Primordial Enemies: Fungal Pathogens in Thrips Societies

Microbial pathogens are ancient selective agents that have driven many aspects of multicellular evolution, including genetic, behavioural, chemical and immune defence systems. It appears that fungi specialised to attack insects were already present in the environments in which social insects first evolved and we hypothesise that if the early stages of social evolution required antifungal defences, then covariance between levels of sociality and antifungal defences might be evident in extant lineages, the defences becoming stronger with group size and increasing social organisation. Thus, we compared the activity of cuticular antifungal compounds in thrips species (Insecta: Thysanoptera) representing a gradient of increasing group size and sociality: solitary, communal, social and eusocial, against the entomopathogen Cordyceps bassiana. Solitary and communal species showed little or no activity. In contrast, the social and eusocial species killed this fungus, suggesting that the evolution of sociality has been accompanied by sharp increases in the effectiveness of antifungal compounds. The antiquity of fungal entomopathogens, demonstrated by fossil finds, coupled with the unequivocal response of thrips colonies to them shown here, suggests two new insights into the evolution of thrips sociality: First, traits that enabled nascent colonies to defend themselves against microbial pathogens should be added to those considered essential for social evolution. Second, limits to the strength of antimicrobials, through resource constraints or self-antibiosis, may have been overcome by increase in the numbers of individuals secreting them, thus driving increases in colony size. If this is the case for social thrips, then we may ask: did antimicrobial traits and microbes such as fungal entomopathogens play an integral part in the evolution of insect sociality in general?     

Antimicrobial strength increases with group size: implications for social evolution

We hypothesize that aggregations of animals are likely to attract pathogenic micro-organisms and that this is especially the case for semisocial and eusocial insects where selection ultimately led to group sizes in the thousands or even millions, attracting the epithet 'superorganism'. Here, we analyse antimicrobial strength, per individual, in eight thrips species (Insecta: Thysanoptera) that present increasing innate group sizes and show that species with the largest group size (100-700) had the strongest antimicrobials, those with smaller groups (10-80) had lower antimicrobial activity, while solitary species showed none. Species with large innate group sizes showed strong antimicrobial activity while the semisocial species showed no activity until group size increased sufficiently to make activity detectable. The eusocial species behaved in a similar way, with detectable activity appearing once group size exceeded 120. These analyses show that antimicrobial strength is determined by innate group size. This suggests that the evolution of sociality that, by definition, increases group size, may have had particular requirements for defences against microbial pathogens. Thus, increase in group size, accompanied by increased antibiotic strength, may have been a critical factor determining the 'point of no return', early in the evolution of social insects, beyond which the evolution of social anatomical and morphological traits was irreversible. Our data suggest that traits that increase group size in general are accompanied by increased antimicrobial strength and that this was critical for transitions from solitary to social and eusocial organization.     

Experimental evolution of external immune defences in the red flour beetle

The red flour beetle, Tribolium castaneum, secretes quinones that control the microbial flora in the surrounding environment. These secretions act as an external immune defence that provides protection against pathogens. At high concentrations, however, these secretions are harmful to the host itself, and selection may thus have optimized the level of expression under natural conditions. Here, we show that the expression of external immunity responded to selection during experimental evolution within a few generations. At the same time, one component of internal immune defence (phenoloxidase activity) was compromised in beetles selected for either high or low external defences. Intriguingly, offspring protection against a natural pathogen was reduced in flour obtained from beetle lines selected for low amounts of secretions. Altogether, this suggests that external and internal immune defences work together efficiently under natural conditions, whereas every manipulation on the side of external immune defence comes with costs to the internal immune defence.     

Selection on an antimicrobial peptide defensin in ants

Ants live in crowded nests with interacting individuals, which makes them particularly prone to infectious diseases. The question is, how do ants cope with the increased risk of pathogen transmission due to sociality? We have studied the molecular evolution of defensin, a gene encoding an antimicrobial protein, in ants. Defensin sequences from several ant species were analyzed with maximum likelihood models of codon substitution to infer selection. Positive selection was detected in the mature region of defensin, whereas the signal and pro regions seem to be evolving neutrally. We also found a significantly higher rate of nonsynonymous substitutions in some phylogenetic lineages, as well as dN/dS >1, suggesting varying selection pressures in different lineages. Earlier studies on the molecular evolution of insect antimicrobial peptide genes have focused on termites and dipteran species, and detected positive selection only in duplicated termicin genes in termites. These findings, together with our present results, provide an indication that the immune systems of social insects (ants and termites) and dipteran insects may have responded differently to the selection pressure caused by microbial pathogens.     

Social complexity and nesting habits are factors in the evolution of antimicrobial defences in wasps

Microbial diseases are important selective agents in social insects and one major defense mechanism is the secretion of cuticular antimicrobial compounds. We hypothesized that given differences in group size, social complexity, and nest type the secretions of these antimicrobials will be under different selective pressures. To test this we extracted secretions from nine wasp species of varying social complexity and nesting habits and assayed their antimicrobial compounds against cultures of Staphylococcus aureus. These data were then combined with phylogenetic data to provide an evolutionary context. Social species showed significantly higher (18x) antimicrobial activity than solitary species and species with paper nests showed significantly higher (11x) antimicrobial activity than those which excavated burrows. Mud-nest species showed no antimicrobial activity. Solitary, burrow-provisioning wasps diverged at more basal nodes of the phylogenetic trees, while social wasps diverged from the most recent nodes. These data suggest that antimicrobial defences may have evolved in response to ground-dwelling pathogens but the most important variable leading to increased antimicrobial strength was increase in group size and social complexity.     

Mechanisms and fitness effects of antibacterial defences in a carrion beetle

Parents of many species care for their offspring by protecting them from a wide range of environmental hazards, including desiccation, food shortages, predators, competitors, and parasites and pathogens. Currently, little is known about the mechanisms and fitness consequences of parental defences against bacterial pathogens and competitors. Here, we combine approaches from microbiology and behavioural ecology to investigate the role and mechanistic basis of antibacterial secretions applied to carcasses by parents of the burying beetle Nicrophorus vespilloides. This species rears its larvae on vertebrate carcasses, where larvae suffer significant fitness costs due to competition with bacterial decomposers. We first confirm that anal secretions produced by parents are potently bactericidal and that their effects are specific to gram-positive bacteria. Next, we identify the source of bacterial killing as a secreted lysozyme and show that its concentration changes throughout the breeding cycle. Finally, we show that secreted lysozyme is crucial for larval development, increasing survival by nearly two-fold compared to offspring reared in its absence. These results demonstrate for the first time that anal secretions applied to carrion is a form of parental care and expand the mechanistic repertoire of defences used by parent insects to protect dependent offspring from microbial threats.     

ECOLOGICAL DRIVERS OF ANTIPREDATOR DEFENSES IN CARNIVORES

Mammals have evolved several morphological and behavioral adaptations to reduce the risk of predation, but we know little about the ecological factors that favor their evolution. For example, some mammalian carnivores have the ability to spray noxious anal secretions in defense, whereas other species lack such weaponry but may instead rely on collective vigilance characteristic of cohesive social groups. Using extensive natural history data on 181 species in the order Carnivora, we created a new estimate of potential predation risk from mammals and birds of prey and used comparative phylogenetic methods to assess how different sources of predation risk and other ecological variables influence the evolution of either noxious weaponry or sociality in this taxon. We demonstrate that the evolution of enhanced spraying ability is favored by increased predation risk from other mammals and by nocturnality, but the evolution of sociality is favored by increased predation risk from birds of prey and by diurnality, which may allow for enhanced early visual detection. These results suggest that noxious defenses and sociality are alternative antipredator strategies targeting different predator guilds under different lighting conditions.     

Ecosystem screening approach for pathogen-associated microorganisms affecting host disease

The microbial community in which a pathogen evolves is fundamental to disease outcome. Species interacting with a pathogen on the host surface shape the distribution, density, and genetic diversity of the inoculum, but the role of these species is rarely determined. The screening method developed here can be used to characterize pathogen-associated species affecting disease. This strategy involves three steps: (i) constitution of the microbial community, using the pathogen as a trap; (ii) community selection, using extracts from the pathogen as the sole nutrient source; and (iii) molecular identification and the screening of isolates focusing on their effects on the growth of the pathogen in vitro and host disease. This approach was applied to a soilborne plant pathogen, Phytophthora parasitica, structured in a biofilm, for screening the microbial community from the rhizosphere of Nicotiana tabacum (the host). Two of the characterized eukaryotes interfered with the oomycete cycle and may affect the host disease. A Vorticella species acted through a mutualistic interaction with P. parasitica, disseminating pathogenic material by leaving the biofilm. A Phoma species established an amensal interaction with P. parasitica, strongly suppressing disease by inhibiting P. parasitica germination. This screening method is appropriate for all nonobligate pathogens. It allows the definition of microbial species as promoters or suppressors of a disease for a given biotope. It should also help to identify important microbial relationships for ecology and evolution of pathogens.     

Antimicrobial Functions of Lactoferrin Promote Genetic Conflicts in Ancient Primates and Modern Humans

Lactoferrin is a multifunctional mammalian immunity protein that limits microbial growth through sequestration of nutrient iron. Additionally, lactoferrin possesses cationic protein domains that directly bind and inhibit diverse microbes. The implications for these dual functions on lactoferrin evolution and genetic conflicts with microbes remain unclear. Here we show that lactoferrin has been subject to recurrent episodes of positive selection during primate divergence predominately at antimicrobial peptide surfaces consistent with long-term antagonism by bacteria. An abundant lactoferrin polymorphism in human populations and Neanderthals also exhibits signatures of positive selection across primates, linking ancient host-microbe conflicts to modern human genetic variation. Rapidly evolving sites in lactoferrin further correspond to molecular interfaces with opportunistic bacterial pathogens causing meningitis, pneumonia, and sepsis. Because microbes actively target lactoferrin to acquire iron, we propose that the emergence of antimicrobial activity provided a pivotal mechanism of adaptation sparking evolutionary conflicts via acquisition of new protein functions.     

Natural variation in priming of basal resistance: from evolutionary origin to agricultural exploitation

Biotic stress has a major impact on the process of natural selection in plants. As plants have evolved under variable environmental conditions, they have acquired a diverse spectrum of defensive strategies against pathogens and herbivores. Genetic variation in the expression of plant defence offers valuable insights into the evolution of these strategies. The 'zigzag' model, which describes an ongoing arms race between inducible plant defences and their suppression by pathogens, is now a commonly accepted model of plant defence evolution. This review explores additional strategies by which plants have evolved to cope with biotic stress under different selective circumstances. Apart from interactions with plant-beneficial micro-organisms that can antagonize pathogens directly, plants have the ability to prime their immune system in response to selected environmental signals. This defence priming offers disease protection that is effective against a broad spectrum of virulent pathogens, as long as the augmented defence reaction is expressed before the invading pathogen has the opportunity to suppress host defences. Furthermore, priming has been shown to be a cost-efficient defence strategy under relatively hostile environmental conditions. Accordingly, it is possible that selected plant varieties have evolved a constitutively primed immune system to adapt to levels of disease pressure. Here, we examine this hypothesis further by evaluating the evidence for natural variation in the responsiveness of basal defence mechanisms, and discuss how this genetic variation can be exploited in breeding programmes to provide sustainable crop protection against pests and diseases.     

Plant-mediated effects in the Brassicaceae on the performance and behaviour of parasitoids

Direct and indirect plant defences are well studied, particularly in the Brassicaceae. Glucosinolates (GS) are secondary plant compounds characteristic in this plant family. They play an important role in defence against herbivores and pathogens. Insect herbivores that are specialists on brassicaceous plant species have evolved adaptations to excrete or detoxify GS. Other insect herbivores may even sequester GS and employ them as defence against their own antagonists, such as predators. Moreover, high levels of GS in the food plants of non-sequestering herbivores can negatively affect the growth and survival of their parasitoids. In addition to allelochemicals, plants produce volatile chemicals when damaged by herbivores. These herbivore induced plant volatiles (HIPV) have been demonstrated to play an important role in foraging behaviour of insect parasitoids. In addition, biosynthetic pathways involved in the production of HIPV are being unraveled using the model plant Arabidopsis thialiana. However, the majority of studies investigating the attractiveness of HIPV to parasitoids are based on experiments mainly using crop plant species in which defence traits may have changed through artificial selection. Field studies with both cultivated and wild crucifers, the latter in which defence traits are intact, are necessary to reveal the relative importance of direct and indirect plant defence strategies on parasitoid and plant fitness. Future research should also consider the potential conflict between direct and indirect plant defences when studying the evolution of plant defences against insect herbivory.     

Source-sink dynamics of virulence evolution

To understand the evolution of genetic diversity within species--bacterial and others--we must dissect the first steps of genetic adaptation to novel habitats, particularly habitats that are suboptimal for sustained growth where there is strong selection for adaptive changes. Here, we present the view that bacterial human pathogens represent an excellent model for understanding the molecular mechanisms of the adaptation of a species to alternative habitats. In particular, bacterial pathogens allow us to develop analytical methods to detect genetic adaptation using an evolutionary 'source-sink' model, with which the evolution of bacterial pathogens can be seen from the angle of continuous switching between permanent (source) and transient (sink) habitats. The source-sink model provides a conceptual framework for understanding the population dynamics and molecular mechanisms of virulence evolution.     

Strategies of chemical anti-predator defences in leaf beetles: is sequestration of plant toxins less costly than de novo synthesis?

The evolution of defensive traits is driven both by benefits gained from protection against enemies and by costs of defence production. We tested the hypothesis that specialisation of herbivores on toxic host plants, accompanied by the ability to acquire plant defensive compounds for herbivore defence, is favoured by the lower costs of sequestration compared to de novo synthesis of defensive compounds. We measured physiological costs of chemical defence as a reduction in larval performance in response to repeated removal of secretions (simulating predator attack) and compared these costs between five species synthesising defences de novo and three species sequestering salicylic glucosides (SGs) from their host plants. Experiments simulating low predator pressure revealed no physiological costs in terms of survival, weight and duration of development in any of study species. However, simulation of high predation caused reduction in relative growth rate in Chrysomela lapponica larvae producing autogenous defences more frequently, than in larvae sequestering SGs. Still meta-analysis of combined data showed no overall difference in costs of autogenous and sequestered defences. However, larvae synthesising their defences de novo demonstrated secretion-conserving behaviour, produced smaller amounts of secretions, replenished them at considerably lower rates and employed other types of defences (regurgitation, evasion) more frequently when compared to sequestering larvae. These latter results provide indirect evidence for biosynthetic constraints for amounts of defensive secretions produced de novo, resulting in low defence effectiveness. Lifting these constraints by sequestration may have driven some leaf beetle lineages toward sequestration of plant allelochemicals as the main defensive strategy.     

Fine‐scale genetic structure reflects sex‐specific dispersal strategies in a population of sociable weavers (Philetairus socius)

Dispersal is a critical driver of gene flow, with important consequences for population genetic structure, social interactions and other biological processes. Limited dispersal may result in kin‐structured populations in which kin selection may operate, but it may also increase the risk of kin competition and inbreeding. Here, we use a combination of long‐term field data and molecular genetics to examine dispersal patterns and their consequences for the population genetics of a highly social bird, the sociable weaver (Philetairus socius), which exhibits cooperation at various levels of sociality from nuclear family groups to its unique communal nests. Using 20 years of data, involving capture of 6508 birds and 3151 recaptures at 48 colonies, we found that both sexes exhibit philopatry and that any dispersal occurs over relatively short distances. Dispersal is female‐biased, with females dispersing earlier, further, and to less closely related destination colonies than males. Genotyping data from 30 colonies showed that this pattern of dispersal is reflected by fine‐scale genetic structure for both sexes, revealed by isolation by distance in terms of genetic relatedness and significant genetic variance among colonies. Both relationships were stronger among males than females. Crucially, significant relatedness extended beyond the level of the colony for both sexes. Such fine‐scale population genetic structure may have played an important role in the evolution of cooperative behaviour in this species, but it may also result in a significant inbreeding risk, against which female‐biased dispersal alone is unlikely to be an effective strategy.     

Antimicrobial peptides from anurans skin secretions

This article is an overview of antimicrobial peptides found in anurans skin secretions. These molecules constitute an initial barrier against microbial infections because of their activity against a large array of microorganisms. These peptides hold remarkable pharmaceutical and technological interest since they selectively kill microorganisms and are unlikely to induce resistance in pathogens. Also, outstanding synergism occurs when these peptides are combined with classic antibiotics and other antimicrobial peptides.     

Adaptive molecular evolution of a defence gene in sexual but not functionally asexual evening primroses

Theory predicts that sexual reproduction provides evolutionary advantages over asexual reproduction by reducing mutational load and increasing adaptive potential. Here, we test the latter prediction in the context of plant defences against pathogens because pathogens frequently reduce plant fitness and drive the evolution of plant defences. Specifically, we ask whether sexual evening primrose plant lineages (Onagraceae) have faster rates of adaptive molecular evolution and altered gene expression of a class I chitinase, a gene implicated in defence against pathogens, than functionally asexual evening primrose lineages. We found that the ratio of amino acid to silent substitutions (K(a) /K(s) = 0.19 vs. 0.11 for sexual and asexual lineages, respectively), the number of sites identified to be under positive selection (four vs. zero for sexual and asexual lineages, respectively) and the expression of chitinase were all higher in sexual than in asexual lineages. Our results are congruent with the conclusion that a loss of sexual recombination and segregation in the Onagraceae negatively affects adaptive structural and potentially regulatory evolution of a plant defence protein.     

Ecology shapes the evolutionary trade‐off between predator avoidance and defence in coral reef butterflyfishes

Antipredator defensive traits are thought to trade‐off evolutionarily with traits that facilitate predator avoidance. However, complexity and scale have precluded tests of this prediction in many groups, including fishes. Using a macroevolutionary approach, we test this prediction in butterflyfishes, an iconic group of coral reef inhabitants with diverse social behaviours, foraging strategies and antipredator adaptations. We find that several antipredator traits have evolved adaptively, dependent primarily on foraging strategy. We identify a previously unrecognised axis of diversity in butterflyfishes where species with robust morphological defences have riskier foraging strategies and lack sociality, while species with reduced morphological defences feed in familiar territories, have adaptations for quick escapes and benefit from the vigilance provided by sociality. Furthermore, we find evidence for the constrained evolution of fin spines among species that graze solely on corals, highlighting the importance of corals, as both prey and structural refuge, in shaping fish morphology.     

Immune defense and host sociality: a comparative study of swallows and martins

Sociality is associated with increased risks of parasitism, predation, and social competition, which may interact because social stress can reduce immunity, and parasitized individuals are more likely to fall prey to a predator. A mechanism allowing evolution of sociality in spite of high costs of parasitism is increased investment in antiparasite defenses. Here we show that the impact of parasites on host reproductive success was positively associated with the degree of sociality in the bird family Hirundinidae. However, the cost of parasitism in highly colonial species was countered by high levels of T- and B-cell immune responses. Investment in immune function among colonial species was particularly strong in nestlings, and among social species, this investment was associated with a relatively prolonged period of development, thereby leading to extended exposure to parasites. Thus, highly social species such as certain species of swallows and martins may cope with strong natural selection arising from parasites by heavy investment in immune function at the cost of a long exposure to nest parasites.     

Reliance on pollinators predicts defensive chemistry across tobacco species

Defensive traits are typically studied in the context of avoiding antagonists, but may also mediate key interactions with mutualists. Plant chemical defences occur in flowers, suggesting pollinators may be agents of selection on defence. We hypothesised that floral defences would deter pollinators, and therefore, pollinators would select for lower defences in outcrossing than self-pollinating species. We measured pollinator reliance and alkaloid levels in 32 greenhouse-grown Nicotiana species. Using a comparative phylogenetic approach, we found significantly lower nectar, floral and leaf nicotine concentrations in outcrossing than selfing species, with a 15-fold decrease in leaf nicotine levels. Nicotine concentrations were positively correlated across tissues, suggesting that selection against floral defences could constrain the evolution of leaf defences. Thus, pollinators could shape the evolution not only of floral defences but also of defences in other tissues where herbivores have traditionally been considered the dominant agent of selection.     

Sociality in New World hystricognath rodents is linked to predators and burrow digging

The importance of predation and burrow digging in explainingthe evolution of sociality is generally unclear. We focusedon New World hystricognath rodents to evaluate three key predictionsof the predation hypothesis. First, large-bodied surface-dwellingspecies will be more vulnerable because they are more detectable;thus sociality should be associated with body size. Second,surface-dwelling, diurnal species would be more vulnerable topredators than nocturnal species; thus sociality should be associatedwith the evolution of diurnality. Third, species living in openhabitats will be more vulnerable; thus sociality should evolvein species living in open habitats. Regarding the importanceof burrows, we tested if species that dig burrows can benefitfrom communal labor; thus, sociality should be associated withburrow digging. All traits had significant phylogenetic signal,thus comparative analyses should explicitly address this. Ina comparative analysis on independent contrasts we found thatsociality was correlated with body size (larger species weremore social), diurnality (diurnal species were more social),and burrowing (burrowing species were more social), but we foundno effect of overhead plant cover of habitat on sociality inhystricognath rodents. Somewhat different results were foundwhen we analyzed the raw data. Taken together, our results providesupport for a link between predation risk, burrow digging, andsociality in this group.