Mining the plant-herbivore interface with a leafmining Drosophila of Arabidopsis

Evolution of chemosensory and detoxification gene families across herbivorous Drosophilidae

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genomic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families-genes directly mediating interactions with plant chemical defenses-underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many insect lineages are ancient (>150 million years ago (mya)), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (<15 mya) herbivore lineage of mustard (Brassicales) specialists and carnation (Caryophyllaceae) specialists, and several nonherbivorous species. Comparative genomic analyses revealed that herbivorous Scaptomyza has among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant-binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on living plants (bitter or electrophilic phytotoxins) or their ancestral diet (fermenting plant volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight gene candidates that have also been linked to other dietary transitions in Drosophila.

Evolution of chemosensory and detoxification gene families across herbivorous Drosophilidae

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genetic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families - genes directly mediating interactions with plant chemical defenses - underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many lineages are ancient (>150 million years ago [mya]), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (<15 mya) herbivore lineage of mustard (Brassicales) specialists and carnation (Caryophyllaceae) specialists, and several non-herbivorous species. Comparative genomic analyses revealed that herbivorous Scaptomyza have among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on plants (bitter or electrophilic phytotoxins) or their ancestral diet (yeast and fruit volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight strong gene candidates that have also been linked to other dietary transitions in Drosophila .

Identifying Soybean Pod Borer (Leguminivora glycinivorella) Resistance QTLs and the Mechanism of Induced Defense Using Linkage Mapping and RNA-Seq Analysis

The soybean pod borer (Leguminivora glycinivorella) (SPB) is a major cause of soybean (Glycine max L.) yield losses in northeast Asia, thus it is desirable to elucidate the resistance mechanisms involved in soybean response to the SPB. However, few studies have mapped SPB-resistant quantitative trait loci (QTLs) and deciphered the response mechanism in soybean. Here, we selected two soybean varieties, JY93 (SPB-resistant) and K6 (SPB-sensitive), to construct F₂ and F_2:3 populations for QTL mapping and collected pod shells before and after SPB larvae chewed on the two parents to perform RNA-Seq, which can identify stable QTLs and explore the response mechanism of soybean to the SPB. The results show that four QTLs underlying SPB damage to seeds were detected on chromosomes 4, 9, 13, and 15. Among them, qESP-9-1 was scanned in all environments, hence it can be considered a stable QTL. All QTLs explained 0.79 to 6.09% of the phenotypic variation. Meanwhile, 2298 and 3509 DEGs were identified for JY93 and K6, respectively, after the SPB attack, and most of these genes were upregulated. Gene Ontology enrichment results indicated that the SPB-induced and differently expressed genes in both parents are involved in biological processes such as wound response, signal transduction, immune response, and phytohormone pathways. Interestingly, secondary metabolic processes such as flavonoid synthesis were only significantly enriched in the upregulated genes of JY93 after SPB chewing compared with K6. Finally, we identified 18 candidate genes related to soybean pod borer resistance through the integration of QTL mapping and RNA-Seq analysis. Seven of these genes had similar expression patterns to the mapping parents in four additional soybean germplasm after feeding by the SPB. These results provide additional knowledge of the early response and induced defense mechanisms against the SPB in soybean, which could help in breeding SPB-resistant soybean accessions.

Evolution of olfactory receptors tuned to mustard oils in herbivorous Drosophilidae

The diversity of herbivorous insects is attributed to their propensity to specialize on toxic plants. In an evolutionary twist, toxins betray the identity of their bearers when herbivores coopt them as cues for host-plant finding, but the evolutionary mechanisms underlying this phenomenon are poorly understood. We focused on Scaptomyza flava, an herbivorous drosophilid specialized on isothiocyanate (ITC)-producing (Brassicales) plants, and identified Or67b paralogs that were triplicated as mustard-specific herbivory evolved. Using in vivo heterologous systems for the expression of olfactory receptors, we found that S. flava Or67bs, but not the homologs from microbe-feeding relatives, responded selectively to ITCs, each paralog detecting different ITC subsets. Consistent with this, S. flava was attracted to ITCs, as was Drosophila melanogaster expressing S. flava Or67b3 in the homologous Or67b olfactory circuit. ITCs were likely coopted as olfactory attractants through gene duplication and functional specialization (neofunctionalization and subfunctionalization) in S. flava, a recently derived herbivore.

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

Genetic adaptation, occurring over a long evolutionary time, enables host-specialized herbivores to develop novel resistance traits and to efficiently counteract the defenses of a narrow range of host plants. In contrast, physiological acclimation, leading to the suppression and/or detoxification of host defenses, is hypothesized to enable broad generalists to shift between plant hosts. However, the host adaptation mechanisms used by generalists composed of host-adapted populations are not known. Two-spotted spider mite (TSSM; Tetranychus urticae) is an extreme generalist herbivore whose individual populations perform well only on a subset of potential hosts. We combined experimental evolution, Arabidopsis thaliana genetics, mite reverse genetics, and pharmacological approaches to examine mite host adaptation upon the shift of a bean (Phaseolus vulgaris)-adapted population to Arabidopsis. We showed that cytochrome P450 monooxygenases are required for mite adaptation to Arabidopsis. We identified activities of two tiers of P450s: general xenobiotic-responsive P450s that have a limited contribution to mite adaptation to Arabidopsis and adaptation-associated P450s that efficiently counteract Arabidopsis defenses. In approximately 25 generations of mite selection on Arabidopsis plants, mites evolved highly efficient detoxification-based adaptation, characteristic of specialist herbivores. This demonstrates that specialization to plant resistance traits can occur within the ecological timescale, enabling the TSSM to shift to novel plant hosts.

The UDP-Glycosyltransferase Family in Drosophila melanogaster: Nomenclature Update, Gene Expression and Phylogenetic Analysis

UDP-glycosyltransferases (UGTs) are important conjugation enzymes found in all kingdoms of life, catalyzing a sugar conjugation with small lipophilic compounds and playing a crucial role in detoxification and homeostasis. The UGT gene family is defined by a signature motif in the C-terminal domain where the uridine diphosphate (UDP)-sugar donor binds. UGTs have been identified in a number of insect genomes over the last decade and much progress has been achieved in characterizing their expression patterns and molecular functions. Here, we present an update of the complete repertoire of UGT genes in Drosophila melanogaster and provide a brief overview of the latest research in this model insect. A total of 35 UGT genes are found in the D. melanogaster genome, localized to chromosomes 2 and 3 with a high degree of gene duplications on the chromosome arm 3R. All D. melanogaster UGT genes have now been named in FlyBase according to the unified UGT nomenclature guidelines. A phylogenetic analysis of UGT genes shows lineage-specific gene duplications. Analysis of anatomical and induced gene expression patterns demonstrate that some UGT genes are differentially expressed in various tissues or after environmental treatments. Extended searches of UGT orthologs from 18 additional Drosophila species reveal a diversity of UGT gene numbers and composition. The roles of Drosophila UGTs identified to date are briefly reviewed, and include xenobiotic metabolism, nicotine resistance, olfaction, cold tolerance, sclerotization, pigmentation, and immunity. Together, the updated genomic information and research overview provided herein will aid further research in this developing field.

The Evolution of Endophagy in Herbivorous Insects

Herbivorous feeding inside plant tissues, or endophagy, is a common lifestyle across Insecta, and occurs in insect taxa that bore, roll, tie, mine, gall, or otherwise modify plant tissues so that the tissues surround the insects while they are feeding. Some researchers have developed hypotheses to explain the adaptive significance of certain endophytic lifestyles (e.g., miners or gallers), but we are unaware of previous efforts to broadly characterize the adaptive significance of endophagy more generally. To fill this knowledge gap, we characterized the limited set of evolutionary selection pressures that could have encouraged phytophagous insects to feed inside plants, and then consider how these factors align with evidence for endophagy in the evolutionary history of orders of herbivorous insects. Reviewing the occurrence of endophytic taxa of various feeding guilds reveals that the pattern of evolution of endophagy varies strongly among insect orders, in some cases being an ancestral trait (e.g., Coleoptera and Lepidoptera) while being more derived in others (e.g., Diptera). Despite the large diversity of endophagous lifestyles and evolutionary trajectories that have led to endophagy in insects, our consideration of selection pressures leads us to hypothesize that nutritionally based factors may have had a stronger influence on evolution of endophagy than other factors, but that competition, water conservation, and natural enemies may have played significant roles in the development of endophagy.

Molecular Evolution of the Glutathione S-Transferase Family in the Bemisia tabaci Species Complex

The glutathione S-transferase (GST) family plays an important role in the adaptation of herbivorous insects to new host plants and other environmental constrains. The family codes for enzymes that neutralize reactive oxygen species and phytotoxins through the conjugation of reduced glutathione. Here, we studied the molecular evolution of the GST family in Bemisia tabaci, a complex of >35 sibling species, differing in their geographic and host ranges. We tested if some enzymes evolved different functionality, by comparing their sequences in six species, representing five of the six major genetic clades in the complex. Comparisons of the nonsynonymous to synonymous substitution ratios detected positive selection events in 11 codons of 5 cytosolic GSTs. Ten of them are located in the periphery of the GST dimer, suggesting a putative involvement in interactions with other proteins. Modeling the tertiary structure of orthologous enzymes, identified additional 19 mutations in 9 GSTs, likely affecting the enzymes' functionality. Most of the mutation events were found in the environmentally responsive classes Delta and Sigma, indicating a slightly different delta/sigma tool box in each species. At a broader genomic perspective, our analyses indicated a significant expansion of the Delta GST class in B. tabaci and a general association between the diet breadth of hemipteran species and their total number of GST genes. We raise the possibility that at least some of the identified changes improve the fitness of the B. tabaci species carrying them, leading to their better adaptation to specific environments.

Chemosensation and Evolution of Drosophila Host Plant Selection

The ability to respond to chemosensory cues is critical for survival of most organisms. Among insects, Drosophila melanogaster has the best characterized olfactory system, and the availability of genome sequences of 30 Drosophila species provides an ideal scenario for studies on evolution of chemosensation. Gene duplications of chemoreceptor genes allow for functional diversification of the rapidly evolving chemoreceptor repertoire. Although some species of the genus Drosophila are generalists for host plant selection, rapid evolution of olfactory receptors, gustatory receptors, odorant-binding proteins, and cytochrome P450s has enabled diverse host specializations of different members of the genus. Here, I review diversification of the chemoreceptor repertoire among members of the genus Drosophila along with co-evolution of detoxification mechanisms that may have enabled occupation of diverse host plant ecological niches.

Horizontal Transfer of Bacterial Cytolethal Distending Toxin B Genes to Insects

Horizontal gene transfer events have played a major role in the evolution of microbial species, but their importance in animals is less clear. Here, we report horizontal gene transfer of cytolethal distending toxin B (cdtB), prokaryotic genes encoding eukaryote-targeting DNase I toxins, into the genomes of vinegar flies (Diptera: Drosophilidae) and aphids (Hemiptera: Aphididae). We found insect-encoded cdtB genes are most closely related to orthologs from bacteriophage that infect Candidatus Hamiltonella defensa, a bacterial mutualistic symbiont of aphids that confers resistance to parasitoid wasps. In drosophilids, cdtB orthologs are highly expressed during the parasitoid-prone larval stage and encode a protein with ancestral DNase activity. We show that cdtB has been domesticated by diverse insects and hypothesize that it functions in defense against their natural enemies.

Evolution of Reproductive Behavior

Behaviors associated with reproduction are major contributors to the evolutionary success of organisms and are subject to many evolutionary forces, including natural and sexual selection, and sexual conflict. Successful reproduction involves a range of behaviors, from finding an appropriate mate, courting, and copulation, to the successful production and (in oviparous animals) deposition of eggs following mating. As a consequence, behaviors and genes associated with reproduction are often under strong selection and evolve rapidly. Courtship rituals in flies follow a multimodal pattern, mediated through visual, chemical, tactile, and auditory signals. Premating behaviors allow males and females to assess the species identity, reproductive state, and condition of their partners. Conflicts between the "interests" of individual males, and/or between the reproductive strategies of males and females, often drive the evolution of reproductive behaviors. For example, seminal proteins transmitted by males often show evidence of rapid evolution, mediated by positive selection. Postmating behaviors, including the selection of oviposition sites, are highly variable and Drosophila species span the spectrum from generalists to obligate specialists. Chemical recognition features prominently in adaptation to host plants for feeding and oviposition. Selection acting on variation in pre-, peri-, and postmating behaviors can lead to reproductive isolation and incipient speciation. Response to selection at the genetic level can include the expansion of gene families, such as those for detecting pheromonal cues for mating, or changes in the expression of genes leading to visual cues such as wing spots that are assessed during mating. Here, we consider the evolution of reproductive behavior in Drosophila at two distinct, yet complementary, scales. Some studies take a microevolutionary approach, identifying genes and networks involved in reproduction, and then dissecting the genetics underlying complex behaviors in D. melanogaster Other studies take a macroevolutionary approach, comparing reproductive behaviors across the genus Drosophila and how these might correlate with environmental cues. A full synthesis of this field will require unification across these levels.

Evolutionary genomics of host plant adaptation: insights from Drosophila

Variation in gene expression in response to the use of alternate host plants can reveal genetic and physiological mechanisms explaining why insect-host relationships vary from host specialism to generalism. Interpreting transcriptome variation relies on well-annotated genomes, making drosophilids valuable model systems, particularly those species with tractable ecological associations. Patterns of whole genome expression and alternate gene splicing in response to growth on different hosts have revealed expression of gene networks of known detoxification genes as well as novel functionally enriched genes of diverse metabolic and structural functions. Integrating trancriptomic responses with fitness differences and levels of phenotypic plasticity in response to alternate hosts will help to reveal the general nature of genotype-phenotype relationships.

Using natural variation to achieve a whole-plant functional understanding of the responses mediated by jasmonate signaling

The dramatic advances in our understanding of the molecular biology and biochemistry of jasmonate (JA) signaling have been the subject of several excellent recent reviews that have highlighted the phytohormonal function of this signaling pathway. Here, we focus on the responses mediated by JA signaling which have consequences for a plant's Darwinian fitness, i.e. the organism-level function of JA signaling. The most diverse module in the signaling cascade, the JAZ proteins, and their interactions with other proteins and transcription factors, allow this canonical signaling cascade to mediate a bewildering array of traits in different tissues at different times; the functional coherence of these diverse responses are best appreciated in an organismal/ecological context. From published work, it appears that jasmonates can function as the 'Swiss Army knife' of plant signaling, mediating many different biotic and abiotic stress and developmental responses that allow plants to contextualize their responses to their frequently changing local environments and optimize their fitness. We propose that a deeper analysis of the natural variation in both within-plant and within-population JA signaling components is a profitable means of attaining a coherent whole-plant functional perspective of this signaling cascade, and provide examples of this approach from the Nicotiana attenuata system.

Transcriptional and Chemical Changes in Soybean Leaves in Response to Long-Term Aphid Colonization

Soybean aphids (Aphis glycines Matsumura) are specialized insects that feed on soybean (Glycine max) phloem sap. Transcriptome analyses have shown that resistant soybean plants mount a fast response that limits aphid feeding and population growth. Conversely, defense responses in susceptible plants are slower and it is hypothesized that aphids block effective defenses in the compatible interaction. Unlike other pests, aphids can colonize plants for long periods of time; yet the effect on the plant transcriptome after long-term aphid feeding has not been analyzed for any plant-aphid interaction. We analyzed the susceptible and resistant (Rag1) transcriptome response to aphid feeding in soybean plants colonized by aphids (biotype 1) for 21 days. We found a reduced resistant response and a low level of aphid growth on Rag1 plants, while susceptible plants showed a strong response consistent with pattern-triggered immunity. GO-term analyses identified chitin regulation as one of the most overrepresented classes of genes, suggesting that chitin could be one of the hemipteran-associated molecular pattern that triggers this defense response. Transcriptome analyses also indicated the phenylpropanoid pathway, specifically isoflavonoid biosynthesis, was induced in susceptible plants in response to long-term aphid feeding. Metabolite analyses corroborated this finding. Aphid-treated susceptible plants accumulated daidzein, formononetin, and genistein, although glyceollins were present at low levels in these plants. Choice experiments indicated that daidzein may have a deterrent effect on aphid feeding. Mass spectrometry imaging showed these isoflavones accumulate likely in the mesophyll cells or epidermis and are absent from the vasculature, suggesting that isoflavones are part of a non-phloem defense response that can reduce aphid feeding. While it is likely that aphid can initially block defense responses in compatible interactions, it appears that susceptible soybean plants can eventually mount an effective defense in response to long-term soybean aphid colonization.

An innovative ovipositor for niche exploitation impacts genital coevolution between sexes in a fruit-damaging Drosophila

Limited attention has been given to ecological factors influencing the coevolution of male and female genitalia. The innovative ovipositor of Drosophila suzukii, an invading fruit pest, represents an appealing case to document this phenomenon. The serrated saw-like ovipositor is used to pierce the hard skin of ripening fruits that are not used by other fruit flies that prefer soft decaying fruits. Here, we highlight another function of the ovipositor related to its involvement in genital coupling during copulation. We compared the morphology and coupling of male and female genitalia in this species to its sibling species, Drosophila subpulchrella, and to an outgroup species, Drosophila biarmipes These comparisons and a surgical manipulation indicated that the shape of male genitalia in D. suzukii has had to be adjusted to ensure tight coupling, despite having to abandon the use of a hook-like structure, paramere, because of the more linearly elongated ovipositor. This phenomenon demonstrates that ecological niche exploitation can directly affect the mechanics of genital coupling and potentially cause incompatibility among divergent forms. This model case provides new insights towards elucidating the importance of the dual functions of ovipositors in other insect species that potentially induce genital coevolution and ecological speciation.

Heritable plant phenotypes track light and herbivory levels at fine spatial scales

Organismal phenotypes often co-vary with environmental variables across broad geographic ranges. Less is known about the extent to which phenotypes match local conditions when multiple biotic and abiotic stressors vary at fine spatial scales. Bittercress (Brassicaceae: Cardamine cordifolia), a perennial forb, grows across a microgeographic mosaic of two contrasting herbivory regimes: high herbivory in meadows (sun habitats) and low herbivory in deeply shaded forest understories (shade habitats). We tested for local phenotypic differentiation in plant size, leaf morphology, and anti-herbivore defense (realized resistance and defensive chemicals, i.e., glucosinolates) across this habitat mosaic through reciprocal transplant-common garden experiments with clonally propagated rhizomes. We found habitat-specific divergence in morphological and defensive phenotypes that manifested as contrasting responses to growth in shade common gardens: weak petiole elongation and attenuated defenses in populations from shade habitats, and strong petiole elongation and elevated defenses in populations from sun habitats. These divergent phenotypes are generally consistent with reciprocal local adaptation: plants from shade habitats that naturally experience low herbivory show reduced investment in defense and an attenuated shade avoidance response, owing to its ineffectiveness within forest understories. By contrast, plants from sun habitats with high herbivory show shade-induced elongation, but no evidence of attenuated defenses canonically associated with elongation in shade-intolerant plant species. Finally, we observed differences in flowering phenology between habitat types that could potentially contribute to inter-habitat divergence by reducing gene flow. This study illuminates how clonally heritable plant phenotypes track a fine-grained mosaic of herbivore pressure and light availability in a native plant.

Arabidopsis glucosinolates trigger a contrasting transcriptomic response in a generalist and a specialist herbivore

Phytophagous insects have to deal with toxic defense compounds from their host plants. Although it is known that insects have evolved genes and mechanisms to detoxify plant allochemicals, how specialist and generalist precisely respond to specific secondary metabolites at the molecular level is less understood. Here we studied the larval performance and transcriptome of the generalist moth Heliothis virescens and the specialist butterfly Pieris brassicae feeding on Arabidopsis thaliana genotypes with different glucosinolate (GS) levels. H. virescens larvae gained significantly more weight on the GS-deficient mutant quadGS compared to wild-type (Col-0) plants. On the contrary, P. brassicae was unaffected by the presence of GS and performed equally well on both genotypes. Strikingly, there was a considerable differential gene expression in H. virescens larvae feeding on Col-0 compared to quadGS. In contrast, compared to H. virescens, P. brassicae displayed a much-reduced transcriptional activation when fed on both plant genotypes. Transcripts coding for putative detoxification enzymes were significantly upregulated in H. virescens, along with digestive enzymes and transposable elements. These data provide an unprecedented view on transcriptional changes that are specifically activated by GS and illustrate differential molecular responses that are linked to adaptation to diet in lepidopteran herbivores.

The Genetics Underlying Natural Variation in the Biotic Interactions of Arabidopsis thaliana: The Challenges of Linking Evolutionary Genetics and Community Ecology

In the context of global change, predicting the responses of plant communities in an ever-changing biotic environment calls for a multipronged approach at the interface of evolutionary genetics and community ecology. However, our understanding of the genetic basis of natural variation involved in mediating biotic interactions, and associated adaptive dynamics of focal plants in their natural communities, is still in its infancy. Here, we review the genetic and molecular bases of natural variation in the response to biotic interactions (viruses, bacteria, fungi, oomycetes, herbivores, and plants) in the model plant Arabidopsis thaliana as well as the adaptive value of these bases. Among the 60 identified genes are a number that encode nucleotide-binding site leucine-rich repeat (NBS-LRR)-type proteins, consistent with early examples of plant defense genes. However, recent studies have revealed an extensive diversity in the molecular mechanisms of defense. Many types of genetic variants associate with phenotypic variation in biotic interactions, even among the genes of large effect that tend to be identified. In general, we found that (i) balancing selection rather than directional selection explains the observed patterns of genetic diversity within A. thaliana and (ii) the cost/benefit tradeoffs of adaptive alleles can be strongly dependent on both genomic and environmental contexts. Finally, because A. thaliana rarely interacts with only one biotic partner in nature, we highlight the benefit of exploring diffuse biotic interactions rather than tightly associated host-enemy pairs. This challenge would help to improve our understanding of coevolutionary quantitative genetics within the context of realistic community complexity.

Aversion and attraction to harmful plant secondary compounds jointly shape the foraging ecology of a specialist herbivore

Most herbivorous insect species are restricted to a narrow taxonomic range of host plant species. Herbivore species that feed on mustard plants and their relatives in the Brassicales have evolved highly efficient detoxification mechanisms that actually prevent toxic mustard oils from forming in the bodies of the animals. However, these mechanisms likely were not present during the initial stages of specialization on mustard plants ~100 million years ago. The herbivorous fly Scaptomyza nigrita (Drosophilidae) is a specialist on a single mustard species, bittercress (Cardamine cordifolia; Brassicaceae) and is in a fly lineage that evolved to feed on mustards only in the past 10–20 million years. In contrast to many mustard specialists, S. nigrita does not prevent formation of toxic breakdown products (mustard oils) arising from glucosinolates (GLS), the primary defensive compounds in mustard plants. Therefore, it is an appealing model for dissecting the early stages of host specialization. Because mustard oils actually form in the bodies of S. nigrita, we hypothesized that in lieu of a specialized detoxification mechanism, S. nigrita may mitigate exposure to high GLS levels within plant tissues using behavioral avoidance. Here, we report that jasmonic acid (JA) treatment increased GLS biosynthesis in bittercress, repelled adult female flies, and reduced larval growth. S. nigrita larval damage also induced foliar GLS, especially in apical leaves, which correspondingly displayed the least S. nigrita damage in controlled feeding trials and field surveys. Paradoxically, flies preferred to feed and oviposit on GLS‐producing Arabidopsis thaliana despite larvae performing worse in these plants versus non‐GLS‐producing mutants. GLS may be feeding cues for S. nigrita despite their deterrent and defensive properties, which underscores the diverse relationship a mustard specialist has with its host when lacking a specialized means of mustard oil detoxification.

Using Drosophila to study the evolution of herbivory and diet specialization

Herbivory evolved many times independently across the insect phylogeny, and its evolution is linked with increased rates of diversification. Plants present many barriers to potential herbivores, among them are the so-called secondary chemicals and other molecular defenses such as protease inhibitors that deter herbivores. To understand the mechanisms behind the emergence of herbivory and subsequent species radiations of insects driven largely by diet specialization, it is important to identify the molecular basis associated with these evolutionary transitions. However, most herbivore species lack the genomic information and genetic tools required to identify functionally important genes. The notable exception is the genus Drosophila in which herbivory evolved at least three times independently, and for which abundant genomic data are available. Furthermore, contained within the family Drosophilidae is Drosophila melanogaster, the first genetic model animal. Here, we provide a synthesis of the salient tools that the D. melanogaster system provides to identify functionally important genes required for herbivory and subsequent diet specialization across insects.

Pseudomonas syringae enhances herbivory by suppressing the reactive oxygen burst in Arabidopsis

Plant-herbivore interactions have evolved in the presence of plant-colonizing microbes. These microbes can have important third-party effects on herbivore ecology, as exemplified by drosophilid flies that evolved from ancestors feeding on plant-associated microbes. Leaf-mining flies in the genus Scaptomyza, which is nested within the paraphyletic genus Drosophila, show strong associations with bacteria in the genus Pseudomonas, including Pseudomonas syringae. Adult females are capable of vectoring these bacteria between plants and larvae show a preference for feeding on P. syringae-infected leaves. Here we show that Scaptomyza flava larvae can also vector P. syringae to and from feeding sites, and that they not only feed more, but also develop faster on plants previously infected with P. syringae. Our genetic and physiological data show that P. syringae enhances S. flava feeding on infected plants at least in part by suppressing anti-herbivore defenses mediated by reactive oxygen species.

Insect-induced effects on plants and possible effectors used by galling and leaf-mining insects to manipulate their host-plant

Gall-inducing insects are iconic examples in the manipulation and reprogramming of plant development, inducing spectacular morphological and physiological changes of host-plant tissues within which the insect feeds and grows. Despite decades of research, effectors involved in gall induction and basic mechanisms of gall formation remain unknown. Recent research suggests that some aspects of the plant manipulation shown by gall-inducers may be shared with other insect herbivorous life histories. Here, we illustrate similarities and contrasts by reviewing current knowledge of metabolic and morphological effects induced on plants by gall-inducing and leaf-mining insects, and ask whether leaf-miners can also be considered to be plant reprogrammers. We review key plant functions targeted by various plant reprogrammers, including plant-manipulating insects and nematodes, and functionally characterize insect herbivore-derived effectors to provide a broader understanding of possible mechanisms used in host-plant manipulation. Consequences of plant reprogramming in terms of ecology, coevolution and diversification of plant-manipulating insects are also discussed.

DNA-based identifications reveal multiple introductions of the vegetable leafminer Liriomyza sativae (Diptera: Agromyzidae) into the Torres Strait Islands and Papua New Guinea

Leafmining flies (Diptera: Agromyzidae) can be serious economic pests of horticultural crops. Some genera such as Liriomyza are particularly problematic with numerous species, some of which are highly polyphagous (wide host range), which can only be confidently identified morphologically from adult males. In our study, DNA barcoding was employed to establish new locality records of the vegetable leafminer fly, Liriomyza sativae, from the islands of Torres Strait (Queensland, Australia) and the central highlands of Papua New Guinea (PNG). These records represent significant range extensions of this highly invasive plant pest. Specimens of immature leafminers (from leaf mines) were collected over a 5-year period during routine plant health surveys in ethanol or on FTA® filter paper cards, both methods proved effective at preserving and transporting insect DNA under tropical conditions, with FTA cards possessing some additional logistical benefits. Specimens were identified through sequencing two sections of the cytochrome oxidase I gene and the utility of each was assessed for the identification of species and intra-specific genetic lineages. Our study indicates that multiple haplotypes of L. sativae occur in PNG, while a different haplotype is present in the Torres Strait, with genetic regionalization between these areas apart from a single possible instance - one haplotype 'S.7' appears to be common between these two regions - interestingly this has also been the most common haplotype detected in previous studies of invasive L. sativae populations. The DNA barcoding methods employed here not only identified multiple introductions of L. sativae, but also appear generally applicable to the identification of other agromyzid leafminers (Phytomyzinae and Agromyzinae) and should decrease the likelihood of potentially co-amplifying internal hymenopteran parasitoids. Currently, L. sativae is still not recorded from the Australian mainland; however, further sampling of leafminer flies from Northern Australia and surrounding areas is required, as surveillance for possible Liriomyza incursions, as well as to characterize endemic species with which Liriomyza species might be confused.

Interaction of plant defense compounds with the insect gut: new insights from genomic and molecular analyses

The co-evolutionary conflict between insect herbivores and their host plants is profoundly influenced by biochemical reactions associated with passage of toxin-laden plant material through the herbivore digestive canal. Insect herbivores provide excellent models in which to understand the mechanistic interplay between nutrition and detoxification, how plant defense compounds hijack these processes, and how insects adapt to host defense chemistry. Expanding genome sequence information and genetic approaches to manipulate gene function in both interacting partners are providing new insights into the genetic underpinnings of host preference and plasticity in gut physiology. Fundamental knowledge gained from these studies has practical application in understanding how insects evolve resistance to pesticides, and may also inform efforts to better understand how plant chemicals impact human health.

Pivoting from Arabidopsis to wheat to understand how agricultural plants integrate responses to biotic stress

In this review, we argue for a research initiative on wheat's responses to biotic stress. One goal is to begin a conversation between the disparate communities of plant pathology and entomology. Another is to understand how responses to a variety of agents of biotic stress are integrated in an important crop. We propose gene-for-gene interactions as the focus of the research initiative. On the parasite's side is an Avirulence (Avr) gene that encodes one of the many effector proteins the parasite applies to the plant to assist with colonization. On the plant's side is a Resistance (R) gene that mediates a surveillance system that detects the Avr protein directly or indirectly and triggers effector-triggered plant immunity. Even though arthropods are responsible for a significant proportion of plant biotic stress, they have not been integrated into important models of plant immunity that come from plant pathology. A roadblock has been the absence of molecular evidence for arthropod Avr effectors. Thirty years after this evidence was discovered in a plant pathogen, there is now evidence for arthropods with the cloning of the Hessian fly's vH13 Avr gene. After reviewing the two models of plant immunity, we discuss how arthropods could be incorporated. We end by showing features that make wheat an interesting system for plant immunity, including 479 resistance genes known from agriculture that target viruses, bacteria, fungi, nematodes, insects, and mites. It is not likely that humans will be subsisting on Arabidopsis in the year 2050. It is time to start understanding how agricultural plants integrate responses to biotic stress.

Evolution of herbivory in Drosophilidae linked to loss of behaviors, antennal responses, odorant receptors, and ancestral diet

Herbivory is a key innovation in insects, yet has only evolved in one-third of living orders. The evolution of herbivory likely involves major behavioral changes mediated by remodeling of canonical chemosensory modules. Herbivorous flies in the genus Scaptomyza (Drosophilidae) are compelling species in which to study the genomic architecture linked to the transition to herbivory because they recently evolved from microbe-feeding ancestors and are closely related to Drosophila melanogaster. We found that Scaptomyza flava, a leaf-mining specialist on plants in the family (Brassicaceae), was not attracted to yeast volatiles in a four-field olfactometer assay, whereas D. melanogaster was strongly attracted to these volatiles. Yeast-associated volatiles, especially short-chain aliphatic esters, elicited strong antennal responses in D. melanogaster, but weak antennal responses in electroantennographic recordings from S. flava. We sequenced the genome of S. flava and characterized this species' odorant receptor repertoire. Orthologs of odorant receptors, which detect yeast volatiles in D. melanogaster and mediate critical host-choice behavior, were deleted or pseudogenized in the genome of S. flava. These genes were lost step-wise during the evolution of Scaptomyza. Additionally, Scaptomyza has experienced gene duplication and likely positive selection in paralogs of Or67b in D. melanogaster. Olfactory sensory neurons expressing Or67b are sensitive to green-leaf volatiles. Major trophic shifts in insects are associated with chemoreceptor gene loss as recently evolved ecologies shape sensory repertoires.

Olfactory specialization in Drosophila suzukii supports an ecological shift in host preference from rotten to fresh fruit

It has been demonstrated that Drosophila suzukii is capable of attacking ripening fruit, making it a unique species within a fly family named for their attraction towards the fermentation products associated with rotten fruits, vinegar, and yeast. It also has been hypothesized that D. suzukii is more attracted to the volatiles associated with the earlier ripening stages of fruit development, and in turn, that D. suzukii is less attracted to fermented food resources, especially when compared with D. melanogaster. Here, we demonstrate that D. suzukii and its close relative D. biarmipes are in fact more sensitive to volatiles associated with the fruit-ripening process; however, in choice-assays, both spotted-wing species are more attracted to fermented fruit than to earlier stages of fruit development, which is similar to the behavioral preferences of D. melanogaster, and thus, fruit developmental stage alone does not explain the ecological niche observed for D. suzukii. In contrast, we show that both D. suzukii and D. biarmipes are more attracted to leaf odors than D. melanogaster in behavioral trials. For D. suzukii, this differential behavioral preference towards leaves appears to be linked to β-cyclocitral, a volatile isoprenoid that we show is most likely a novel ligand for the “ab3A” neuron. In addition, this compound is not detected by either of the other two tested fly species.

Microbial interactions and the ecology and evolution of Hawaiian Drosophilidae

Adaptive radiations are characterized by an increased rate of speciation and expanded range of habitats and ecological niches exploited by those species. The Hawaiian Drosophilidae is a classic adaptive radiation; a single ancestral species colonized Hawaii approximately 25 million years ago and gave rise to two monophyletic lineages, the Hawaiian Drosophila and the genus Scaptomyza. The Hawaiian Drosophila are largely saprophagous and rely on approximately 40 endemic plant families and their associated microbes to complete development. Scaptomyza are even more diverse in host breadth. While many species of Scaptomyza utilize decomposing plant substrates, some species have evolved to become herbivores, parasites on spider egg masses, and exploit microbes on living plant tissue. Understanding the origin of the ecological diversity encompassed by these nearly 700 described species has been a challenge. The central role of microbes in drosophilid ecology suggests bacterial and fungal associates may have played a role in the diversification of the Hawaiian Drosophilidae. Here we synthesize recent ecological and microbial community data from the Hawaiian Drosophilidae to examine the forces that may have led to this adaptive radiation. We propose that the evolutionary success of the Hawaiian Drosophilidae is due to a combination of factors, including adaptation to novel ecological niches facilitated by microbes.

Overexpression of glutathione transferase E7 in Drosophila differentially impacts toxicity of organic isothiocyanates in males and females

Organic isothiocyanates (ITCs) are allelochemicals produced by plants in order to combat insects and other herbivores. The compounds are toxic electrophiles that can be inactivated and conjugated with intracellular glutathione in reactions catalyzed by glutathione transferases (GSTs). The Drosophila melanogaster GSTE7 was heterologously expressed in Escherichia coli and purified for functional studies. The enzyme showed high catalytic activity with various isothiocyanates including phenethyl isothiocyanate (PEITC) and allyl isothiocyanate (AITC), which in millimolar dietary concentrations conferred toxicity to adult D. melanogaster leading to death or a shortened life-span of the flies. In situ hybridization revealed a maternal contribution of GSTE7 transcripts to embryos, and strongest zygotic expression in the digestive tract. Transgenesis involving the GSTE7 gene controlled by an actin promoter produced viable flies expressing the GSTE7 transcript ubiquitously. Transgenic females show a significantly increased survival when subjected to the same PEITC treatment as the wild-type flies. By contrast, transgenic male flies show a significantly lower survival rate. Oviposition activity was enhanced in transgenic flies. The effect was significant in transgenic females reared in the absence of ITCs as well as in the presence of 0.15 mM PEITC or 1 mM AITC. Thus the GSTE7 transgene elicits responses to exposure to ITC allelochemicals which differentially affect life-span and fecundity of male and female flies.

Application of two-spotted spider mite Tetranychus urticae for plant-pest interaction studies

The two-spotted spider mite, Tetranychus urticae, is a ubiquitous polyphagous arthropod herbivore that feeds on a remarkably broad array of species, with more than 150 of economic value. It is a major pest of greenhouse crops, especially in Solanaceae and Cucurbitaceae (e.g., tomatoes, eggplants, peppers, cucumbers, zucchini) and greenhouse ornamentals (e.g., roses, chrysanthemum, carnations), annual field crops (such as maize, cotton, soybean, and sugar beet), and in perennial cultures (alfalfa, strawberries, grapes, citruses, and plums)1,2. In addition to the extreme polyphagy that makes it an important agricultural pest, T. urticae has a tendency to develop resistance to a wide array of insecticides and acaricides that are used for its control3-7. T. urticae is an excellent experimental organism, as it has a rapid life cycle (7 days at 27 °C) and can be easily maintained at high density in the laboratory. Methods to assay gene expression (including in situ hybridization and antibody staining) and to inactivate expression of spider mite endogenous genes using RNA interference have been developed8-10. Recently, the whole genome sequence of T. urticae has been reported, creating an opportunity to develop this pest herbivore as a model organism with equivalent genomic resources that already exist in some of its host plants (Arabidopsis thaliana and the tomato Solanum lycopersicum)11. Together, these model organisms could provide insights into molecular bases of plant-pest interactions. Here, an efficient method for quick and easy collection of a large number of adult female mites, their application on an experimental plant host, and the assessment of the plant damage due to spider mite feeding are described. The presented protocol enables fast and efficient collection of hundreds of individuals at any developmental stage (eggs, larvae, nymphs, adult males, and females) that can be used for subsequent experimental application.

Jasmonate-triggered plant immunity

The plant hormone jasmonate (JA) exerts direct control over the production of chemical defense compounds that confer resistance to a remarkable spectrum of plant-associated organisms, ranging from microbial pathogens to vertebrate herbivores. The underlying mechanism of JA-triggered immunity (JATI) can be conceptualized as a multi-stage signal transduction cascade involving: i) pattern recognition receptors (PRRs) that couple the perception of danger signals to rapid synthesis of bioactive JA; ii) an evolutionarily conserved JA signaling module that links fluctuating JA levels to changes in the abundance of transcriptional repressor proteins; and iii) activation (de-repression) of transcription factors that orchestrate the expression of myriad chemical and morphological defense traits. Multiple negative feedback loops act in concert to restrain the duration and amplitude of defense responses, presumably to mitigate potential fitness costs of JATI. The convergence of diverse plant- and non-plant-derived signals on the core JA module indicates that JATI is a general response to perceived danger. However, the modular structure of JATI may accommodate attacker-specific defense responses through evolutionary innovation of PRRs (inputs) and defense traits (outputs). The efficacy of JATI as a defense strategy is highlighted by its capacity to shape natural populations of plant attackers, as well as the propensity of plant-associated organisms to subvert or otherwise manipulate JA signaling. As both a cellular hub for integrating informational cues from the environment and a common target of pathogen effectors, the core JA module provides a focal point for understanding immune system networks and the evolution of chemical diversity in the plant kingdom.

Evolution in an ancient detoxification pathway is coupled with a transition to herbivory in the drosophilidae

Chemically defended plant tissues present formidable barriers to herbivores. Although mechanisms to resist plant defenses have been identified in ancient herbivorous lineages, adaptations to overcome plant defenses during transitions to herbivory remain relatively unexplored. The fly genus Scaptomyza is nested within the genus Drosophila and includes species that feed on the living tissue of mustard plants (Brassicaceae), yet this lineage is derived from microbe-feeding ancestors. We found that mustard-feeding Scaptomyza species and microbe-feeding Drosophila melanogaster detoxify mustard oils, the primary chemical defenses in the Brassicaceae, using the widely conserved mercapturic acid pathway. This detoxification strategy differs from other specialist herbivores of mustard plants, which possess derived mechanisms to obviate mustard oil formation. To investigate whether mustard feeding is coupled with evolution in the mercapturic acid pathway, we profiled functional and molecular evolutionary changes in the enzyme glutathione S-transferase D1 (GSTD1), which catalyzes the first step of the mercapturic acid pathway and is induced by mustard defense products in Scaptomyza. GSTD1 acquired elevated activity against mustard oils in one mustard-feeding Scaptomyza species in which GstD1 was duplicated. Structural analysis and mutagenesis revealed that substitutions at conserved residues within and near the substrate-binding cleft account for most of this increase in activity against mustard oils. Functional evolution of GSTD1 was coupled with signatures of episodic positive selection in GstD1 after the evolution of herbivory. Overall, we found that preexisting functions of generalized detoxification systems, and their refinement by natural selection, could play a central role in the evolution of herbivory.

Cytokinin-induced phenotypes in plant-insect interactions: learning from the bacterial world

Recently, a renewed interest in cytokinins (CKs) has allowed the characterization of these phytohormones as key regulatory molecules in plant biotic interactions. They have been proved to be instrumental in microbe- and insect-mediated plant phenotypes that can be either beneficial or detrimental for the host-plant. In parallel, insect endosymbiotic bacteria have emerged as key players in plant-insect interactions mediating directly or indirectly fundamental aspects of insect nutrition, such as insect feeding efficiency or the ability to manipulate plant physiology to overcome food nutritional imbalances. However, mechanisms that regulate CK production and the role played by insects and their endosymbionts remain largely unknown. Against this backdrop, studies on plant-associated bacteria have revealed fascinating and complex molecular mechanisms that lead to the production of bacterial CKs and the modulation of plant-borne CKs which ultimately result in profound metabolic and morphological plant modifications. This review highlights major strategies used by plant-associated bacteria that impact the CK homeostasis of their host-plant, to raise parallels with strategies used by phytophagous insects and to discuss the possible role played by endosymbiotic bacteria in these CK-mediated plant phenotypes. We hypothesize that insects employ a CK-mix production strategy that manipulates the phytohormonal balance of their host-plant and overtakes plant gene expression causing a metabolic and morphological habitat modification. In addition, insect endosymbiotic bacteria may prove to be instrumental in these manipulations through the production of bacterial CKs, including specific forms that challenge the CK-degrading capacity of the plant (thus ensuring persistent effects) and the CK-mediated plant defenses.

The making of a pest: the evolution of a fruit-penetrating ovipositor in Drosophila suzukii and related species

Evolutionary innovation can allow a species access to a new ecological niche, potentially reducing competition with closely related species. While the vast majority of Drosophila flies feed on rotting fruit and other decaying matter, and are harmless to human activity, Drosophila suzukii, which has a morphologically modified ovipositor, is capable of colonizing live fruit that is still in the process of ripening, causing massive agricultural damage. Here, we conducted the first comparative analysis of this species and its close relatives, analysing both ovipositor structure and fruit susceptibility. We found that the ovipositor of the species most closely related to D. suzukii, Drosophila subpulchrella, has a similar number of enlarged, evolutionarily derived bristles, but a notably different overall shape. Like D. suzukii, D. subpulchrella flies are capable of puncturing the skin of raspberries and cherries, but we found no evidence that they could penetrate the thicker skin of two varieties of grapes. More distantly related species, one of which has previously been mistaken for D. suzukii, have blunt ovipositors with small bristles. While they did not penetrate fruit skin in any of the assays, they readily colonized fruit interiors where the skin was broken. Our results suggest that considering evolutionary context may be beneficial to the management of invasive species.

Pathogen-triggered ethylene signaling mediates systemic-induced susceptibility to herbivory in Arabidopsis

Multicellular eukaryotic organisms are attacked by numerous parasites from diverse phyla, often simultaneously or sequentially. An outstanding question in these interactions is how hosts integrate signals induced by the attack of different parasites. We used a model system comprised of the plant host Arabidopsis thaliana, the hemibiotrophic bacterial phytopathogen Pseudomonas syringae, and herbivorous larvae of the moth Trichoplusia ni (cabbage looper) to characterize mechanisms involved in systemic-induced susceptibility (SIS) to T. ni herbivory caused by prior infection by virulent P. syringae. We uncovered a complex multilayered induction mechanism for SIS to herbivory. In this mechanism, antiherbivore defenses that depend on signaling via (1) the jasmonic acid-isoleucine conjugate (JA-Ile) and (2) other octadecanoids are suppressed by microbe-associated molecular pattern-triggered salicylic acid (SA) signaling and infection-triggered ethylene signaling, respectively. SIS to herbivory is, in turn, counteracted by a combination of the bacterial JA-Ile mimic coronatine and type III virulence-associated effectors. Our results show that SIS to herbivory involves more than antagonistic signaling between SA and JA-Ile and provide insight into the unexpectedly complex mechanisms behind a seemingly simple trade-off in plant defense against multiple enemies.

Detoxification activity and energy cost is attenuated in whiteflies feeding on tomato yellow leaf curl China virus-infected tobacco plants

The begomovirus Tomato yellow leaf curl China virus (TYLCCNV) can benefit its vector, the whitefly Bemisia tabaci, through suppressing the defences of their shared host plants. However, the mechanisms of this vector-virus mutualism remain largely unknown on the insect side of the interaction. Here, we compared the transcriptional profiles of female adult whiteflies of B. tabaci Middle East-Asia Minor 1 feeding on TYLCCNV-free and TYLCCNV-infected tobacco plants using the next-generation sequencing technique and quantitative real-time PCR. Interestingly, the genes involved in the oxidative phosphorylation (OXPHOS) pathway and detoxification enzyme were down-regulated in whiteflies feeding on virus-infected plants. Decreased detoxification activity costs less energy, which may reduce OXPHOS activity. Moreover, the genes involved in redox activity were also down-regulated, which may indicate that the reduced OXPHOS activity decreased reactive oxygen species production. Reduced detoxification activity is likely to attenuate energy costs, thereby enhancing the performance of whiteflies on virus-infected plants. These results provide further insight into the mechanisms of the plant-mediated whitefly-virus mutualism. Moreover, our study suggests that investigating the transcriptional profiles on the insect side of the interaction can advance our understanding of the tripartite interactions.

Maintenance of genetic diversity through plant-herbivore interactions

Identifying the factors governing the maintenance of genetic variation is a central challenge in evolutionary biology. New genomic data, methods and conceptual advances provide increasing evidence that balancing selection, mediated by antagonistic species interactions, maintains genome-wide functionally important genetic variation within species and natural populations. Because diverse interactions between plants and herbivorous insects dominate terrestrial communities, they provide excellent systems to address this hypothesis. Population genomic studies of Arabidopsis thaliana and its relatives suggest spatial variation in herbivory maintains adaptive genetic variation controlling defense phenotypes, both within and among populations. Conversely, inter-species variation in plant defenses promotes adaptive genetic variation in herbivores. Emerging genomic model herbivores of Arabidopsis could illuminate how genetic variation in herbivores and plants interact simultaneously.

Diversification and dispersal of the Hawaiian Drosophilidae: the evolution of Scaptomyza

The genus Scaptomyza is emerging as a model lineage in which to study biogeography and ecological adaptation. To place future research on these species into an evolutionary framework we present the most comprehensive phylogeny of Scaptomyza to date, based on 5042 bp of DNA sequence data and representatives from 13 of 21 subgenera. We infer strong support for the monophyly of almost all subgenera with exceptions corroborating hypotheses of conflict inferred from previous taxonomic studies. We find evidence that the lineage originated in the Hawaiian Islands and subsequently dispersed to the mainland and other remote oceanic islands. We also identify that many of the unique ecological niches exploited by this lineage (e.g., herbivory, spider predation) arose singly and independently.

A pharm-ecological perspective of terrestrial and aquatic plant-herbivore interactions

We describe some recent themes in the nutritional and chemical ecology of herbivores and the importance of a broad pharmacological view of plant nutrients and chemical defenses that we integrate as "Pharm-ecology". The central role that dose, concentration, and response to plant components (nutrients and secondary metabolites) play in herbivore foraging behavior argues for broader application of approaches derived from pharmacology to both terrestrial and aquatic plant-herbivore systems. We describe how concepts of pharmacokinetics and pharmacodynamics are used to better understand the foraging phenotype of herbivores relative to nutrient and secondary metabolites in food. Implementing these concepts into the field remains a challenge, but new modeling approaches that emphasize tradeoffs and the properties of individual animals show promise. Throughout, we highlight similarities and differences between the historic and future applications of pharm-ecological concepts in understanding the ecology and evolution of terrestrial and aquatic interactions between herbivores and plants. We offer several pharm-ecology related questions and hypotheses that could strengthen our understanding of the nutritional and chemical factors that modulate foraging behavior of herbivores across terrestrial and aquatic systems.

Genes involved in the evolution of herbivory by a leaf-mining, Drosophilid fly

Herbivorous insects are among the most successful radiations of life. However, we know little about the processes underpinning the evolution of herbivory. We examined the evolution of herbivory in the fly, Scaptomyza flava, whose larvae are leaf miners on species of Brassicaceae, including the widely studied reference plant, Arabidopsis thaliana (Arabidopsis). Scaptomyza flava is phylogenetically nested within the paraphyletic genus Drosophila, and the whole genome sequences available for 12 species of Drosophila facilitated phylogenetic analysis and assembly of a transcriptome for S. flava. A time-calibrated phylogeny indicated that leaf mining in Scaptomyza evolved between 6 and 16 million years ago. Feeding assays showed that biosynthesis of glucosinolates, the major class of antiherbivore chemical defense compounds in mustard leaves, was upregulated by S. flava larval feeding. The presence of glucosinolates in wild-type (WT) Arabidopsis plants reduced S. flava larval weight gain and increased egg-adult development time relative to flies reared in glucosinolate knockout (GKO) plants. An analysis of gene expression differences in 5-day-old larvae reared on WT versus GKO plants showed a total of 341 transcripts that were differentially regulated by glucosinolate uptake in larval S. flava. Of these, approximately a third corresponded to homologs of Drosophila melanogaster genes associated with starvation, dietary toxin-, heat-, oxidation-, and aging-related stress. The upregulated transcripts exhibited elevated rates of protein evolution compared with unregulated transcripts. The remaining differentially regulated transcripts also contained a higher proportion of novel genes than the unregulated transcripts. Thus, the transition to herbivory in Scaptomyza appears to be coupled with the evolution of novel genes and the co-option of conserved stress-related genes.