxins for their defence. This necessitates specific mechanisms, especially carrier proteins that regulate uptake and transport to specific storage sites or protect sensitive tissues from noxious compounds. We identified three ATP-binding cassette subfamily B (ABCB) transporters from the transcriptome of the cardenolide-sequestering leaf beetle Chrysochus auratus and analysed their functional role in the sequestration process. These were heterologously expressed and tested for their ability to interact with various potential substrates: verapamil (standard ABCB substrate), the cardenolides digoxin (commonly used), cymarin (present in the species's host plant) and calotropin (present in the ancestral host plants). Verapamil stimulated all three ABCBs and each was activated by at least one cardenolide, however, they differed as to which they were activated by. While the expression of the most versatile transporter fits with a protective role in the blood-brain barrier, the one specific for cymarin shows an extreme abundance in the elytra, coinciding with the location of the defensive glands. Our data thus suggest a key role of ABCBs in the transport network needed for cardenolide sequestration.

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.

Toxicity and effects of four insecticides on Na+, K+-ATPase of western flower thrips, Frankliniella occidentalis

Western flower thrips (WFT), Frankliniella occidentalis, has become an important pest of vegetables worldwide, due to its economic damage to crop production. In order to control WFT, chemical insecticides are widely used. However, WFT has developed a high resistance against many kinds of insecticides. Na⁺, K⁺-ATPase, playing an important role in the ionic transmission across the membrane, is commonly considered to be the target of several xenobiotic compounds. However, whether the Na⁺, K⁺-ATPase can be used as one of the target sites for controlling WFT is still unknown. In this study, resistance levels of WFT to four insecticides (chlorpyrifos, beta cypermethrin, abamectin, and thiamethoxam) were measured. It was found that all four insecticides exhibited significant inhibitory effects on WFT, especially on nymphs. The activity of Na⁺, K⁺-ATPase was estimated after the treatment of four insecticides. Additionally, mRNA expression levels of three Na⁺, K⁺-ATPase α-subunit isoforms (X1, X2 and X3) were detected using RT-qPCR. The transcription profile of three Na⁺, K⁺-ATPase α-subunit isoforms were diverse after treatment by these four insecticides, which indicated that these isoforms might play different roles in the tolerance to insecticides. The results suggested that Na⁺, K⁺-ATPase can obviously be inhibited by these four classes of insecticide, and may serve as the new target for controlling WFT.

Genome editing retraces the evolution of toxin resistance in the monarch butterfly

Identifying the genetic mechanisms of adaptation requires the elucidation of links between the evolution of DNA sequence, phenotype, and fitness¹. Convergent evolution can be used as a guide to identify candidate mutations that underlie adaptive traits^2-4, and new genome editing technology is facilitating functional validation of these mutations in whole organisms^1,5. We combined these approaches to study a classic case of convergence in insects from six orders, including the monarch butterfly (Danaus plexippus), that have independently evolved to colonize plants that produce cardiac glycoside toxins^6-11. Many of these insects evolved parallel amino acid substitutions in the α-subunit (ATPα) of the sodium pump (Na⁺/K⁺-ATPase)^7-11, the physiological target of cardiac glycosides¹². Here we describe mutational paths involving three repeatedly changing amino acid sites (111, 119 and 122) in ATPα that are associated with cardiac glycoside specialization^13,14. We then performed CRISPR-Cas9 base editing on the native Atpα gene in Drosophila melanogaster flies and retraced the mutational path taken across the monarch lineage^11,15. We show in vivo, in vitro and in silico that the path conferred resistance and target-site insensitivity to cardiac glycosides¹⁶, culminating in triple mutant 'monarch flies' that were as insensitive to cardiac glycosides as monarch butterflies. 'Monarch flies' retained small amounts of cardiac glycosides through metamorphosis, a trait that has been optimized in monarch butterflies to deter predators^17-19. The order in which the substitutions evolved was explained by amelioration of antagonistic pleiotropy through epistasis^13,14,20-22. Our study illuminates how the monarch butterfly evolved resistance to a class of plant toxins, eventually becoming unpalatable, and changing the nature of species interactions within ecological communities^{2,6-11,15,17-19}.

Adaptive substitutions underlying cardiac glycoside insensitivity in insects exhibit epistasis in vivo

Predicting how species will respond to selection pressures requires understanding the factors that constrain their evolution. We use genome engineering of Drosophila to investigate constraints on the repeated evolution of unrelated herbivorous insects to toxic cardiac glycosides, which primarily occurs via a small subset of possible functionally-relevant substitutions to Na⁺,K⁺-ATPase. Surprisingly, we find that frequently observed adaptive substitutions at two sites, 111 and 122, are lethal when homozygous and adult heterozygotes exhibit dominant neural dysfunction. We identify a phylogenetically correlated substitution, A119S, that partially ameliorates the deleterious effects of substitutions at 111 and 122. Despite contributing little to cardiac glycoside-insensitivity in vitro, A119S, like substitutions at 111 and 122, substantially increases adult survivorship upon cardiac glycoside exposure. Our results demonstrate the importance of epistasis in constraining adaptive paths. Moreover, by revealing distinct effects of substitutions in vitro and in vivo, our results underscore the importance of evaluating the fitness of adaptive substitutions and their interactions in whole organisms.

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.

Predictability in the evolution of Orthopteran cardenolide insensitivity

The repeated evolutionary specialization of distantly related insects to cardenolide-containing host plants provides a stunning example of parallel adaptation. Hundreds of herbivorous insect species have independently evolved insensitivity to cardenolides, which are potent inhibitors of the alpha-subunit of Na⁺,K⁺-ATPase (ATPα). Previous studies investigating ATPα-mediated cardenolide insensitivity in five insect orders have revealed remarkably high levels of parallelism in the evolution of this trait, including the frequent occurrence of parallel amino acid substitutions at two sites and recurrent episodes of duplication followed by neo-functionalization. Here we add data for a sixth insect order, Orthoptera, which includes an ancient group of highly aposematic cardenolide-sequestering grasshoppers in the family Pyrgomorphidae. We find that Orthopterans exhibit largely predictable patterns of evolution of insensitivity established by sampling other insect orders. Taken together the data lend further support to the proposal that negative pleiotropic constraints are a key determinant in the evolution of cardenolide insensitivity in insects. Furthermore, analysis of our expanded taxonomic survey implicates positive selection acting on site 111 of cardenolide-sequestering species with a single-copy of ATPα, and sites 115, 118 and 122 in lineages with neo-functionalized duplicate copies, all of which are sites of frequent parallel amino acid substitution. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Robust reference gene design and validation for expression studies in the large milkweed bug, Oncopeltus fasciatus, upon cardiac glycoside stress

Despite its history as a developmental and evolutionary model organism, gene expression analysis in the large milkweed bug, Oncopeltus fasciatus, has rarely been explored using quantitative real-time PCR. The strength of this method depends greatly on the endogenous controls used for normalization, which are lacking for the milkweed bug system. Here, to fill in this gap in our knowledge, we validated the stability of a set of ten candidate reference genes identified from the O. fasciatus transcriptome, and did so upon exposure to a dietary toxin, a cardiac glycoside, and across four different exposure periods. To increase robustness against gDNA contaminants, genome resources were used to design intron-bridging primers. A comprehensive stability validation by the Bestkeeper, Normfinder, geNorm and comparative ΔCt methods identified ef1a and tubulin as the most stable genes across treatments and time points, whereas 18S rRNA was the most unstable. However, accounting for the temporal scale indicated that time point confined normalizers might enable higher quantification accuracy for treatment comparison. Overall this study demonstrates: (i) a robust RT-qPCR primer design approach is possible for non-model organisms where genome annotation is often incomplete, and (ii) the importance of detailed reference gene stability exploration in multifactorial experimental designs.

Toxicity of Milkweed Leaves and Latex: Chromatographic Quantification Versus Biological Activity of Cardenolides in 16 Asclepias Species

Cardenolides are classically studied steroidal defenses in chemical ecology and plant-herbivore coevolution. Although milkweed plants (Asclepias spp.) produce up to 200 structurally different cardenolides, all compounds seemingly share the same well-characterized mode of action, inhibition of the ubiquitous Na⁺/K⁺ ATPase in animal cells. Over their evolutionary radiation, milkweeds show a quantitative decline of cardenolide production and diversity. This reduction is contrary to coevolutionary predictions and could represent a cost-saving strategy, i.e. production of fewer but more toxic cardenolides. Here we test this hypothesis by tandem cardenolide quantification using HPLC (UV absorption of the unsaturated lactone) and a pharmacological assay (in vitro inhibition of a sensitive Na⁺/K⁺ ATPase) in a comparative study of 16 species of Asclepias. We contrast cardenolide concentrations in leaf tissue to the subset of cardenolides present in exuding latex. Results from the two quantification methods were strongly correlated, but the enzymatic assay revealed that milkweed cardenolide mixtures often cause stronger inhibition than equal amounts of a non-milkweed reference cardenolide, ouabain. Cardenolide concentrations in latex and leaves were positively correlated across species, yet latex caused 27% stronger enzyme inhibition than equimolar amounts of leaf cardenolides. Using a novel multiple regression approach, we found three highly potent cardenolides (identified as calactin, calotropin, and voruscharin) to be primarily responsible for the increased pharmacological activity of milkweed cardenolide mixtures. However, contrary to an expected trade-off between concentration and toxicity, later-diverging milkweeds had the lowest amounts of these potent cardenolides, perhaps indicating an evolutionary response to milkweed's diverse community of specialist cardenolide-sequestering insect herbivores.

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.

Toxicity of the spiny thick-foot Pachypodium

PREMISE OF THE STUDY

Pachypodium (Apocynaceae) is a genus of iconic stem-succulent and poisonous plants endemic to Madagascar and southern Africa. We tested hypotheses about the mode of action and macroevolution of toxicity in this group. We further hypothesized that while monarch butterflies are highly resistant to cardenolide toxins (a type of cardiac glycoside) from American Asclepias, they may be negatively affected by Pachypodium defenses, which evolved independently.

METHODS

We grew 16 of 21 known Pachypodium spp. and quantified putative cardenolides by HPLC and also by inhibition of animal Na⁺ /K⁺ -ATPase (the physiological target of cardiac glycosides) using an in vitro assay. Pachypodium extracts were tested against monarch caterpillars in a feeding bioassay. We also tested four Asclepias spp. and five Pachypodium spp. extracts, contrasting inhibition of the cardenolide-sensitive porcine Na⁺ /K⁺ -ATPase to the monarch's resistant form.

KEY RESULTS

We found evidence for low cardenolides by HPLC, but substantial toxicity when extracts were assayed on Na⁺ /K⁺ -ATPases. Toxicity showed phylogenetic signal, and taller species showed greater toxicity (this was marginal after phylogenetic correction). Application of Pachypodium extracts to milkweed leaves reduced monarch growth, and this was predicted by inhibition of the sensitive Na⁺ /K⁺ -ATPase in phylogenetic analyses. Asclepias extracts were 100-fold less potent against the monarch compared to the porcine Na⁺ /K⁺ -ATPase, but this difference was absent for Pachypodium extracts.

CONCLUSIONS

Pachypodium contains potent toxicity capable of inhibiting sensitive and cardenolide-adapted Na⁺ /K⁺ -ATPases. Given the monarch's sensitivity to Pachypodium, we suggest that these plants contain novel cardiac glycosides or other compounds that facilitate toxicity by binding to Na⁺ /K⁺ -ATPases.

Does host plant adaptation lead to pesticide resistance in generalist herbivores?

Most herbivorous arthropods feed on one or a few closely related plant species; however, certain insect and mite species have a greatly expanded host range. Several of these generalists also show a remarkable propensity to evolve resistance to chemical pesticides. In this review, we ask if the evolution of mechanisms to tolerate the diversity of plant secondary metabolites that generalist herbivores encounter, has pre-adapted them to resist synthetic pesticides. Critical examination of the evidence suggests that a generalist life-style per se is not a predictor of rapid resistance evolution to pesticides. Rather the prevalence of pesticide resistance in generalist herbivores probably reflects their economic importance as pests and thus the strong selection imposed by intensive pesticide use.

Substitutions in the cardenolide binding site and interaction of subunits affect kinetics besides cardenolide sensitivity of insect Na,K-ATPase

Substitutions within the cardenolide target site of several insects' Na,K-ATPase α-subunits may confer resistance against toxic cardenolides. However, to which extent these substitutions alter the Na,K-ATPase's kinetic properties and how they interact with different β-subunits is not clear. The cardenolide-adapted milkweed bug Oncopeltus fasciatus possesses three paralogs of the α-subunit (A, B, and C) that differ in number and identity of resistance-conferring substitutions. We introduced these substitutions into the α-subunit of Drosophila melanogaster and combined them with the β-subunits Nrv2.2 and Nrv3. The substitutions Q111T-N122H-F786N-T797A (A-copy mimic) and Q111T-N122H-F786N (B-copy mimic) mediated high insensitivity to ouabain, yet they drastically lowered ATPase activity. Remarkably, the identity of the β-subunit was decisive and all α-subunits were less active when combined with Nrv3 than when combined with Nrv2.2. Both the substitutions and the co-expressed β-subunit strongly affected the enyzme's affinity for Na⁺ and K⁺. Na⁺ affinity was considerably higher for all enzymes expressed with nrv3 while expression with nrv2.2 mostly increased K⁺ affinity. Our results provide the first evidence that resistance against cardenolides comes at the cost of significantly altered kinetic properties of the Na,K-ATPase. The β-subunit can strongly modulate these properties but cannot fully compensate for the effect of the substitutions.