Cooperation among selfish individuals in insect societies

Kin selection is the key to understanding the evolution of cooperation in insect societies. However, kin selection also predicts potential kin conflict, and understanding how these conflicts are resolved is a major goal of current research on social insects

Amino acid coevolution reveals three-dimensional structure and functional domains of insect odorant receptors.

Insect odorant receptors (ORs) comprise an enormous protein family that translates environmental chemical signals into neuronal electrical activity. These heptahelical receptors are proposed to function as ligand-gated ion channels and/or to act metabotropically as G protein-coupled receptors (GPCRs). Resolving their signalling mechanism has been hampered by the lack of tertiary structural information and primary sequence similarity to other proteins. We use amino acid evolutionary covariation across these ORs to define restraints on structural proximity of residue pairs, which permit de novo generation of three-dimensional models. The validity of our analysis is supported by the location of functionally important residues in highly constrained regions of the protein. Importantly, insect OR models exhibit a distinct transmembrane domain packing arrangement to that of canonical GPCRs, establishing the structural unrelatedness of these receptor families. The evolutionary couplings and models predict odour binding and ion conduction domains, and provide a template for rationale structure-activity dissection.

Social insect genomes exhibit dramatic evolution in gene composition and regulation while preserving regulatory features linked to sociality.

Genomes of eusocial insects code for dramatic examples of phenotypic plasticity and social organization. We compared the genomes of seven ants, the honeybee, and various solitary insects to examine whether eusocial lineages share distinct features of genomic organization. Each ant lineage contains ∼4000 novel genes, but only 64 of these genes are conserved among all seven ants. Many gene families have been expanded in ants, notably those involved in chemical communication (e.g., desaturases and odorant receptors). Alignment of the ant genomes revealed reduced purifying selection compared with Drosophila without significantly reduced synteny. Correspondingly, ant genomes exhibit dramatic divergence of noncoding regulatory elements; however, extant conserved regions are enriched for novel noncoding RNAs and transcription factor-binding sites. Comparison of orthologous gene promoters between eusocial and solitary species revealed significant regulatory evolution in both cis (e.g., Creb) and trans (e.g., fork head) for nearly 2000 genes, many of which exhibit phenotypic plasticity. Our results emphasize that genomic changes can occur remarkably fast in ants, because two recently diverged leaf-cutter ant species exhibit faster accumulation of species-specific genes and greater divergence in regulatory elements compared with other ants or Drosophila. Thus, while the "socio-genomes" of ants and the honeybee are broadly characterized by a pervasive pattern of divergence in gene composition and regulation, they preserve lineage-specific regulatory features linked to eusociality. We propose that changes in gene regulation played a key role in the origins of insect eusociality, whereas changes in gene composition were more relevant for lineage-specific eusocial adaptations.

Insect eggs suppress plant defence against chewing herbivores.

Plants activate direct and indirect defences in response to insect egg deposition. However, whether eggs can manipulate plant defence is unknown. In Arabidopsis thaliana, oviposition by the butterfly Pieris brassicae triggers cellular and molecular changes that are similar to the changes caused by biotrophic pathogens. In the present study, we found that the plant defence signal salicylic acid (SA) accumulates at the site of oviposition. This is unexpected, as the SA pathway controls defence against fungal and bacterial pathogens and negatively interacts with the jasmonic acid (JA) pathway, which is crucial for the defence against herbivores. Application of P. brassicae or Spodoptera littoralis egg extract onto leaves reduced the induction of insect-responsive genes after challenge with caterpillars, suggesting that egg-derived elicitors suppress plant defence. Consequently, larval growth of the generalist herbivore S. littoralis, but not of the specialist P. brassicae, was significantly higher on plants treated with egg extract than on control plants. In contrast, suppression of gene induction and enhanced S. littoralis performance were not seen in the SA-deficient mutant sid2-1, indicating that it is SA that mediates this phenomenon. These data reveal an intriguing facet of the cross-talk between SA and JA signalling pathways, and suggest that insects have evolved a way to suppress the induction of defence genes by laying eggs that release elicitors. We show here that egg-induced SA accumulation negatively interferes with the JA pathway, and provides an advantage for generalist herbivores.

Ablation of caterpillar labial salivary glands: technique for determining the role of saliva in insect-plant interactions.

There has been an ardent interest in herbivore saliva due to its roles in inducing plant defenses and its impact on herbivore fitness. Two techniques are described that inhibit the secretion of labial saliva from the caterpillar, Helicoverpa zea, during feeding. The methods rely on cauterizing the caterpillar's spinneret, the principal secretory structure of the labial glands, or surgically removing the labial salivary gland. Both methods successfully inhibit secretion of saliva and the principal salivary enzyme glucose oxidase. Caterpillars with inhibited saliva production feed at similar rates as the untreated caterpillars, pupate, and emerge as adults. Glucose oxidase has been suggested to increase the caterpillar's survival through the suppression of inducible anti-herbivore defenses in plants. Tobacco (Nicotiana tabacum) leaves fed on by caterpillars with ablated salivary glands had significantly higher levels of nicotine, an inducible anti-herbivore defense compound of tobacco, than leaves fed upon by caterpillars with intact labial salivary glands. Tomato (Lycopersicon esculentum) leaves fed upon by caterpillars with suppressed salivary secretions showed greatly reduced evidence of hydrogen peroxide formation compared to leaves fed upon by intact caterpillars. These two methods are useful techniques for determining the role that saliva plays in manipulating plant anti-herbivore defenses.

Task-dependent influence of genetic architecture and mating frequency on division of labour in social insect societies.

Division of labour is one of the most prominent features of social insects. The efficient allocation of individuals to different tasks requires dynamic adjustment in response to environmental perturbations. Theoretical models suggest that the colony-level flexibility in responding to external changes and internal perturbation may depend on the within-colony genetic diversity, which is affected by the number of breeding individuals. However, these models have not considered the genetic architecture underlying the propensity of workers to perform the various tasks. Here, we investigated how both within-colony genetic variability (stemming from variation in the number of matings by queens) and the number of genes influencing the stimulus (threshold) for a given task at which workers begin to perform that task jointly influence task allocation efficiency. We used a numerical agent-based model to investigate the situation where workers had to perform either a regulatory task or a foraging task. One hundred generations of artificial selection in populations consisting of 500 colonies revealed that an increased number of matings always improved colony performance, whatever the number of loci encoding the thresholds of the regulatory and foraging tasks. However, the beneficial effect of additional matings was particularly important when the genetic architecture of queens comprised one or a few genes for the foraging task's threshold. By contrast, a higher number of genes encoding the foraging task reduced colony performance with the detrimental effect being stronger when queens had mated with several males. Finally, the number of genes encoding the threshold for the regulatory task only had a minor effect on colony performance. Overall, our numerical experiments support the importance of mating frequency on efficiency of division of labour and also reveal complex interactions between the number of matings and genetic architecture.

Differential gene expression in response to mechanical wounding and insect feeding in Arabidopsis.

Wounding in multicellular eukaryotes results in marked changes in gene expression that contribute to tissue defense and repair. Using a cDNA microarray technique, we analyzed the timing, dynamics, and regulation of the expression of 150 genes in mechanically wounded leaves of Arabidopsis. Temporal accumulation of a group of transcripts was correlated with the appearance of oxylipin signals of the jasmonate family. Analysis of the coronatine-insensitive coi1-1 Arabidopsis mutant that is also insensitive to jasmonate allowed us to identify a large number of COI1-dependent and COI1-independent wound-inducible genes. Water stress was found to contribute to the regulation of an unexpectedly large fraction of these genes. Comparing the results of mechanical wounding with damage by feeding larvae of the cabbage butterfly (Pieris rapae) resulted in very different transcript profiles. One gene was specifically induced by insect feeding but not by wounding; moreover, there was a relative lack of water stress-induced gene expression during insect feeding. These results help reveal a feeding strategy of P. rapae that may minimize the activation of a subset of water stress-inducible, defense-related genes.

Decoupled post-glacial history in mutualistic plant-insect interactions: insights from the yellow loosestrife (Lysimachia vulgaris) and its associated oil-collecting bees (Macropis europaea and M-fulvipes)

AimWe take a comparative phylogeographical approach to assess whether three species involved in a specialized oil-rewarding pollination system (i.e. Lysimachia vulgaris and two oil-collecting bees within the genus Macropis) show congruent phylogeographical trajectories during post-glacial colonization processes. Our working hypothesis is that within specialized mutualistic interactions, where each species relies on the co-occurrence of the other for survival and/or reproduction, partners are expected to show congruent evolutionary trajectories, because they are likely to have followed parallel migration routes and to have shared glacial refugia. LocationWestern Palaearctic. MethodsOur analysis relies on the extensive sampling of 104 Western Palaearctic populations (totalling 434, 159 and 74 specimens of Lysimachiavulgaris, Macropiseuropaea and Macropisfulvipes, respectively), genotyped with amplified fragment length polymorphism. Based on this, we evaluated the regional genetic diversity (Shannon diversity and allele rarity index) and genetic structure (assessed using structure, population networks, isolation-by-distance and spatial autocorrelation metrics) of each species. Finally, we compared the general phylogeographical patterns obtained. ResultsContrary to our expectations, the analyses revealed phylogeographical signals suggesting that the investigated organisms demonstrate independent post-glacial trajectories as well as distinct contemporaneous demographic parameters, despite their mutualistic interaction. Main conclusionsThe mutualistic partners investigated here are likely to be experiencing distinct and independent evolutionary dynamics because of their contrasting life-history traits (e.g. dispersal abilities), as well as distinct hubs and migration routes. Such conditions would prevent and/or erase any signature of co-structuring of lineages in space and time. As a result, the lack of phylogeographical congruence driven by differences in life-history traits might have arisen irrespective of the three species having shared similar Pleistocene glacial refugia.

Control and host-dependent activation of insect toxin expression in a root-associated biocontrol pseudomonad.

Pseudomonas fluorescens CHA0 is a root-associated biocontrol agent that suppresses soil-borne fungal diseases of crops. Remarkably, the pseudomonad is also endowed with systemic and oral activity against pest insects which depends on the production of the insecticidal Fit toxin. The toxin gene (fitD) is part of a virulence cassette encoding three regulators (FitF, FitG, FitH) and a type I secretion system (FitABC-E). Immunoassays with a toxin-specific antibody and transcriptional analyses involving fitG and fitH deletion and overexpression mutants identified LysR family regulator FitG and response regulator FitH as activator and repressor, respectively, of Fit toxin and transporter expression. To visualize and quantify toxin expression in single live cells by fluorescence microscopy, we developed reporters which in lieu of the native toxin protein express a fusion of the Fit toxin with red fluorescent mCherry. In a wild-type background, expression of the mCherry-tagged Fit toxin was activated at high levels in insect hosts, i.e. when needed, yet not on plant roots or in batch culture. By contrast, a derepressed fitH mutant expressed the toxin in all conditions. P. fluorescens hence can actively induce insect toxin production in response to the host environment, and FitH and FitG are key regulators in this mechanism.

Conflict resolution and evolution of social structures in insect societies

In colonies of social Hymenoptera (which include all ants, as well as some wasp and bee species), only queens reproduce whereas workers generally perform other tasks. The evolution of worker's reproductive altruism can be explained by kin selection, which states that workers can indirectly transmit copies of their genes by helping the reproduction of relatives. The relatedness between queens and workers may however be low, particularly when there are multiple queens per colony, which limits the transmission of copies of workers genes and increases potential conflicts between colony members. In this thesis, we investigated the link between social structure variations and conflicts, and explored the mechanisms involved in variation of colony queen number in ants. According to kin selection, workers should rear the brood they are most related to. In social Hymenoptera, males are haploid whereas females (workers and queens) are diploid. As a result, workers can be up to three times more related to females than males in some colonies, where they should consequently favour the production of females. In contrast, queens are equally related to daughters and sons in all types of colonies and therefore should favour a balanced sex ratio. In a meta-analysis across all studies of social Hymenoptera, we showed that colony sex ratio is generally largely influenced by workers. Hence, the evolution of social structures where queens and workers are equally related to males and females may contribute to decrease the conflict between the two castes over colony sex ratio. Another conflict between queens and workers can occur over male production. Many species contain workers that still have the ability to lay haploid eggs. In some social structures, workers are on average more related to sons of queens than to sons of other workers. As a result, workers should eliminate worker-laid eggs to favour queen-laid eggs. We showed that in the ant Formica selysi, workers eliminate more worker-laid than queen-laid eggs, independently of colony social structure. These results therefore suggest that worker policing can evolve independently from relatedness, potentially because of costs of worker reproduction at the colony-level. Colony queen number is a key parameter that influences relatedness between group members. Queen body size is generally linked to the success of independent colony foundation by single queens and may influence the number of queens in the new colony. In the ant F. selysi, single-queen colonies produce larger queens than multiple-queen colonies. We showed that this association results from genes or maternal effects transmitted to the eggs. However, we also found that queens produced in colonies of the two social forms did not differ in their general ability to found new colonies independently. Queen body size may also influence queen dispersal ability and constrain small queens to be re-adopted in their original nest after mating at proximity. We tested the acceptance of new queens in another ant species, Formica paralugubris, which has numerous queens per colony. Our results show that workers do not discriminate between nestmate and foreign queens, and more generally accept new queens at a limited rate. To conclude, this thesis shows that mechanisms influencing variation in colony queen number and the influence of these changes on conflict resolution are complex. Data gathered in this thesis therefore constitute a solid background for further research on the evolution and the maintenance of complex organisations in insect societies.

Stick insect genomes reveal natural selection's role in parallel speciation.

Natural selection can drive the repeated evolution of reproductive isolation, but the genomic basis of parallel speciation remains poorly understood. We analyzed whole-genome divergence between replicate pairs of stick insect populations that are adapted to different host plants and undergoing parallel speciation. We found thousands of modest-sized genomic regions of accentuated divergence between populations, most of which are unique to individual population pairs. We also detected parallel genomic divergence across population pairs involving an excess of coding genes with specific molecular functions. Regions of parallel genomic divergence in nature exhibited exceptional allele frequency changes between hosts in a field transplant experiment. The results advance understanding of biological diversification by providing convergent observational and experimental evidence for selection's role in driving repeatable genomic divergence.

Binding of low concentration of peptide to H-2Kd produced in insect cells requires mouse beta 2-microglobulin co-expression.

A recombinant baculovirus expressing the murine class I MHC heavy chain H-2Kd cDNA under the transcriptional control of Autografa californica nuclear polyhedrosis virus (AcNPV) polyhedrin promoter has been isolated and used to infect Sf9 lepidopteran cells either alone or in association with a previously isolated virus expressing mouse beta 2-microglobulina (beta 2-ma). When infected with the heavy chain-encoding virus alone, H-2Kd was produced in a beta 2-m-free conformation detected on the surface of infected cells by conformation-independent antibodies. When Sf9 cells were co-infected with both viruses, approximately 10% of the heavy chain pool was engaged in the formation of native heterodimeric MHC class I molecules, which were glycosylated and transported to the cell surface as demonstrated by radio-binding experiments and flow cytometry. The assembly of the recombinant class I molecule was dependent on peptide, since heterodimer formation was brought about by H-2Kd-specific peptide ligands both in vivo, upon incubation with dually infected cells, and in vitro, in cell-free detergent extracts. In addition, a change in heavy chain conformation was brought about upon incubation with high concentrations (100 microM) of an H-2Kd-restricted octapeptide epitope from Plasmodium berghei. Furthermore, using low concentrations (3 nM) of a photoaffinity label derivative of this peptide, we show direct binding to cells co-expressing class I heavy chain and mouse beta 2-m but not to cells expressing free heavy chain only.

Domain shuffling in a sensor protein contributed to the evolution of insect pathogenicity in plant-beneficial Pseudomonas protegens.

Pseudomonas protegens is a biocontrol rhizobacterium with a plant-beneficial and an insect pathogenic lifestyle, but it is not understood how the organism switches between the two states. Here, we focus on understanding the function and possible evolution of a molecular sensor that enables P. protegens to detect the insect environment and produce a potent insecticidal toxin specifically during insect infection but not on roots. By using quantitative single cell microscopy and mutant analysis, we provide evidence that the sensor histidine kinase FitF is a key regulator of insecticidal toxin production. Our experimental data and bioinformatic analyses indicate that FitF shares a sensing domain with DctB, a histidine kinase regulating carbon uptake in Proteobacteria. This suggested that FitF has acquired its specificity through domain shuffling from a common ancestor. We constructed a chimeric DctB-FitF protein and showed that it is indeed functional in regulating toxin expression in P. protegens. The shuffling event and subsequent adaptive modifications of the recruited sensor domain were critical for the microorganism to express its potent insect toxin in the observed host-specific manner. Inhibition of the FitF sensor during root colonization could explain the mechanism by which P. protegens differentiates between the plant and insect host. Our study establishes FitF of P. protegens as a prime model for molecular evolution of sensor proteins and bacterial pathogenicity.

Born to be bee, fed to be worker? The caste system of a primitively eusocial insect.

ABSTRACT: INTRODUCTION: Primitively eusocial halictid bees are excellent systems to study the origin of eusociality, because all individuals have retained the ancestral ability to breed independently. In the sweat bee Halictus scabiosae, foundresses overwinter, establish nests and rear a first brood by mass-provisioning each offspring with pollen and nectar. The mothers may thus manipulate the phenotype of their offspring by restricting their food provisions. The first brood females generally help their mother to rear a second brood of males and gynes that become foundresses. However, the first brood females may also reproduce in their maternal or in other nests, or possibly enter early diapause. Here, we examined if the behavioural specialization of the first and second brood females was associated with between-brood differences in body size, energetic reserves and pollen provisions. RESULTS: The patterns of variation in adult body size, weight, fat content and food provisioned to the first and second brood indicate that H. scabiosae has dimorphic females. The first-brood females were significantly smaller, lighter and had lower fat reserves than the second-brood females and foundresses. The first-brood females were also less variable in size and fat content, and developed on homogeneously smaller pollen provisions. Foundresses were larger than gynes of the previous year, suggesting that small females were less likely to survive the winter. CONCLUSIONS: The marked size dimorphism between females produced in the first and second brood and the consistently smaller pollen provisions provided to the first brood suggest that the first brood females are channelled into a helper role during their pre-imaginal development. As a large body size is needed for successful hibernation, the mother may promote helping in her first brood offspring by restricting their food provisions. This pattern supports the hypothesis that parental manipulation may contribute to promote worker behaviour in primitively eusocial halictids.