Defensive Symbiosis Research Articles

Arsenophonusinsect symbionts are commonly infected with APSE, a bacteriophage involved in protective symbiosis

Insects commonly have intimate associations with maternally inherited bacterial symbionts. While many inherited symbionts are not essential for host survival, they often act as conditional mutualists, conferring protection against certain environmental stresses. The defensive symbiont Hamiltonella defensa which protects aphids against attacks by parasitoid wasps is one of these conditional mutualists. The protection afforded by Hamiltonella depends on the presence of a lysogenic bacteriophage, called APSE, encoding homologs of toxins that are suspected to target wasp cells. In this study, an important diversity of APSE variants is reported from another heritable symbiont, Arsenophonus, which is exceptionally widespread in insects. APSE was found in association with two-thirds of the Arsenophonus strains examined and from a variety of insect groups such as aphids, white flies, parasitoid wasps, triatomine bugs, louse flies, and bat flies. No APSE was, however, found from Arsenophonus relatives such as the recently described Aschnera chinzeii and ALO-3 endosymbionts. Phylogenetic investigations revealed that APSE has a long evolutionary history in heritable symbionts, being secondarily acquired by Hamiltonella through lateral transfer from Arsenophonus. Overall, this highlights the role of lateral transfer as a major evolutionary process shaping the emergence of defensive symbiosis in heritable bacteria.

Occurrence of Bacillus amyloliquefaciens as a systemic endophyte of vanilla orchids.

We report the occurrence of Bacillus amyloliquefaciens in vanilla orchids (Vanilla phaeantha) and cultivated hybrid vanilla (V. planifolia × V. pompona) as a systemic bacterial endophyte. We determined with light microscopy and isolations that tissues of V. phaeantha and the cultivated hybrid were infected by a bacterial endophyte and that shoot meristems and stomatal areas of stems and leaves were densely colonized. We identified the endophyte as B. amyloliquefaciens using DNA sequence data. Since additional endophyte-free plants and seed of this orchid were not available, additional studies were performed on surrogate hosts Amaranthus caudatus, Ipomoea tricolor, and I. purpurea. Plants of A. caudatus inoculated with B. amyloliquefaciens demonstrated intracellular colonization of guard cells and other epidermal cells, confirming the pattern observed in the orchids. Isolations and histological studies suggest that the bacterium may penetrate deeply into developing plant tissues in shoot meristems, forming endospores in maturing tissues. B. amyloliquefaciens produced fungal inhibitors in culture. In controlled experiments using morning glory seedlings we showed that the bacterium promoted seedling growth and reduced seedling necrosis due to pathogens. We detected the gene for phosphopantetheinyl transferase (sfp), an enzyme in the pathway for production of antifungal lipopeptides, and purified the lipopeptide "surfactin" from cultures of the bacterium. We hypothesize that B. amyloliquefaciens is a robust endophyte and defensive mutualist of vanilla orchids. Whether the symbiosis between this bacterium and its hosts can be managed to protect vanilla crops from diseases is a question that should be evaluated in future research.

Factors limiting the spread of the protective symbiont Hamiltonella defensa in Aphis craccivora Aphids.

Many insects are associated with heritable symbionts that mediate ecological interactions, including host protection against natural enemies. The cowpea aphid, Aphis craccivora, is a polyphagous pest that harbors Hamiltonella defensa, which defends against parasitic wasps. Despite this protective benefit, this symbiont occurs only at intermediate frequencies in field populations. To identify factors constraining H. defensa invasion in Ap. craccivora, we estimated symbiont transmission rates, performed fitness assays, and measured infection dynamics in population cages to evaluate effects of infection. Similar to results with the pea aphid, Acyrthosiphon pisum, we found no consistent costs to infection using component fitness assays, but we did identify clear costs to infection in population cages when no enemies were present. Maternal transmission rates of H. defensa in Ap. craccivora were high (ca. 99%) but not perfect. Transmission failures and infection costs likely limit the spread of protective H. defensa in Ap. craccivora. We also characterized several parameters of H. defensa infection potentially relevant to the protective phenotype. We confirmed the presence of H. defensa in aphid hemolymph, where it potentially interacts with endoparasites, and performed real-time quantitative PCR (qPCR) to estimate symbiont and phage abundance during aphid development. We also examined strain variation of H. defensa and its bacteriophage at multiple loci, and despite our lines being collected in different regions of North America, they were infected with a nearly identical strains of H. defensa and APSE4 phage. The limited strain diversity observed for these defensive elements may result in relatively static protection profile for this defensive symbiosis.

Posttranslationally modified small-peptide signals in plants.

Cell-to-cell signaling is essential for many processes in plant growth and development, including coordination of cellular responses to developmental and environmental cues. Cumulative studies have demonstrated that peptide signaling plays a greater-than-anticipated role in such intercellular communication. Some peptides act as signals during plant growth and development, whereas others are involved in defense responses or symbiosis. Peptides secreted as signals often undergo posttranslational modification and proteolytic processing to generate smaller peptides composed of approximately 10 amino acid residues. Such posttranslationally modified small-peptide signals constitute one of the largest groups of secreted peptide signals in plants. The location of the modification group incorporated into the peptides by specific modification enzymes and the peptide chain length defined by the processing enzymes are critical for biological function and receptor interaction. This review covers 20 years of research into posttranslationally modified small-peptide signals in plants.

Partner choice and fidelity stabilize coevolution in a Cretaceous-age defensive symbiosis.

Many insects rely on symbiotic microbes for survival, growth, or reproduction. Over evolutionary timescales, the association with intracellular symbionts is stabilized by partner fidelity through strictly vertical symbiont transmission, resulting in congruent host and symbiont phylogenies. However, little is known about how symbioses with extracellular symbionts, representing the majority of insect-associated microorganisms, evolve and remain stable despite opportunities for horizontal exchange and de novo acquisition of symbionts from the environment. Here we demonstrate that host control over symbiont transmission (partner choice) reinforces partner fidelity between solitary wasps and antibiotic-producing bacteria and thereby stabilizes this Cretaceous-age defensive mutualism. Phylogenetic analyses show that three genera of beewolf wasps (Philanthus, Trachypus, and Philanthinus) cultivate a distinct clade of Streptomyces bacteria for protection against pathogenic fungi. The symbionts were acquired from a soil-dwelling ancestor at least 68 million years ago, and vertical transmission via the brood cell and the cocoon surface resulted in host-symbiont codiversification. However, the external mode of transmission also provides opportunities for horizontal transfer, and beewolf species have indeed exchanged symbiont strains, possibly through predation or nest reuse. Experimental infection with nonnative bacteria reveals that--despite successful colonization of the antennal gland reservoirs--transmission to the cocoon is selectively blocked. Thus, partner choice can play an important role even in predominantly vertically transmitted symbioses by stabilizing the cooperative association over evolutionary timescales.

Defensive symbiosis: a microbial perspective

Department of Biology, Indiana University, Bloomington, IN, USAThe enemy of my enemy is my friend (Ancient Proverb)Defensive symbioses are indirect interactions thatinvolve at least three species (host, symbiont and enemy)where the net benefits of symbiosis are contingent on thepresence of enemies (Fig. 1, Clay, Holah & Rudgers 2005;Lively et al. 2005). The performance of host and non-hostpopulations in the presence and the absence of natural ene-mies distinguishes the direct benefits of symbiosis (such asnutrient provisioning) from indirect benefits arising fromprotection from natural enemies. Defensive symbiosisrequires that the fitness of hosts is proportionally higherthan non-hosts in the presence of natural enemies relativeto enemy-free conditions. In the absence of enemies, thedefensive symbiont may decline in frequency or be lostfrom the host population (Janzen 1973; Lively et al. 2005;Palmer et al. 2008).Protective or defensive mutualisms have long been rec-ognized. Thomas Belt (1874), in The Naturalist in Nicara-gua, first described the defence of acacia trees fromherbivores by ants (see also Boucher, James & Keeler1982; Janzen 1985; Palmer et al. 2008). We now know thatextrafloral nectaries, Beltian bodies and other traits thatattract ants for defence against herbivores are widespreadin plants (Bentley 1977; Beattie 1985; Rudgers & Strauss2004; Rico-Grey & Oliveira 2007). Outside of ant–plantprotective interactions, which are based on physicalaggression, defensive mutualisms involving microbial sym-bionts often involve the production of toxic secondarymetabolites. One well-understood example is grassesinfected by endophytic fungi (family Clavicipitaceae) thatgrow systemically in above-ground plant tissues, are verti-cally transmitted through seeds and produce a variety ofalkaloid compounds deterrent to herbivores (Clay 1988;Clay & Schardl 2002). The endophytes are under strongselection for alkaloid diversification which may improvetheir protective function (Schardl et al. 2013). Defensivesymbioses between insects and bacteria are also wide-spread. For example, Currie and colleagues (Currie, Muel-ler & Malloch 1999a; Currie et al. 1999b, 2006) reportedthat leaf cutter ants (Atta spp.) harbour antibiotic-produc-ing actinobacteria (Pseudocardinia spp.) that inhibit Escov-opsis spp. pathogens of their fungal gardens and helpmaintain the mutualism between ants and their fungal gar-dens. Many sap-sucking insects like aphids also harbourdefensive symbionts which provide protection against arange of natural enemies (Oliver et al. 2003, 2008; Scarbor-ough, Ferrari & Godfray 2005; Oliver & Moran 2009;Tsuchida et al. 2010; Lukasik et al. 2013).Microbial interactions with plants and animals are typi-cally invisible to the naked eye, but their impacts on hostsand host communities can be very large. Higher organismshost diverse microbial communities (Arnold et al. 2000;Costello et al. 2009; Rodriguez et al. 2009; Hawlena et al.2013), and microbial dependency on the host will favourtraits that help protect that resource and ensure theirtransmission (Lukasik et al. 2013). Microbes have highrates of evolutionary change, and horizontal transfer ofadaptive genes or genomes is common (McCutcheon,McDonald & Moran 2009; Oliver et al. 2010; Werrenet al. 2010; Smillie et al. 2011; Zhu et al. 2013). Newapproaches and technological advances are providingnovel insights into plant and animal microbiomes, andmore demonstrated and hypothesized examples of defen-sive symbiosis (Turnbaugh et al. 2009; White & Torres2009; Zhu et al. 2011; Lundberg et al. 2012). Identifyingthe key microbial players and the underlying mechanismsof protection will improve our understanding of factorsaffecting the dynamics of ecological communities andprovide applications for agriculture and human health(Wicklow et al. 2005; Mazmanian, Round & Kasper 2008;Mao-Jones et al. 2010).

The role of birds in the acacia—ant interaction: New insights from nest predation

The association of the swollen-thorn acacia (Acacia cornigera) and ants is an example of defensive mutualism. It has been shown that some bird species prefer to nest in myrmecophyte acacias, suggesting that their nests are protected from predators by the associated ants. Based on previous knowledge regarding the existence of extended benefits for birds in the acacia-ant interaction, the role of the rufous-naped wren was analyzed within the ant-acacia system. Nest predation was assessed taking into account tree species with and without myrmecophytic interactions, tree cover and height, the presence and abundance of ants, predator type, and the distance between each nest and the closest myrmecophyte acacia. The probability of survivorship of artificial nests using a known fate analysis was calculated. The results showed non-significant differences in the probability of survivorship regardless of the presence and density of myrmecophyte acacias, tree cover and height, the presence and abundance of ants, or predator type. Interestingly, the highest degree of predation in the artificial nests was related to the rufous-naped wren. These results suggest that the acacia-ant-bird interaction is more complex than previously perceived.

Plant size and reproductive state affect the quantity and quality of rewards to animal mutualists

Summary Many plants engage ants in defensive mutualisms by offering extrafloral nectar (EFN). Identifying sources of variation in EFN quantity (amount) and quality (composition) is important because they can affect ant visitation and identity and hence effectiveness of plant defence. I investigated plant size and reproductive state (vegetative or flowering) as sources of variation in EFN quantity and quality. I focused on Opuntia imbricata and two ant partners, Crematogaster opuntiae and Liometopum apiculatum. I tested the influence of plant size and nectary type (vegetative vs. reproductive structure) on the probability and rate of EFN secretion, concentrations of total carbohydrates (CH) and amino acids (AAs), and relative abundances of constituent CH and AAs. I also examined how traits of individual nectaries scaled up to influence total plant‐level rewards. Parallel observations documented associations between plant demographic state and ant visitation and species identity. EFN quantity and quality were generally greater for larger, reproductive plants. At the scale of individual nectaries, probability of EFN secretion was positively size‐dependent and greater for nectaries on reproductive vs. vegetative structures. Rate of EFN secretion, carbohydrate and amino acid concentrations, and the relative abundance of disaccharide vs. monosaccharide sugars were greater for reproductive nectaries but were unaffected by plant size. Nectary‐level traits scaled up to influence rewards at the whole‐plant level in ways that corresponded to ant visitation: the probability of ant occupancy increased with plant size and reproduction, as did the likelihood of being tended by the superior guard, L. apiculatum. Variability in EFN traits may contribute to changes in ant occupancy and identity across plant sizes and reproductive states. Synthesis. This study provides a thorough examination of how plant investment in biotic defence varies over the life cycle. Explicit consideration of plant demography may enhance understanding of ant–plant mutualisms. Populations of long‐lived plants are demographically heterogeneous, spanning sizes and reproductive states. The rewards offered to animal mutualists can track demographic heterogeneity with consequences for plant defence and the dynamics of multispecies mutualisms.

Mutualistic ants as an indirect defence against leaf pathogens

Mutualistic ants are commonly considered as an efficient indirect defence against herbivores. Nevertheless, their indirect protective role against plant pathogens has been scarcely investigated. We compared the protective role against pathogens of two different ant partners, a mutualistic and a parasitic ant, on the host plant Acacia hindsii (Fabaceae). The epiphytic bacterial community on leaves was evaluated in the presence and absence of both ant partners by cultivation and by 454 pyrosequencing of the 16S rRNA gene. Pathogen-inflicted leaf damage, epiphytic bacterial abundance (colony-forming units) and number of operational taxonomic units (OTUs) were significantly higher in plants inhabited by parasitic ants than in plants inhabited by mutualistic ants. Unifrac unweighted and weighted principal component analyses showed that the bacterial community composition on leaves changed significantly when mutualistic ants were removed from plants or when plants were inhabited by parasitic ants. Direct mechanisms provided by ant-associated bacteria would contribute to the protective role against pathogens. The results suggest that the indirect defence of mutualistic ants also covers the protection from bacterial plant pathogens. Our findings highlight the importance of considering bacterial partners in ant-plant defensive mutualisms, which can contribute significantly to ant-mediated protection from plant pathogens.

Transcriptional responses in a Drosophila defensive symbiosis

Inherited symbionts are ubiquitous in insects and can have important consequences for the fitness of their hosts. Many inherited symbionts defend their hosts against parasites or other natural enemies; however, the means by which most symbionts confer protection is virtually unknown. We examine the mechanisms of defence in a recently discovered case of symbiont-mediated protection, where the bacterial symbiont Spiroplasma defends the fly Drosophila neotestacea from a virulent nematode parasite, Howardula aoronymphium. Using quantitative PCR of Spiroplasma infection intensities and whole transcriptome sequencing, we attempt to distinguish between the following modes of defence: symbiont-parasite competition, host immune priming and the production of toxic factors by Spiroplasma. Our findings do not support a model of exploitative competition between Howardula and Spiroplasma to mediate defence, nor do we find strong support for host immune priming during Spiroplasma infection. Interestingly, we recovered sequence for putative toxins encoded by Spiroplasma, including a novel putative ribosome-inactivating protein, transcripts of which are up-regulated in response to nematode exposure. Protection via the production of toxins may be a widely used and important mechanism in heritable defensive symbioses in insects.

Extended disease resistance emerging from the faecal nest of a subterranean termite

Social insects nesting in soil environments are in constant contact with entomopathogens but have evolved a range of defence mechanisms, resulting in both individual and social immunity that reduce the chance for epizootics in the colony, as in the case of subterranean termites. Coptotermes formosanus uses its faeces as building material for its nest structure that result into a ‘carton material’, and here, we report that the faecal nest supports the growth of Actinobacteria which provide another level of protection to the social group against entomopathogens. A Streptomyces species with in vivo antimicrobial activity against fungal entomopathogens was isolated from the nest material of multiple termite colonies. Termite groups were exposed to Metarhizium anisopliae, a fungal entomopathogen, during their foraging activity and the presence of Streptomyces within the nest structure provided a significant survival benefit to the termites. Therefore, this report describes a non-nutritional exosymbiosis in a termite, in the form of a defensive mutualism which has emerged from the use of faecal material in the nesting structure of Coptotermes. The association with an Actinobacteria community in the termite faecal material provides an extended disease resistance to the termite group as another level of defence, in addition to their individual and social immunity.

Hardship Strengthens Mutual Bonds

Hardship Strengthens Mutual Bonds

Partner manipulation stabilises a horizontally transmitted mutualism

Mutualisms require protection from non-reciprocating exploiters. Pseudomyrmex workers that engage in an obligate defensive mutualism with Acacia hosts feed exclusively on the sucrose-free extrafloral nectar (EFN) that is secreted by their hosts, a behaviour linking ant energy supply directly to host performance and thus favouring reciprocating behaviour. We tested the hypothesis that Acacia hosts manipulate this digestive specialisation of their ant mutualists. Invertase (sucrose hydrolytic) activity in the ant midguts was inhibited by chitinase, a dominant EFN protein. The inhibition occurred quickly in cell-free gut liquids and in native gels and thus likely results from an enzyme-enzyme interaction. Once a freshly eclosed worker ingests EFN as the first diet available, her invertase becomes inhibited and she, thus, continues feeding on host-derived EFN. Partner manipulation acts at the phenotypic level and means that one partner actively controls the phenotype of the other partner to enhance its dependency on host-derived rewards.

Defensive mutualisms: do microbial interactions within hosts drive the evolution of defensive traits?

Summary We examine theoretical and empirical results to determine the importance of microbe–microbe interactions in the evolution of defensive traits. Theoretical models show that the evolution of parasitism and the maintenance of mutualisms in multispecies interactions will depend on interactions with the host as well as with the defensive symbionts. At the community level, selection for defensive traits will vary greatly with ecological context and such spatial or temporal variation itself may stabilize defensive mutualisms. Studies of fungal endophytes within plant hosts demonstrate that endophytes acting as defensive mutualists may derive fitness benefits from the parasite as well as the host and suggest that interactions between co‐occurring symbionts within hosts may lead to the evolution of virulence.

Chemical defensive symbioses in the marine environment

Summary Marine organisms are a prolific source of natural products, many of which have become the target of pharmacological investigations. As well as being bioactive, these compounds can play an important role in the survival of the organisms and potentially affect community structure. Sessile invertebrates such as sponges, cnidarians, bryozoans and tunicates, in particular, are an especially rich source of ecologically relevant compounds. These organisms are typically soft‐bodied and unable to escape predators, and as such, they rely on chemical defence for their persistence. These invertebrates are also frequently hosts for microbial symbionts. Marine microbes are a prolific source of bioactive natural products, many of which can be allelopathic and prevent the growth of pathogens. For sessile marine invertebrates, which often have both bioactive natural products and microbial symbionts, it is logical to hypothesize that microbial symbionts may produce the secondary metabolites. While symbionts are often thought to be responsible for producing bioactive natural products that play a role in host defence, relatively few studies have experimentally demonstrated that symbiont‐produced compounds defend the hosts. One reason for this is the difficulty of manipulating the host–symbiont relationship, which is often obligate for one or both partners. Given the importance of natural products to marine invertebrates for defence, the prevalence of symbiosis in the marine environment and the diverse metabolic capabilities of micro‐organisms, symbiont‐mediated host chemical defence may be more prevalent than currently understood. In this review, I document evidence supporting chemical defensive symbioses in marine organisms, discuss commonalities and differences among the diverse relationships, and provide future research directions. Epibiont‐produced defensive compounds would seem to result in the most efficient manner of protection, as the compounds are most accessible to the predators. By contrast, endosymbiont‐produced compounds may need to be transported to exposed tissues. Elucidating mechanisms to transport symbiont‐produced defensive compounds within the host will provide greater insight into the breadth and complexity of host–symbiont interactions. Another important unexplored issue is how the hosts are able to tolerate symbiont‐produced metabolites that are often eucaryotic cell effectors. A greater understanding of defensive symbiosis will result from detailed studies of the co‐evolution of predator, host and symbiont, and if or how predators can influence the production of defensive metabolites.

Defensive Bacteriome Symbiont with a Drastically Reduced Genome

Diverse insect species harbor symbiotic bacteria, which playimportant roles such as provisioning nutrients and providing defense against natural enemies [1-6]. Whereas nutritional symbioses are often indispensable for both partners, defensive symbioses tend to be of a facultative nature [1-12]. The Asian citrus psyllid Diaphorina citri is a notorious agricultural pest that transmits Liberibacter spp.(Alphaproteobacteria), causing the devastating citrus greening disease or Huanglongbing [13, 14]. In a symbiotic organ called the bacteriome, D.citri harbors two distinct intracellular symbionts: a putative nutrition provider, Carsonella_DC (Gammaproteobacteria), and an unnamed betaproteobacterium with unknown function [15], for which we propose the name "Candidatus Profftella armatura." Here we report that Profftella is a defensive symbiont presumably of an obligate nature with an extremely streamlined genome. The genomes of Profftella and Carsonella_DC were drastically reduced to 464,857bp and 174,014bp, respectively, suggesting their ancient and mutually indispensible association with the host. Strikingly, 15% of the small Profftella genome encoded horizontally acquired genes forsynthesizing a novel polyketide toxin. The toxin was extracted, pharmacologically and structurally characterized, and designated diaphorin. The presence of Profftella and its diaphorin-biosynthetic genes was perfectly conserved in the world's D.citri populations.

Is the pathogenic ergot fungus a conditional defensive mutualist for its host grass?

It is well recognized, that outcomes of mutualistic plant-microorganism interactions are often context dependent and can range from mutualistic to antagonistic depending on conditions. Instead, seemingly pathogenic associations are generally considered only harmful to plants. The ergot fungus (Claviceps purpurea) is a common seed pathogen of grasses and cereals. Ergot sclerotia contain alkaloids which can cause severe toxicity in mammals when ingested, and thus the fungal infection might provide protection for the host plant against mammalian herbivores. Theoretically, the net effect of ergot infection would positively affect host seed set if the cost is not too high and the defensive effect is strong enough. According to our empirical data, this situation is plausible. First, we found no statistically significant seed loss in wild red fescue (Festuca rubra) inflorescences due to ergot infection, but the seed succession decreased along increasing number of sclerotia. Second, in a food choice experiment, sheep showed avoidance against forage containing ergot. Third, the frequency of ergot-infected inflorescences was higher in sheep pastures than surrounding ungrazed areas, indicating a protective effect against mammalian grazing. We conclude that, although ergot can primarily be categorized as a plant pathogen, ergot infection may sometimes represent indirect beneficial effects for the host plant. Ergot may thus serve as a conditional defensive mutualist for its host grass, and the pathogenic interaction may range from antagonistic to mutualistic depending on the situation.

Defensive symbiosis in the real world – advancing ecological studies of heritable, protective bacteria in aphids and beyond

Summary Symbiotic microbes have become increasingly recognized to mediate interactions between natural enemies and their hosts. The ecologies of these symbioses, however, are poorly understood in many systems, and a predictive framework is needed to guide future studies. To achieve this, we focus on heritable, defensive microbes of insects. Our review of laboratory‐based studies identifies diverse bacterial species that have independently evolved to protect a range of insects against parasitoids, parasites, predators and pathogens. Although defensive mechanisms are typically unknown, some involve toxins or the upregulation of host immunity. Despite substantial benefits of infection in the presence of natural enemies, the protective symbionts of insects are often found at intermediate levels in natural populations. Using a host‐centred population genetics approach made possible by the host restriction and cytoplasmic inheritance of these microbes, we propose that balancing selection plays a major role in symbiont maintenance, with protective benefits in the presence of enemies and infection costs in their absence. Other mediating factors are likely to be important, including temperature, superinfections and transmission dynamics. While few studies have provided evidence for defence in the field, several studies have shown symbiont infection frequencies to be dynamic, varying across temporal and spatial gradients and food–plant associations. Newly presented data from our pea aphid research reveal that temporal shifts in defensive symbiont prevalence can be quite rapid, with Hamiltonella defensa showing 10–20% shifts around a seasonal average of c. 50%. Such findings contrast with more unidirectional changes seen in laboratory population cages, suggesting temporal changes in the costs and benefits of symbionts in the field. To frame future research on defensive symbiont ecology, we briefly consider a range of studies needed to test laboratory‐ and field‐derived predictions on defensive symbiosis. Included are investigations of defensive mechanisms, symbiont‐driven co‐evolution and community‐level effects. We also consider the need for more thorough and highly resolved molecular diagnostics of natural infections, laboratory studies on functional differences between symbiont strains and species and studies on the relative costs and benefits of defenders in nature. The emerging theme of symbiont‐mediated defence across eukaryotes suggests that knowledge of the functional mechanisms behind protection and natural symbiont dynamics could be key to understanding many of the world's antagonistic species interactions. Thus, the development of insects as a model for such studies holds promise for these organisms and beyond.

Chemical Ecology Mediated by Fungal Endophytes in Grasses

Defensive mutualism is widely accepted as providing the best framework for understanding how seed-transmitted, alkaloid producing fungal endophytes of grasses are maintained in many host populations. Here, we first briefly review current knowledge of bioactive alkaloids produced by systemic grass-endophytes. New findings suggest that chemotypic diversity of the endophyte-grass symbiotum is far more complex, involving multifaceted signaling and chemical cross-talk between endophyte and host cells (e.g., reactive oxygen species and antioxidants) or between plants, herbivores, and their natural enemies (e.g., volatile organic compounds, and salicylic acid and jasmonic acid pathways). Accumulating evidence also suggests that the tight relationship between the systemic endophyte and the host grass can lead to the loss of grass traits when the lost functions, such as plant defense to herbivores, are compensated for by an interactive endophytic fungal partner. Furthermore, chemotypic diversity of a symbiotum appears to depend on the endophyte and the host plant life histories, as well as on fungal and plant genotypes, abiotic and biotic environmental conditions, and their interactions. Thus, joint approaches of (bio)chemists, molecular biologists, plant physiologists, evolutionary biologists, and ecologists are urgently needed to fully understand the endophyte-grass symbiosis, its coevolutionary history, and ecological importance. We propose that endophyte-grass symbiosis provides an excellent model to study microbially mediated multirophic interactions from molecular mechanisms to ecology.

Antifungal Agents from Pseudallescheria boydii SNB-CN73 Isolated from a Nasutitermes sp. Termite

Defense mutualisms between social insects and microorganisms have been described in the literature. The present article describes the discovery of a Pseudallescheria boydii strain isolated from Nasutitermes sp. The microbial symbiont produces two antifungal metabolites: tyroscherin and N-methyltyroscherin, a compound not previously described in the literature. Methylation of tyroscherin has confirmed the structure of N-methyltyroscherin. Both compounds are effective antifungal agents with favorable selectivity indices for Candida albicans and Trichophyton rubrum.