Genomic dissection of an extended phenotype: Oak galling by a cynipid gall wasp

Comparative transcriptome reprogramming in oak galls containing asexual or sexual generations of gall wasps

Oak gall wasps have evolved strategies to manipulate the developmental pathways of their host to induce gall formation. This provides shelter and nutrients for the developing larva. Galls are entirely host tissue; however, the initiation, development, and physical appearance are controlled by the inducer. The underlying molecular mechanisms of gall formation, by which one or a small number of cells are reprogrammed and commit to a novel developmental path, are poorly understood. In this study, we sought a deeper insight into the molecular underpinnings of this process. Oak gall wasps have two generations each year, one sexual, and one asexual. Galls formed by these two generations exhibit a markedly different appearance. We sequenced transcriptomes of both the asexual and sexual generations of Neuroterus quercusbaccarum and Neuroterus numismalis. We then deployed Nanopore sequencing to generate long-read sequences to test the hypothesis that gall wasps introduce DNA insertions to determine gall development. We detected potential genome rearrangements but did not uncover any non-host DNA insertions. Transcriptome analysis revealed that transcriptomes of the sexual generations of distinct species of wasp are more similar than inter-generational comparisons from the same species of wasp. Our results highlight the intricate interplay between the host leaves and gall development, suggesting that season and requirements of the gall structure play a larger role than species in controlling gall development and structure.

Cynipid wasps systematically reprogram host metabolism and restructure cell walls in developing galls

Many insects have evolved the ability to manipulate plant growth to generate extraordinary structures called galls, in which insect larva can develop while being sheltered and feeding on the plant. In particular, cynipid (Hymenoptera: Cynipidae) wasps have evolved to form morphologically complex galls and generate an astonishing array of gall shapes, colors, and sizes. However, the biochemical basis underlying these remarkable cellular and developmental transformations remains poorly understood. A key determinant in plant cellular development is cell wall deposition that dictates the physical form and physiological function of newly developing cells, tissues, and organs. However, it is unclear to what degree cell walls are restructured to initiate and support the formation of new gall tissue. Here, we characterize the molecular alterations underlying gall development using a combination of metabolomic, histological, and biochemical techniques to elucidate how valley oak (Quercus lobata) leaf cells are reprogrammed to form galls. Strikingly, gall development involves an exceptionally coordinated spatial deposition of lignin and xylan to form de novo gall vasculature. Our results highlight how cynipid wasps can radically change the metabolite profile and restructure the cell wall to enable the formation of galls, providing insights into the mechanism of gall induction and the extent to which plants can be entirely reprogrammed to form unique structures and organs.

Molecular Aspects of Gall Formation Induced by Mites and Insects

Recent publications on gall formation induced on the leaves of dicotyledonous flowering plants by eriophyoid mites (Eriophyoidea) and representatives of four insect orders (Diptera, Hemiptera, Hymenoptera, Lepidoptera) are analyzed. Cellular and molecular level data on the stimuli that induce and sustain the development of both mite and insect galls, the expression of host plant genes during gallogenesis, and the effects of these galling arthropods on photosynthesis are considered. A hypothesis is proposed for the relationship between the size of galls and the volume of secretions injected by a parasite. Multistep, varying patterns of plant gene expression and accompanying histo-morphological changes in the transformed gall tissues are apparent. The main obstacle to better elucidating the nature of the induction of gallogenesis is the impossibility of collecting a sufficient amount of saliva for analysis, which is especially important in the case of microscopic eriophyoids. The use of modern omics technologies at the organismal level has revealed a spectrum of genetic mechanisms of gall formation at the molecular level but has not yet answered the questions regarding the nature of gall-inducing agents and the features of events occurring in plant cells at the very beginning of gall growth.

Genomic Assessment of the Contribution of the Wolbachia Endosymbiont of Eurosta solidaginis to Gall Induction

We explored the genome of the Wolbachia strain, wEsol, symbiotic with the plant-gall-inducing fly Eurosta solidaginis with the goal of determining if wEsol contributes to gall induction by its insect host. Gall induction by insects has been hypothesized to involve the secretion of the phytohormones cytokinin and auxin and/or proteinaceous effectors to stimulate cell division and growth in the host plant. We sequenced the metagenome of E. solidaginis and wEsol and assembled and annotated the genome of wEsol. The wEsol genome has an assembled length of 1.66 Mbp and contains 1878 protein-coding genes. The wEsol genome is replete with proteins encoded by mobile genetic elements and shows evidence of seven different prophages. We also detected evidence of multiple small insertions of wEsol genes into the genome of the host insect. Our characterization of the genome of wEsol indicates that it is compromised in the synthesis of dimethylallyl pyrophosphate (DMAPP) and S-adenosyl L-methionine (SAM), which are precursors required for the synthesis of cytokinins and methylthiolated cytokinins. wEsol is also incapable of synthesizing tryptophan, and its genome contains no enzymes in any of the known pathways for the synthesis of indole-3-acetic acid (IAA) from tryptophan. wEsol must steal DMAPP and L-methionine from its host and therefore is unlikely to provide cytokinin and auxin to its insect host for use in gall induction. Furthermore, in spite of its large repertoire of predicted Type IV secreted effector proteins, these effectors are more likely to contribute to the acquisition of nutrients and the manipulation of the host's cellular environment to contribute to growth and reproduction of wEsol than to aid E. solidaginis in manipulating its host plant. Combined with earlier work that shows that wEsol is absent from the salivary glands of E. solidaginis, our results suggest that wEsol does not contribute to gall induction by its host.

Host plant intraspecific variation determines gall traits

Galls display a multiplicity of traits, including colours, which are driven by pigment accumulation. Their conspicuousness has attracted researchers' attention and several hypotheses have been raised. However, plants themselves vary intra-specifically, including in their pigment concentrations. As galls are a result of host tissue development, colours may be a by-product of the host's own traits, being more conspicuous simply because the sites where galls develop already have the predisposition to accumulate more pigment. Here, we call this the host variation hypothesis. We test this hypothesis using the system of galls induced by Palaeomystella oligophaga on Macairea radula host plant. Using spectrophotometry, we calculated the Anthocyanin Reflectance Index (ARI) of gall projections, which are responsible for their characteristic colours. We tested the influence of occupant identity (galling insect or any natural enemy), gall volume, parenchyma thickness, height from the ground, ARI of leaf, ARI of gall surface and ARI of the respective stem. We corroborated the host variation hypothesis since the anthocyanin content in stems and in galls' projections were positively related. Moreover, anthocyanin in galls' projections was positively related to anthocyanin in the gall surface and negatively related to gall volume and parenchyma thickness. This shows that, besides the host specificities, galls' own traits may also be responsible for pigment accumulation, influencing their colours. In this study, using colour as an example, we show that although galls tend to be considered complex expressions of galling insects' stimuli, their traits may be simply influenced by previous and specific attributes of the host organs.

The Arthropod Associates of 155 North American Cynipid Oak Galls

The identities of most arthropod associates of cynipid-induced oak galls in the western Palearctic are generally known. However, a comprehensive accounting of associates has been performed for only a small number of the galls induced by the estimated 700 species of cynipid gall wasps in the Nearctic. This gap in knowledge stymies many potential studies of diversity, coevolution, and community ecology, for which oak gall systems are otherwise ideal models. We report rearing records of insects and other arthropods from more than 527,306 individual galls representing 201 different oak gall types collected from 32 oak tree species in North America. Of the 201 gall types collected, 155 produced one or more arthropods. A total of 151,075 arthropods were found in association with these 155 gall types, and of these 61,044 (40.4%) were gall wasps while 90,031 (59.6%) were other arthropods. We identified all arthropods to superfamily, family, or, where possible, to genus. We provide raw numbers and summaries of collections, alongside notes on natural history, ecology, and previously published associations for each taxon. For eight common gall-associated genera (Synergus, Ceroptres, Euceroptres, Ormyrus, Torymus, Eurytoma, Sycophila, and Euderus), we also connect rearing records to gall wasp phylogeny, geography, and ecology -including host tree and gall location (host organ), and their co-occurrence with other insect genera. Though the diversity of gall wasps and the large size of these communities is such that many Nearctic oak gall-associated insects still remain undescribed, this large collection and identification effort should facilitate the testing of new and varied ecological and evolutionary hypotheses in Nearctic oak galls.

Delimiting the cryptic diversity and host preferences of Sycophila parasitoid wasps associated with oak galls using phylogenomic data

Cryptic species diversity is a major challenge for the species-rich community of parasitoids attacking oak gall wasps due to a high degree of sexual dimorphism, morphological plasticity, small size, and poorly known biology. As such, we know very little about the number of species present, nor the evolutionary forces responsible for generating this diversity. One hypothesis is that trait diversity in the gall wasps, including the morphology of the galls they induce, has evolved in response to selection imposed by the parasitoid community, with reciprocal selection driving diversification of the parasitoids. Using a rare, continental-scale data set of Sycophila parasitoid wasps reared from 44 species of cynipid galls from 18 species of oak across the US, we combined mitochondrial DNA barcodes, Ultraconserved Elements (UCEs), morphological, and natural history data to delimit putative species. Using these results, we generate the first large-scale assessment of ecological specialization and host association in this species-rich group, with implications for evolutionary ecology and biocontrol. We find most Sycophila target specific subsets of available cynipid host galls with similar morphologies, and generally attack larger galls. Our results suggest that parasitoid wasps such as Sycophila have adaptations allowing them to exploit particular host trait combinations, while hosts with contrasting traits are resistant to attack. These findings support the tritrophic niche concept for the structuring of plant-herbivore-parasitoid communities.

A Tale of Two Tissues: probing gene expression in a complex insect‐induced gall

Plant galls are novel and sometimes dramatic plant organs whose development is initiated and controlled by parasitic microbes, nematodes, insects and mites. For arthropods, galls provide relative safety from enemies and abiotic stresses while providing nutrition. Galls are formed entirely by the plant, whose transcriptional pathways are modified and coopted to produce a structure specific to the galler species; they comprise a classic example of Dawkins’ “extended phenotype”. Arthropod‐elicited galls are unique in that they are often anatomically complex (Figure 1a), with multiple differentiated tissue types (Figure 1b). A growing number of investigators have studied changes in hostplant gene expression to understand arthropod gall development. In this issue of Molecular Ecology, Martinson et al. (2021) report using RNA sequencing to explore tissue‐specific gene expression associated with anatomical and functional gall complexity, demonstrating for the first time that gall tissues are as different transcriptionally as they are anatomically.

Comparison of Auxin and Cytokinins Concentrations, and the Structure of Bacterial Community between Host Twigs and Lithosaphonecrus arcoverticus Galls

Simple Summary

Insect galls are characterized by high concentrations of auxins and cytokinins. We calculated the correlation between the concentrations of indoleacetic acid (IAA), trans-zeatin riboside (tZR) and isopentenyladenine (iP) and the bacterial community structure of Lithosaphonecrus arcoverticus galls. Our results indicated the concentrations of IAA, tZR and iP were positively correlated with the bacterial community structure of L. arcoverticus galls. We suggest the high concentrations of IAA, tZR and iP may affect the bacterial community structure of L. arcoverticus galls.

Abstract

Insect galls are the abnormal growth of plant tissues induced by a wide variety of galling insects and characterized by high concentrations of auxins and cytokinins. It remains unclear whether the auxins and cytokinins affect the bacterial community structure of insect galls. We determined the concentrations of indoleacetic acid (IAA) as an example of auxin, trans-zeatin riboside (tZR) and isopentenyladenine (iP) as cytokinins in Lithosaphonecrus arcoverticus (Hymenoptera: Cynipidae) galls and the galled twigs of Lithocarpus glaber (Fagaceae) using liquid chromatography–tandem mass spectrometry. Moreover, for the first time, we compared the bacterial community structure of L. arcoverticus galls and galled twigs by high-throughput sequencing, and calculated the Spearman correlation and associated degree of significance between the IAA, tZR and iP concentrations and the bacterial community structure. Our results indicated the concentrations of IAA, tZR and iP were higher in L. arcoverticus galls than in galled twigs, and positively correlated with the bacterial community structure of L. arcoverticus galls. We suggest the high concentrations of IAA, tZR and iP may affect the bacterial community structure of L. arcoverticus galls.

Anatomy and Ultrastructure of Galls Induced by Neuroterus quercusbaccarum (Hymenoptera: Cynipidae) on Oak Leaves (Quercus robur)

Simple Summary

Galls induced by insects are commonly found on oak leaves in temperate climates. Their induction and development are, however, still only partially known. Our aim was to present a detailed description of their anatomy and fine structure. Using scanning electron microscopy (SEM), we observed the wax cover of a gall and two types of openings on its surface: one similar to degenerated stomata and the other one that we called “large openings on epidermal protuberance”. Nutritive tissue underwent marked changes with time: in a young gall, the cellular walls were not visible, and a distinct cellular structure appeared only later. In the mound of the gall, there was a distinct subepidermal layer of cells containing dense cytoplasm. Tannins occurred in vacuoles or in cell walls in granular form or in compact formations. The leaf subtending the gall showed additional cell divisions and strong lignification of cell walls in the tissue surrounding the peduncle of the gall.

Abstract

The structure and ultrastructure of two developmental stages of the spangle gall induced by Neuroterus quercusbaccarum (Hymenoptera, Cynipidae) were investigated using light microscopy (LM), fluorescence microscopy (FM), and transmission (TEM) and scanning (SEM) electron microscopy. The general design of the gall structure was typical of Cynipidae, but some structural features distinguished the spangle gall. Previously undescribed, characteristic multicellular epidermal protuberances with large openings were observed in autumn on the surface of galls. These may facilitate the gas exchange between the atmosphere and the inside of the gall, thus assisting larval respiration. The larval chamber is surrounded by both a sclerenchymatous capsule and numerous cells containing calcium oxalate crystals that may both serve as protective barriers. In young galls, the nutritive tissue is a wall-less protoplasmic mass, potentially easily accessible to young larvae with delicate mandibles. Cell walls only develop at a later stage. The nutritive tissue was found to be rich in proteins and lipids, but starch grains were not observed. Cellular topology suggests that spangle galls grow by anticlinal division of marginal epidermal cells and periclinal division of subepidermal cells. Cellular proliferation (hyperplasia) also occurs in the leaf tissue near the connection with the gall peduncle, which eventually lignifies.

Tissue-specific gene expression shows a cynipid wasp repurposes oak host gene networks to create a complex and novel parasite-specific organ

Every organism on Earth depends on interactions with other organisms to survive. In each of these interactions, an organism must utilize the limited toolbox of genes and proteins it possesses to successfully manipulate or cooperate with another species, but it can also co-opt the genome machinery of its partner to expand its available tools. Insect-induced plant galls are an extreme example of this, wherein an insect hijacks the plant's genome to direct the initiation and development of galls consisting of plant tissue. However, previous transcriptomic studies have not evaluated individual tissues within a gall to determine the full extent to which a galling insect manipulates its host plant. Here we demonstrate that the cynipid wasp Dryocosmus quercuspalustris creates a complex parasite-specific organ from red oak tissue via massive changes in host gene expression. Our results show that the gall wasp is not merely modifying oak leaf tissue but creating extensive changes in gene expression between galled and ungalled tissue (differential expression in 28% of genes) and distinct gall tissue types (20% of genes). The outer gall tissue shows increases in various plant defence systems, which is consistent with its predicted functional role of protecting the wasp larva. The inner larval capsule shows suppression of large parts of the plant innate immune system and evidence for the wasp utilizing the plant's RNA interference mechanisms, which may be a potential mechanism for the wasp's control on gall growth.

Interactions Between Figs and Gall-Inducing Fig Wasps: Adaptations, Constraints, and Unanswered Questions

The ancient interaction between figs (Ficus, Moraceae) and their pollinating fig wasps is an unusual example of a mutualism between plants and gall-inducing insects. This review intends to offer fresh perspectives into the relationship between figs and the diversity of gall-inducing sycophiles which inhabit their enclosed globular inflorescences that function as microcosms. Besides gall-inducing pollinators, fig inflorescences are also inhabited by other gall-inducing wasps. This review evaluates the state of current knowledge on gall-induction by fig wasps and exposes the many lacunae in this area. This review makes connections between fig and gall-inducing wasp traits, and suggests relatively unexplored research avenues. This manuscript calls for an integrated approach that incorporates such diverse fields as life-history theory, plant mate choice, wasp sexual selection and local mate competition, plant embryology as well as seed and fruit dispersal. It calls for collaboration between researchers such as plant developmental biologists, insect physiologists, chemical ecologists and sensory biologists to jointly solve the many valuable questions that can be addressed in community ecology, co-evolution and species interaction biology using the fig inflorescence microcosm, that is inhabited by gall-inducing mutualistic and parasitic wasps, as a model system.

The Localization of Phytohormones within the Gall-inducing Insect Eurosta solidaginis (Diptera: Tephritidae)

The phytohormone production hypothesis suggests that organisms, including insects, induce galls by producing and secreting plant growth hormones. Auxins and cytokinins are classes of phytohormones that induce cell growth and cell division, which could contribute to the plant tissue proliferation which constitutes the covering gall. Bacteria, symbiotic with insects, may also play a part in gall induction by insects through the synthesis of phytohormones or other effectors. Past studies have shown that concentrations of cytokinins and auxins in gall-inducing insects are higher than in their host plants. However, these analyses have involved whole-body extractions. Using immunolocalization of cytokinin and auxin, in the gall inducing stage of Eurosta solidaginis, we found both phytohormones to localize almost exclusively to the salivary glands. Co-localization of phytohormone label with a nucleic acid stain in the salivary glands revealed the absence of Wolbachia sp., the bacterial symbiont of E. solidaginis, which suggests that phytohormone production is symbiont independent. Our findings are consistent with the hypothesis that phytohormones are synthesized in and secreted from the salivary glands of E. solidaginis into host-plant tissues for the purpose of manipulating the host plant.

A novel family of secreted insect proteins linked to plant gall development

In an elaborate form of inter-species exploitation, many insects hijack plant development to induce novel plant organs called galls that provide the insect with a source of nutrition and a temporary home. Galls result from dramatic reprogramming of plant cell biology driven by insect molecules, but the roles of specific insect molecules in gall development have not yet been determined. Here, we study the aphid Hormaphis cornu, which makes distinctive "cone" galls on leaves of witch hazel Hamamelis virginiana. We found that derived genetic variants in the aphid gene determinant of gall color (dgc) are associated with strong downregulation of dgc transcription in aphid salivary glands, upregulation in galls of seven genes involved in anthocyanin synthesis, and deposition of two red anthocyanins in galls. We hypothesize that aphids inject DGC protein into galls and that this results in differential expression of a small number of plant genes. dgc is a member of a large, diverse family of novel predicted secreted proteins characterized by a pair of widely spaced cysteine-tyrosine-cysteine (CYC) residues, which we named BICYCLE proteins. bicycle genes are most strongly expressed in the salivary glands specifically of galling aphid generations, suggesting that they may regulate many aspects of gall development. bicycle genes have experienced unusually frequent diversifying selection, consistent with their potential role controlling gall development in a molecular arms race between aphids and their host plants.

Cecidology: Anatomy of a biohack

Galls are abnormal plant growths induced by animals, especially insects, to foster their own developing offspring. Which molecular malware gall-inducers use to hack into the plant's genetic programs is unclear. In an aphid, a gene affecting gall color has been discovered, part of a large family of putative effectors.

The Diversity and Dynamics of Fungi in Dryocosmus kuriphilus Community

Simple Summary

Dryocosmus kuriphilus is an invasive pest species which is native to China and is widely distributed in Asia, Europe and North America. D. kuriphilus induces insect galls on chestnut trees, and fungi can cause the necrosis of chestnut trees and the death of D. kuriphilus. The aim of this research was to investigate the potential role of D. kuriphilus in the transmission of fungi. We provide the first evidence that D. kuriphilus adults shared most fungal species with associated insect galls and the galled twigs of Castanea mollissima, and were dominated by Botryosphaeria sp., Aspergillus sp. and Diaporthe sp. Furthermore, we suggest that D. kuriphilus adults may be potential vectors of plant pathogens and mediate the transmission of fungi between chestnut trees.

Abstract

Dryocosmus kuriphilus (Hymenoptera: Cynipidae) is a gall wasp that induces insect galls on chestnut trees and results in massive yield losses worldwide. Fungi can cause the necrosis of chestnut trees and the death of gall wasps. The aim of this research was to investigate the potential role of D. kuriphilus in the transmission of fungi. We sequenced the ribosomal RNA internal transcribed spacer region 1 of fungi in D. kuriphilus adults, associated insect galls and the galled twigs of Castanea mollissima, using high-throughput sequencing. We compared the species richness, α-diversity and community structure of fungi in D. kuriphilus adults, insect galls and the galled twigs. We provide the first evidence that D. kuriphilus adults shared most fungal species with associated insect galls and the galled twigs, and were dominated by Botryosphaeria sp., Aspergillus sp. and Diaporthe sp. We suggest D. kuriphilus adults may be potential vectors of plant pathogens and may facilitate the transmission of fungi between chestnut trees. Furthermore, the fungi may horizontally transmit among D. kuriphilus adults, associated insect galls and the galled twigs.

A novel family of secreted insect proteins linked to plant gall development

In an elaborate form of inter-species exploitation, many insects hijack plant development to induce novel plant organs called galls that provide the insect with a source of nutrition and a temporary home. Galls result from dramatic reprogramming of plant cell biology driven by insect molecules, but the roles of specific insect molecules in gall development have not yet been determined. Here we study the aphidHormaphis cornu, which makes distinctive “cone” galls on leaves of witch hazelHamamelis virginiana. We found that derived genetic variants in the aphid genedeterminant of gall color(dgc) are associated with strong downregulation ofdgctranscription in aphid salivary glands, upregulation in galls of seven genes involved in anthocyanin synthesis, and deposition of two red anthocyanins in galls. We hypothesize that aphids inject DGC protein into galls, and that this results in differential expression of a small number of plant genes.Dgcis a member of a large, diverse family of novel predicted secreted proteins characterized by a pair of widely spaced cysteine-tyrosine-cysteine (CYC) residues, which we named BICYCLE proteins.Bicyclegenes are most strongly expressed in the salivary glands specifically of galling aphid generations, suggesting that they may regulate many aspects of gall development.Bicyclegenes have experienced unusually frequent diversifying selection, consistent with their potential role controlling gall development in a molecular arms race between aphids and their host plants.

One Sentence Summary

Aphidbicyclegenes, which encode diverse secreted proteins, contribute to plant gall development.

Microbiomes of willow-galling sawflies: effects of host plant, gall type, and phylogeny on community structure and function

While free-living herbivorous insects are thought to harbor microbial communities composed of transient bacteria derived from their diet, recent studies indicate that insects that induce galls on plants may be involved in more intimate host-microbe relationships. We used 16S rDNA metabarcoding to survey larval microbiomes of 20 nematine sawfly species that induce bud or leaf galls on 13 Salix species. The 391 amplicon sequence variants (ASVs) detected represented 69 bacterial genera in six phyla. Multi-variate statistical analyses showed that the structure of larval microbiomes is influenced by willow host species as well as by gall type. Nevertheless, a "core" microbiome composed of 58 ASVs is shared widely across the focal galler species. Within the core community, the presence of many abundant, related ASVs representing multiple distantly related bacterial taxa is reflected as a statistically significant effect of bacterial phylogeny on galler-microbe associations. Members of the core community have a variety of inferred functions, including degradation of phenolic compounds, nutrient supplementation, and production of plant hormones. Hence, our results support suggestions of intimate and diverse interactions between galling insects and microbes and add to a growing body of evidence that microbes may play a role in the induction of insect galls on plants.

Comprehensive phylogenomic analyses re-write the evolution of parasitism within cynipoid wasps

BACKGROUND

Parasitoidism, a specialized life strategy in which a parasite eventually kills its host, is frequently found within the insect order Hymenoptera (wasps, ants and bees). A parasitoid lifestyle is one of two dominant life strategies within the hymenopteran superfamily Cynipoidea, with the other being an unusual plant-feeding behavior known as galling. Less commonly, cynipoid wasps exhibit inquilinism, a strategy where some species have adapted to usurp other species' galls instead of inducing their own. Using a phylogenomic data set of ultraconserved elements from nearly all lineages of Cynipoidea, we here generate a robust phylogenetic framework and timescale to understand cynipoid systematics and the evolution of these life histories.

RESULTS

Our reconstructed evolutionary history for Cynipoidea differs considerably from previous hypotheses. Rooting our analyses with non-cynipoid outgroups, the Paraulacini, a group of inquilines, emerged as sister-group to the rest of Cynipoidea, rendering the gall wasp family Cynipidae paraphyletic. The families Ibaliidae and Liopteridae, long considered archaic and early-branching parasitoid lineages, were found nested well within the Cynipoidea as sister-group to the parasitoid Figitidae. Cynipoidea originated in the early Jurassic around 190 Ma. Either inquilinism or parasitoidism is suggested as the ancestral and dominant strategy throughout the early evolution of cynipoids, depending on whether a simple (three states: parasitoidism, inquilinism and galling) or more complex (seven states: parasitoidism, inquilinism and galling split by host use) model is employed.

CONCLUSIONS

Our study has significant impact on understanding cynipoid evolution and highlights the importance of adequate outgroup sampling. We discuss the evolutionary timescale of the superfamily in relation to their insect hosts and host plants, and outline how phytophagous galling behavior may have evolved from entomophagous, parasitoid cynipoids. Our study has established the framework for further physiological and comparative genomic work between gall-making, inquiline and parasitoid lineages, which could also have significant implications for the evolution of diverse life histories in other Hymenoptera.

From Inquilines to Gall Inducers: Genomic Signature of a Life-Style Transition in Synergus Gall Wasps

Gall wasps (Hymenoptera: Cynipidae) induce complex galls on oaks, roses, and other plants, but the mechanism of gall induction is still unknown. Here, we take a comparative genomic approach to revealing the genetic basis of gall induction. We focus on Synergus itoensis, a species that induces galls inside oak acorns. Previous studies suggested that this species evolved the ability to initiate gall formation recently, as it is deeply nested within the genus Synergus, whose members are mostly inquilines that develop inside the galls of other species. We compared the genome of S. itoensis with that of three related Synergus inquilines to identify genomic changes associated with the origin of gall induction. We used a novel Bayesian selection analysis, which accounts for branch-specific and gene-specific selection effects, to search for signatures of selection in 7,600 single-copy orthologous genes shared by the four Synergus species. We found that the terminal branch leading to S. itoensis had more genes with a significantly elevated dN/dS ratio (positive signature genes) than the other terminal branches in the tree; the S. itoensis branch also had more genes with a significantly decreased dN/dS ratio. Gene set enrichment analysis showed that the positive signature gene set of S. itoensis, unlike those of the inquiline species, is enriched in several biological process Gene Ontology terms, the most prominent of which is “Ovarian Follicle Cell Development.” Our results indicate that the origin of gall induction is associated with distinct genomic changes, and provide a good starting point for further characterization of the genes involved.

UCE data reveal multiple origins of rose gallers in North America: Global phylogeny of Diplolepis Geoffroy (Hymenoptera: Cynipidae)

Gall wasps in the genus Diplolepis Geoffroy are specialized herbivores that induce galls exclusively on roses. Despite their wide distribution across the Holarctic, little is known about their evolutionary history. Here we present the first phylogenomic tree of global Diplolepis reconstructed using Ultraconserved Elements (UCEs), resulting in a robust phylogeny based on 757 genes. Results support the existence of two principal clades: a Nearctic stem-galler clade, and a Holarctic leaf-galler clade that further splits into two Palearctic groups and one Nearctic group. This topology is congruent with a previous study based on the mitochondrial gene COI, an unexpected result given the common occurrence of mitonuclear discordance in closely related oak gall wasp lineages. Most Diplolepis species were recovered as reciprocally monophyletic, with some notable exceptions such as the D. polita and the D. ignota complex, for which species boundaries remain unresolved. Historical biogeographic reconstruction was unable to pinpoint the origin of Diplolepis, but confirms two independent incursions into the Nearctic. Ancestral state reconstruction analysis highlights the conservatism of gall location on the host plants, as shifts to different host organs are relatively rare. We suggest that Diplolepis were originally leaf gallers, with a Nearctic stem-galler clade undergoing a major plant organ switch onto rose stems. Host organ switch or reversal is uncommon, which suggests a level of conservatism. Our study showcases the resolving power of UCEs at the species level while also suggesting improvements to advance future Cynipoidea phylogenomics. Our results also highlight the additional sampling needed to clarify taxonomic relationships in the Nearctic and eastern Palearctic regions.

Gall-Inducing Parasites: Convergent and Conserved Strategies of Plant Manipulation by Insects and Nematodes

Gall-inducing insects and nematodes engage in sophisticated interactions with their host plants. These parasites can induce major morphological and physiological changes in host roots, leaves, and other tissues. Sedentary endoparasitic nematodes, root-knot and cyst nematodes in particular, as well as gall-inducing and leaf-mining insects, manipulate plant development to form unique organs that provide them with food from feeding cells. Sometimes, infected tissues may undergo a developmental switch resulting in the formation of aberrant and spectacular structures (clubs or galls). We describe here the complex interactions between these plant-reprogramming sedentary endoparasites and their infected hosts, focusing on similarities between strategies of plant manipulation. We highlight progress in our understanding of the host plant response to infection and focus on the nematode and insect molecules secreted in planta. We suggest thatlooking at similarities may identify convergent and conserved strategies and shed light on the promise they hold for the development of new management strategies in agriculture and forestry.

Daphnia magna modifies its gene expression extensively in response to caloric restriction revealing a novel effect on haemoglobin isoform preference

Caloric restriction (CR) produces clear phenotypic effects within and between generations of the model crustacean Daphnia magna. We have previously established that micro-RNAs and cytosine methylation change in response to CR in this organism, and we demonstrate here that CR has a dramatic effect on gene expression. Over 6,000 genes were differentially expressed between CR and well-fed D. magna, with a bias towards up-regulation of genes under caloric restriction. We identified a highly expressed haemoglobin gene that responds to CR by changing isoform proportions. Specifically, a transcript containing three haem-binding erythrocruorin domains was strongly down-regulated under CR in favour of transcripts containing fewer or no such domains. This change in the haemoglobin mix is similar to the response to hypoxia in Daphnia, which is mediated through the transcription factor hypoxia-inducible factor 1, and ultimately the mTOR signalling pathway. This is the first report of a role for haemoglobin in the response to CR. We also observed high absolute expression of superoxide dismutase (SOD) in normally fed individuals, which contrasts with observations of high SOD levels under CR in other taxa. However, key differentially expressed genes, like SOD, were not targeted by differentially expressed micro-RNAs. Whether the link between haemoglobin and CR occurs in other organisms, or is related to the aquatic lifestyle, remains to be tested. It suggests that one response to CR may be to simply transport less oxygen and lower respiration.

Plants use identical inhibitors to protect their cell wall pectin against microbes and insects

As fundamentally different as phytopathogenic microbes and herbivorous insects are, they enjoy plant-based diets. Hence, they encounter similar challenges to acquire nutrients. Both microbes and beetles possess polygalacturonases (PGs) that hydrolyze the plant cell wall polysaccharide pectin. Countering these threats, plant proteins inhibit PGs of microbes, thereby lowering their infection rate. Whether PG-inhibiting proteins (PGIPs) play a role in defense against herbivorous beetles is unknown. To investigate the significance of PGIPs in insect-plant interactions, feeding assays with the leaf beetle Phaedon cochleariae on Arabidopsis thaliana pgip mutants were performed. Fitness was increased when larvae were fed on mutant plants compared to wild-type plants. Moreover, PG activity was higher, although PG genes were downregulated in larvae fed on PGIP-deficient plants, strongly suggesting that PGIPs impair PG activity. As low PG activity resulted in delayed larval growth, our data provide the first in vivo correlative evidence that PGIPs act as defense against insects.

Plants make galls to accommodate foreigners: some are friends, most are foes

At the colonization site of a foreign entity, plant cells alter their trajectory of growth and development. The resulting structure - a plant gall - accommodates various needs of the foreigner, which are phylogenetically diverse: viruses, bacteria, protozoa, oomycetes, true fungi, parasitic plants, and many types of animals, including rotifers, nematodes, insects, and mites. The plant species that make galls also are diverse. We assume gall production costs the plant. All is well if the foreigner provides a gift that makes up for the cost. Nitrogen-fixing nodule-inducing bacteria provide nutritional services. Gall wasps pollinate fig trees. Unfortunately for plants, most galls are made for foes, some of which are deeply studied pathogens and pests: Agrobacterium tumefaciens, Rhodococcus fascians, Xanthomonas citri, Pseudomonas savastanoi, Pantoea agglomerans, 'Candidatus' phytoplasma, rust fungi, Ustilago smuts, root knot and cyst nematodes, and gall midges. Galls are an understudied phenomenon in plant developmental biology. We propose gall inception for discovering unifying features of the galls that plants make for friends and foes, talk about molecules that plants and gall-inducers use to get what they want from each other, raise the question of whether plants colonized by arbuscular mycorrhizal fungi respond in a gall-like manner, and present a research agenda.