Parasitic Fungi and their Interactions with the Insect Immune System

Fungal infection alters collective nutritional intake of ant colonies

In animals, parasitic infections impose significant fitness costs.1,2,3,4,5,6 Infected animals can alter their feeding behavior to resist infection,7,8,9,10,11,12 but parasites can manipulate animal foraging behavior to their own benefits.13,14,15,16 How nutrition influences host-parasite interactions is not well understood, as studies have mainly focused on the host and less on the parasite.9,12,17,18,19,20,21,22,23 We used the nutritional geometry framework24 to investigate the role of amino acids (AA) and carbohydrates (C) in a host-parasite system: the Argentine ant, Linepithema humile, and the entomopathogenic fungus, Metarhizium brunneum. First, using 18 diets varying in AA:C composition, we established that the fungus performed best on the high-amino-acid diet 1:4. Second, we found that the fungus reached this optimal diet when given various diet pairings, revealing its ability to cope with nutritional challenges. Third, we showed that the optimal fungal diet reduced the lifespan of healthy ants when compared with a high-carbohydrate diet but had no effect on infected ants. Fourth, we revealed that infected ant colonies, given a choice between the optimal fungal diet and a high-carbohydrate diet, chose the optimal fungal diet, whereas healthy colonies avoided it. Lastly, by disentangling fungal infection from host immune response, we demonstrated that infected ants foraged on the optimal fungal diet in response to immune activation and not as a result of parasite manipulation. Therefore, we revealed that infected ant colonies chose a diet that is costly for survival in the long term but beneficial in the short term-a form of collective self-medication.

Adverse effects of the fungal biopesticide Beauveria bassiana on a predatory social wasp

Biopesticides are considered eco-friendly alternatives to synthetic agrochemicals. However, their impact on non-target organisms is still poorly understood. Social wasps, in particular, are a largely neglected group when it comes to risk assessment of plant protection products, despite the relevant ecological and economic services provided by these insects. In the present study, we evaluated the impact of a common biopesticide, the entomopathogenic fungus Beauveria bassiana, on the paper wasp Polistes dominula. We adopted a holistic approach in ecotoxicology by focusing not only on the detrimental effects on isolated individuals, but also on the whole colony. Both adult wasps belonging to different castes and immature larvae were topically exposed to a field-realistic concentration of fungal spores from the commercial strain of B. bassiana ATCC 74040 to assess the impact of the biopesticide on their survival, behavior and physiology. Our results showed that the fungus causes a number of adverse effects on P. dominula, that include increased mortality, altered locomotion and feeding rate, selective ejection of exposed larvae from nests, reduced oviposition rate and ovary development in foundresses, and colony failure. Our findings provide new insights on the often-neglected sublethal effects of pollutants that can jeopardize not only individual beneficial insects, but also the delicate social balance of their colonies and their valuable ecosystem services, highlighting that the natural origin of plant-protection products does not always guarantee environmental safety.

Cuticle melanization and the expression of immune-related genes in the honeybee Apis mellifera (Hymenoptera: Apidae) adult workers

The global decline of bee populations has several factors, including pathogens, which need overcome the insect defenses such as the physical barriers, the body cuticle and peritrophic matrix (primary defenses), as well as the secondary defenses with antimicrobial peptides (AMPs) and the enzyme lysozyme. The regulation of immune defenses according to the infection risks raises questions about the immunity of social bees due to their exposition to different pathogens pressures during the adult lifespan and tasks performed. This study evaluated the primary (body cuticle melanization, peritrophic matrix and cpr14 expression) and secondary (AMPs and lysozyme expression) defenses of the honeybee Apis mellifera workers according to the age and tasks. The expression of malvolio was used to detect precocious forage tasks outside the colony. Forager workers have higher amount of cuticular melanization in the body cuticle than nurse, but not when the age effect is retired, indicating the gradual acquisition of this compound in the integument of adult bees. The relative value of chitin in the peritrophic matrix and cpr14 mRNA are similar in all bees evaluated, suggesting that these components of primary defenses do not change according to the task and age. Differential expression of genes for AMPs in workers performing different tasks, within the same age group, indicates that the behavior stimulates expression of genes related to secondary immune defense. The expression of malvolio gene, accelerating the change in workers behavior, and those related to immune defense suggest the investment in secondary defense mechanisms when the primary defense of the body cuticle is not yet completed.

In vitro screening of 65 mycotoxins for insecticidal potential

The economic losses and threats to human and animal health caused by insects and the pathogens transmitted by them require effective and environmentally-friendly methods of controlling them. One such group of natural biocontrol agents which may be used as biopesticides is that of the entomopathogenic fungi and their toxic secondary metabolites (mycotoxins). The present in vitro work examined the insecticidal potential of 65 commercially-available mycotoxins against the insect Sf-9 cell line. Mammalian Caco-2 and THP-1 cell lines served as reference controls to select insecticidal mycotoxins harmless to mammalian cells. All tested mycotoxins significantly reduced the in vitro proliferation of the Sf-9 cells and evoked morphological changes. Ten of the mycotoxins found to strongly inhibit Sf-9 proliferation also had moderate or no effect on Caco-2 cells. The THP-1 cells were highly resistant to the tested mycotoxins: doses 10³ times higher were needed to affect viability and morphology (1 μg/ml for THP-1 versus 1 ng/ml for Sf-9 and Caco-2). Nine mycotoxins significantly decreased Sf-9 cell proliferation with minor effects on mammalian cells: cyclosporins B and D, cytochalasin E, gliotoxin, HC toxin, paxilline, penitrem A, stachybotrylactam and verruculogen. These may be good candidates for future biopesticide formulations.

The interaction between cuticle free fatty acids (FFAs) of the cockroaches Blattella germanica and Blatta orientalis and hydrolases produced by the entomopathogenic fungus Conidiobolus coronatus

The interactions between entomopathogenic fungi and insects serve a classic example of a co-evolutionary arms race between pathogens and their target host. The cuticle, site of the first contact between insects and entomopathogenic fungus, is an important defensive barrier against pathogens. It is covered by a layer of lipids that appears to play a key role in these processes and cuticular free fatty acid (FFA) profiles are consider as a determinant of susceptibility, or resistance, to fungal infections. These profiles are species-specific. The cockroaches Blattella germanica (Blattodea: Blattidae) and Blatta orientalis (Blattodea: Ectobiidae) are unsusceptible to the soil fungus Conidiobolus coronatus (Entomophthorales: Ancylistaceae) infection, therefore we studied the profiles of FFAs in order to understand the defensive capabilities of the cockroaches. The fungus was cultivated for three weeks in minimal medium. Cell-free filtrate was obtained, assayed for elastase, N-acetylglucosaminidase, chitobiosidase and lipase activity, and then used for in vitro hydrolysis of the cuticle from wings and thoraces of adults and oothecae. The amounts of amino acids, N-glucosamine and FFAs released from the hydrolysed cuticle samples were measured after eight hours of incubation. The FFA profiles of the cuticle of adults, and the wings, thoraces and oothecae of both species were established using GC-MS and the results were correlated with the effectiveness of fungal proteases, chitinases and lipases in the hydrolyzation of cuticle samples. Positive correlations would suggest the existence of compounds used by the fungus as nutrients, whereas negative correlations may indicate that these compounds could be engaged in insect defence.

External Immune Inhibitory Efficiency of External Secretions and Their Metabolic Profiling in Red Palm Weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

External secretions play a vital role in external immune defense. However, the functions and components of these exudates are largely unknown in the red palm weevil, Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae). In order to determine their role in external immunity, the immunosuppressive efficacy of the secretions in vitro against microbes, including bacteria and fungi, was clarified. In the present study, we found that these secretions had antimicrobial activity in vitro, implying external immunizing potency against pathogens. Surprisingly, all liquid phases of secretions could not significantly inhibit the growth of microbes in vitro compared to solid phases. To explain this phenomenon, the composition and emission differentia of secretions from the exocrine glands associated with different developmental stages, secretory regions, and phases were identified and analyzed based on metabonomics techniques. A total of more than 200 compounds, including quinines, phenols, aldehydes, acids, alcohols, saccharides, ketones, esters, amines, salts, ureas, and heterocycles, were identified in the secretions of larvae and adults. The liquid phase shared a number of metabolites with the solid phase, but the emission types and amounts were significantly different in the two phases, resulting in differences in external immunological activity. Tyrosine and p-benzoquinone were the dominant metabolites in all of the secretions, accounting for approximately 11.29% of emissions, with the portion in the solid phase being generally higher than that in the liquid phase. Moreover, only p-benzoquinone was entirely significantly upregulated in the solid phase compared to the liquid phase. Therefore, metabolome analysis suggested that p-benzoquinone, which may potentially be developed to be a valuable marker for determining external immunity, was considered to be the main substance responsible for external immune functions. This hypothesis was further demonstrated by the antimicrobial activity of p-benzoquinone.

Theoretical analysis of spatial nonhomogeneous patterns of entomopathogenic fungi growth on insect pest

This paper presents the study of the dynamics of intrahost (insect pests)-pathogen [entomopathogenic fungi (EPF)] interactions. The interaction between the resources from the insect pest and the mycelia of EPF is represented by the Holling and Powell type II functional responses. Because the EPF's growth is related to the instability of the steady state solution of our system, particular attention is given to the stability analysis of this steady state. Initially, the stability of the steady state is investigated without taking into account diffusion and by considering the behavior of the system around its equilibrium states. In addition, considering small perturbation of the stable singular point due to nonlinear diffusion, the conditions for Turing instability occurrence are deduced. It is observed that the absence of the regeneration feature of insect resources prevents the occurrence of such phenomena. The long time evolution of our system enables us to observe both spot and stripe patterns. Moreover, when the diffusion of mycelia is slightly modulated by a weak periodic perturbation, the Floquet theory and numerical simulations allow us to derive the conditions in which diffusion driven instabilities can occur. The relevance of the obtained results is further discussed in the perspective of biological insect pest control.

Natural biocide disrupts nestmate recognition in honeybees

Honeybee colonies are under the threat of many stressors, biotic and abiotic factors that strongly affect their survival. Recently, great attention has been directed at chemical pesticides, including their effects at sub-lethal doses on bee behaviour and colony success; whereas the potential side effects of natural biocides largely used in agriculture, such as entomopathogenic fungi, have received only marginal attention. Here, we report the impact of the fungus Beauveria bassiana on honeybee nestmate recognition ability, a crucial feature at the basis of colony integrity. We performed both behavioural assays by recording bee guards’ response towards foragers (nestmate or non-nestmate) either exposed to B. bassiana or unexposed presented at the hive entrance, and GC-MS analyses of the cuticular hydrocarbons (CHCs) of fungus-exposed versus unexposed bees. Our results demonstrated that exposed bees have altered cuticular hydrocarbons and are more easily accepted into foreign colonies than controls. Since CHCs are the main recognition cues in social insects, changes in their composition appear to affect nestmate recognition ability at the colony level. The acceptance of chemically unrecognizable fungus-exposed foragers could therefore favour forager drift and disease spread across colonies.

Epigenetic mechanisms mediate the experimental evolution of resistance against parasitic fungi in the greater wax moth Galleria mellonella

Recent concepts in evolutionary biology suggest that epigenetic mechanisms can translate environmental selection pressures into heritable changes in phenotype. To determine whether experimental selection for a complex trait in insects involves epigenetic modifications, we carried out a generation-spanning experiment using larvae of the greater wax moth Galleria mellonella as a model host to investigate the role of epigenetics in the heritability of resistance against the parasitic fungus Metarhizium robertsii. We investigated differences in DNA methylation, histone acetylation and microRNA (miRNA) expression between an experimentally resistant population and an unselected, susceptible line, revealing that the survival of G. mellonella larvae infected with M. robertsii correlates with tissue-specific changes in DNA methylation and histone modification and the modulation of genes encoding the corresponding enzymes. We also identified miRNAs differentially expressed between resistant and susceptible larvae and showed that these regulatory molecules target genes encoding proteinases and proteinase inhibitors, as well as genes related to cuticle composition, innate immunity and metabolism. These results support our hypothesis that epigenetic mechanisms facilitate, at least in part, the heritable manifestation of parasite resistance in insects. The reciprocal adaptations underlying host–parasite coevolution therefore extend beyond the genetic level to encompass epigenetic modifications.

Identification of a new Cordyceps javanica fungus isolate and its toxicity evaluation against Asian citrus psyllid

The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most serious pest of citrus worldwide. It acts as a vector for a group of phloem‐limited bacteria (Candidatus Liberibacter spp.) that causes Huanglongbing (HLB) disease. Thus, D. citri management is an important strategy against HLB, and biological control is currently considered as the most effective method because of the unsustainable and negative side effects of chemical control. Here, we isolated a new strain of entomopathogenic fungus, Cordyceps javanica (GZQ‐1), from one cadaver of D. citri adult based on its morphological and phylogenetic data. Five conidial concentrations of the C. javanica pathogen (1 × 10³, 1 × 10⁴, 1 × 10⁵, 1 × 10⁶, and 1 × 10⁷ conidia/ml) were assessed against six life stages of D. citri (1st‐5th instar nymphs and adults). Results showed that C. javanica GZQ‐1 was highly pathogenic to D. citri nymphs (69.49%–90.87% mortality) and adults (69.98% mortality). The LC₅₀ and LT₅₀ values of C. javanica against 1st‐2nd instar (younger), 3rd‐4th instar (middle aged), 5th instar (older), and adults were 1.20 × 10⁵, 1.10 × 10⁶, 4.47 × 10⁶, 8.12 × 10⁶ conidia/ml and 4.25, 4.51, 5.17, 5.49 days, respectively. Moreover, glasshouse experiments indicated that this C. javanica GZQ‐1 caused higher infection rates of D. citri adults compared to two other fungal strains we previously isolated in the laboratory, Cordyceps fumosorosea (IF010) and Metarhizium anisopliae (CNGD7).

The entomopathogenic fungus Metarhizium robertsii communicates with the insect host Galleria mellonella during infection

Parasitic fungi are the only pathogens that can infect insect hosts directly through their proteinaceous exoskeleton. Penetration of the cuticle requires the release of fungal enzymes, including proteinases, which act as virulence factors. Insects can sense fungal infections and activate innate immune responses, including the synthesis of antifungal peptides and proteinase inhibitors that neutralize the incoming proteinases. This well-studied host response is epigenetically regulated by histone acetylation/deacetylation. Here we show that entomopathogenic fungi can in turn sense the presence of insect-derived antifungal peptides and proteinase inhibitors, and respond by inducing the synthesis of chymotrypsin-like proteinases and metalloproteinases that degrade the host-derived defense molecules. The rapidity of this response is dependent on the virulence of the fungal strain. We confirmed the specificity of the pathogen response to host-derived defense molecules by LC/MS and RT-PCR analysis, and correlated this process with the epigenetic regulation of histone acetylation/deacetylation. This cascade of responses reveals that the coevolution of pathogens and hosts can involve a complex series of attacks and counterattacks based on communication between the invading fungal pathogen and its insect host. The resolution of this process determines whether or not pathogenesis is successful.

Non-host larvae negatively impact persistence of the entomopathogen Beauveria bassiana in soil

A better understanding of the ecology of the insect pathogenic fungus, Beauveria bassiana, in soil is needed to identify reasons behind the variable efficacy often seen after field application. A transformed strain of a candidate commercial strain of B. bassiana (F418 gfp tr3), expressing the green fluorescent protein and the hygromycin B resistance gene, was used to assess the effects of the larvae of a host insect, Tenebrio molitor L. (Coleoptera: Tenebrionidae), a non-host, Costelytra zealandica (Coleoptera: Scarabaeidae) and the absence of larvae on the persistence of F418 gfp tr3 in pasteurised and non-sterile soil over 4 months. In the presence of a T. molitor larvae, F418 gfp tr3 populations increased significantly in pasteurised and non-sterile soil; however, populations increased less in non-sterile soil than in pasteurised soil. Lower populations of F418 gfp tr3 were recovered in pasteurised soil in the presence of C. zealandica larvae than in pasteurised soil without larvae. No difference was observed between F418 gfp tr3 populations in non-sterile soil with a non-host larvae or without larvae. Accompanying studies showed that F418 gfp tr3 conidia germinated and produced appressoria on live and excised cuticle of non-host (C. zealandica) larvae but infection did not occur, leading to a net loss of viable conidia in the soil. Conidia administrated orally to C. zealandica larvae were viable on recovery from faecal samples, suggesting that ingestion of the fungus by the larvae had little impact on the viable fungal population. Soil bacterial and fungal community patterns were analysed using Single-Strand Conformation Polymorphism (SSCP) and showed a correlation between changes in F418 gfp tr3 persistence in pasteurised and non-sterile soil and changes in soil communities in the presence of a host insect, non-host insect or in the absence of insect. In pasteurised soil, non-specific germination of F418 gfp tr3 conidia on the non-host larval cuticle and the presence of antagonistic bacteria introduced with the field-collected larvae are most likely responsible for the differences observed. The more complex microbial community structures in non-sterile soil could lead to fungistasis, preventing potentially antagonistic bacteria degrading conidia or inhibiting attachment and germination on the non-host larval cuticle, resulting in the observed lack of difference between non-host and no larval treatments.

Recurrent symbiont recruitment from fungal parasites in cicadas

Cicadas are dependent on the essential bacterial symbionts Sulcia and Hodgkinia. The symbiont genomes are extremely streamlined for provisioning of essential amino acids and other nutrients. In some cicada lineages, Hodgkinia genomes are fragmented into numerous minicircles, which may represent a critical stage of genomic erosion close to collapse. What would happen subsequently? Our survey of the Japanese cicada diversity revealed that while Sulcia is conserved among all species, the majority of them have lost Hodgkinia and instead harbor yeast-like fungal associates. The fungal symbionts are phylogenetically intermingled with cicada-parasitizing Ophiocordyceps fungi, indicating recurrent symbiont replacements by entomopathogens in cicadas and providing insights into the mechanisms underlying the parasitism-symbiosis evolutionary continuum, compensation of symbiont genome erosion, and diversification of host-symbiont associations.

Diverse insects are associated with ancient bacterial symbionts, whose genomes have often suffered drastic reduction and degeneration. In extreme cases, such symbiont genomes seem almost unable to sustain the basic cellular functioning, which comprises an open question in the evolution of symbiosis. Here, we report an insect group wherein an ancient symbiont lineage suffering massive genome erosion has experienced recurrent extinction and replacement by host-associated pathogenic microbes. Cicadas are associated with the ancient bacterial co-obligate symbionts Sulcia and Hodgkinia, whose streamlined genomes are specialized for synthesizing essential amino acids, thereby enabling the host to live on plant sap. However, our inspection of 24 Japanese cicada species revealed that while all species possessed Sulcia, only nine species retained Hodgkinia, and their genomes exhibited substantial structural instability. The remaining 15 species lacked Hodgkinia and instead harbored yeast-like fungal symbionts. Detailed phylogenetic analyses uncovered repeated Hodgkinia-fungus and fungus-fungus replacements in cicadas. The fungal symbionts were phylogenetically intermingled with cicada-parasitizing Ophiocordyceps fungi, identifying entomopathogenic origins of the fungal symbionts. Most fungal symbionts of cicadas were uncultivable, but the fungal symbiont of Meimuna opalifera was cultivable, possibly because it is at an early stage of fungal symbiont replacement. Genome sequencing of the fungal symbiont revealed its metabolic versatility, presumably capable of synthesizing almost all amino acids, vitamins, and other metabolites, which is more than sufficient to compensate for the Hodgkinia loss. These findings highlight a straightforward ecological and evolutionary connection between parasitism and symbiosis, which may provide an evolutionary trajectory to renovate deteriorated ancient symbiosis via pathogen domestication.

Reprogramming the virulence: Insect defense molecules navigating the epigenetic landscape of Metarhizium robertsii

Metarhizium species are the leading bio-control agents well characterized regarding pathogenicity to agricultural, forest, public health, stored grains and urban insect pests. They infect the target host through the tight conidial adherence with the insect cuticle. Conidial binding to the insect cuticle drive the systematic integrated disease development events in target host to impart pathogenesis. However, there is growing evidence that virulence of the pathogen is directly related with proteolytic enzymes including metalloproteinases, chymotrypsin-like proteinases and subtilisin-like proteinases. Successful host pathogenesis is the selection of right set of virulence-related proteinases, which evolved as a result of host-pathogen coevolution.

Cuticular fatty acids of Galleria mellonella (Lepidoptera) inhibit fungal enzymatic activities of pathogenic Conidiobolus coronatus

The entomopathogenic fungus Conidiobolus coronatus produces enzymes that may hydrolyze the cuticle of Galleria mellonella. Of these enzymes, elastase activity was the highest: this figure being 24 times higher than NAGase activity 553 times higher than chitinase activity and 1844 times higher than lipase activity. The present work examines the differences in the hydrolysis of cuticles taken from larvae, pupae and adults (thorax and wings), by C. coronatus enzymes. The cuticles of the larvae and adult thorax were the most susceptible to digestion by proteases and lipases. Moreover, the maximum concentration of free N-glucosamine was in the hydrolysis of G. mellonella thorax. These differences in the digestion of the various types of cuticle may result from differences in their composition. GC-MS analysis of the cuticular fatty acids isolated from pupae of G. mellonella confirmed the presence of C 8:0, C 9:0, C 12:0, C 14:0, C 15:0, C 16:1, C 16:0, C 17:0, C 18:1, C 18:0, with C 16:0 and C 18:0 being present in the highest concentrations. Additional fatty acids were found in extracts from G. mellonella imagines: C 10:0, C 13:0, C 20:0 and C 20:1, with a considerable dominance of C 16:0 and C 18:1. In larvae, C 16:0 and C 18:1 predominated. Statistically significant differences in concentration (p≤0.05) were found between the larvae, pupae and imago for each fatty acid. The qualitative and quantitative differences in the fatty acid composition of G. mellonella cuticle occurring throughout normal development might be responsible for the varied efficiency of fungal enzymes in degrading larval, pupal and adult cuticles.

Ant-infecting Ophiocordyceps genomes reveal a high diversity of potential behavioral manipulation genes and a possible major role for enterotoxins

Much can be gained from revealing the mechanisms fungal entomopathogens employ. Especially intriguing are fungal parasites that manipulate insect behavior because, presumably, they secrete a wealth of bioactive compounds. To gain more insight into their strategies, we compared the genomes of five ant-infecting Ophiocordyceps species from three species complexes. These species were collected across three continents, from five different ant species in which they induce different levels of manipulation. A considerable number of (small) secreted and pathogenicity-related proteins were only found in these ant-manipulating Ophiocordyceps species, and not in other ascomycetes. However, few of those proteins were conserved among them, suggesting that several different methods of behavior modification have evolved. This is further supported by a relatively fast evolution of previously reported candidate manipulation genes associated with biting behavior. Moreover, secondary metabolite clusters, activated during biting behavior, appeared conserved within a species complex, but not beyond. The independent co-evolution between these manipulating parasites and their respective hosts might thus have led to rather diverse strategies to alter behavior. Our data indicate that specialized, secreted enterotoxins may play a major role in one of these strategies.

The effect of entomopathogenic fungal culture filtrate on the immune response of the greater wax moth, Galleria mellonella

Galleria mellonella is a well-established model species regularly employed in the study of the insect immune response at cellular and humoral levels to investigate fungal pathogenesis and biocontrol agents. A cellular and proteomic analysis of the effect of culture filtrate of three entomopathogenic fungi (EPF) species on the immune system of G. mellonella was performed. Treatment with Beauveria caledonica and Metarhizium anisopliae 96h culture filtrate facilitated a significantly increased yeast cell density in larvae (3-fold and 3.8-fold, respectively). Larvae co-injected with either M. anisopliae or B. caledonica culture filtrate and Candida albicans showed significantly increased mortality. The same was not seen for larvae injected with Beauveria bassiana filtrate. Together these results suggest that B. caledonica and M. anisopliae filtrate are modulating the insect immune system allowing a subsequent pathogen to proliferate. B. caledonica and M. anisopliae culture filtrates impact upon the larval prophenoloxidase (ProPO) cascade (e.g. ProPO activating factor 3 and proPO activating enzyme 3 were increased in abundance relative to controls), while B. bassiana treated larvae displayed higher abundances of alpha-esterase when compared to control larvae (2.4-fold greater) and larvae treated with M. anisopliae and B. caledonica. Treatment with EPF culture filtrate had a significant effect on antimicrobial peptide abundances particularly in M. anisopliae treated larvae where cecropin-D precursor, hemolin and gloverin were differentially abundant in comparison to controls. Differences in proteomic profiles for different treatments may reflect or even partially explain the differences in their immunomodulatory potential. Screening EPF for their ability to modulate the insect immune response represents a means of assessing EPF for use as biocontrol agents, particularly if the goal is to use them in combination with other control agents. Additionally EPF represent a valuable resource pool in our search for natural products with insect immunomodulatory and biocontrol properties.

Insect pathogenic fungus interacts with the gut microbiota to accelerate mosquito mortality

The insect gut microbiota plays crucial roles in modulating the interactions between the host and intestinal pathogens. Unlike viruses, bacteria, and parasites, which need to be ingested to cause disease, entomopathogenic fungi infect insects through the cuticle and proliferate in the hemolymph. However, interactions between the gut microbiota and entomopathogenic fungi are unknown. Here we show that the pathogenic fungus Beauveria bassiana interacts with the gut microbiota to accelerate mosquito death. After topical fungal infection, mosquitoes with gut microbiota die significantly faster than mosquitoes without microbiota. Furthermore, fungal infection causes dysbiosis of mosquito gut microbiota with a significant increase in gut bacterial load and a significant decrease in bacterial diversity. In particular, the opportunistic pathogenic bacterium Serratia marcescens overgrows in the midgut and translocates to the hemocoel, which promotes fungal killing of mosquitoes. We further reveal that fungal infection down-regulates antimicrobial peptide and dual oxidase expression in the midgut. Duox down-regulation in the midgut is mediated by secretion of the toxin oosporein from B. bassiana Our findings reveal the important contribution of the gut microbiota in B. bassiana-killing activity, providing new insights into the mechanisms of fungal pathogenesis in insects.

Life-history strategy determines constraints on immune function

Determining the factors governing investment in immunity is critical to understanding host-pathogen ecological and evolutionary dynamics. Studies often consider disease resistance in the context of life-history theory, with the expectation that investment in immunity will be optimized in anticipation of disease risk. Immunity, however, is constrained by context-dependent fitness costs. How the costs of immunity vary across life-history strategies has yet to be considered. Pea aphids are typically unwinged but produce winged offspring in response to high population densities and deteriorating conditions. This is an example of polyphenism, a strategy used by many organisms to adjust to environmental cues. The goal of this study was to examine the relationship between the fitness costs of immunity, pathogen resistance and the strength of an immune response across aphid morphs that differ in life-history strategy but are genetically identical. We measured fecundity of winged and unwinged aphids challenged with a heat-inactivated fungal pathogen, and found that immune costs are limited to winged aphids. We hypothesized that these costs reflect stronger investment in immunity in anticipation of higher disease risk, and that winged aphids would be more resistant due to a stronger immune response. However, producing wings is energetically expensive. This guided an alternative hypothesis - that investing resources into wings could lead to a reduced capacity to resist infection. We measured survival and pathogen load after live fungal infection, and we characterized the aphid immune response to fungi by measuring immune cell concentration and gene expression. We found that winged aphids are less resistant and mount a weaker immune response than unwinged aphids, demonstrating that winged aphids pay higher costs for a less effective immune response. Our results show that polyphenism is an understudied factor influencing the expression of immune costs. More generally, our work shows that in addition to disease resistance, the costs of immunity vary between individuals with different life-history strategies. We discuss the implications of these findings for understanding how organisms invest optimally in immunity in the light of context-dependent constraints.

Coronatin-2 from the entomopathogenic fungus Conidiobolus coronatus kills Galleria mellonella larvae and incapacitates hemocytes

Coronatin-2, a 14.5 kDa protein, was isolated from culture filtrates of the entomopathogenic fungus Conidiobolus coronatus (Costantin) Batko (Entomophthoramycota: Entomophthorales). After LC-MS/MS (liquid chromatography tandem mass spectrometry) analysis of the tryptic peptide digest of coronatin-2 and a mass spectra database search no orthologs of this protein could be found in fungi. The highest homology was observed to the partial translation elongation factor 1a from Sphaerosporium equinum (protein sequence coverage, 21%), with only one peptide sequence, suggesting that coronatin-2 is a novel fungal protein that has not yet been described. In contrast to coronatin-1, an insecticidal 36 kDa protein, which shows both elastolytic and chitinolytic activity, coronatin-2 showed no enzymatic activity. Addition of coronatin-2 into cultures of hemocytes taken from larvae of Galleria mellonella Linnaeus (Lepidoptera: Pyralidae), resulted in progressive disintegration of nets formed by granulocytes and plasmatocytes due to rapid degranulation of granulocytes, extensive vacuolization of plasmatocytes accompanied by cytoplasm expulsion, and cell disintegration. Spherulocytes remained intact, while oenocytes rapidly disintegrated. Coronatin-2 produced 80% mortality when injected into G. mellonella at 5 µg larva-1. Further study is warranted to determine the relevance of the acute toxicity of coronatin-2 and its effects on hemocytes in vitro to virulence of C. coronatus against its hosts.

Coevolution of parasitic fungi and insect hosts

Parasitic fungi and their insect hosts provide an intriguing model system for dissecting the complex co-evolutionary processes, which result in Red Queen dynamics. To explore the genetic basis behind host-parasite coevolution we chose two parasitic fungi (Beauveria bassiana and Metarhizium anisopliae, representing the most important entomopathogenic fungi used in the biological control of pest or vector insects) and two established insect model hosts (the greater wax moth Galleria mellonella and the red flour beetle Tribolium castaneum) for which sequenced genomes or comprehensive transcriptomes are available. Focusing on these model organisms, we review the knowledge about the interactions between fungal molecules operating as virulence factors and insect host-derived defense molecules mediating antifungal immunity. Particularly the study of the intimate interactions between fungal proteinases and corresponding host-derived proteinase inhibitors elucidated novel coevolutionary mechanisms such as functional shifts or diversification of involved effector molecules. Complementarily, we compared the outcome of coevolution experiments using the parasitic fungus B. bassiana and two different insect hosts which were initially either susceptible (Galleria mellonella) or resistant (Tribolium castaneum). Taking a snapshot of host-parasite coevolution, we show that parasitic fungi can overcome host barriers such as external antimicrobial secretions just as hosts can build new barriers, both within a relatively short time of coevolution.

Cuticle Fatty Acid Composition and Differential Susceptibility of Three Species of Cockroaches to the Entomopathogenic Fungi Metarhizium anisopliae (Ascomycota, Hypocreales)

Differences in free fatty acids (FFAs) chemical composition of insects may be responsible for susceptibility or resistance to fungal infection. Determination of FFAs found in cuticular lipids can effectively contribute to the knowledge concerning insect defense mechanisms. In this study, we have evaluated the susceptibility of three species of cockroaches to the entomopathogenic fungi Metarhizium anisopliae (Metschnikoff) Sorokin by topical application. Mortality due to M. anisopliae was highly significant on adults and nymphs of Blattella germanica L. (Blattodea: Blattellidae). However, mortality was faster in adults than in nymphs. Adults of Blatta orientalis L. (Blattodea: Blattidae) were not susceptible to the fungus, and nymphs of Blaptica dubia Serville (Blattodea: Blaberidae) were more susceptible to the fungus than adults. The composition of cuticular FFAs in the three species of cockroaches was also studied. The analysis indicated that all of the fatty acids were mostly straight-chain, long-chain, saturated or unsaturated. Cuticular lipids of three species of cockroaches contained 19 FFAs, ranging from C14:0 to C24:0. The predominant fatty acids found in the three studied species of cockroaches were oleic, linoleic, palmitic, and stearic acid. Only in adults of Bl. orientalis, myristoleic acid, γ-linolenic acid, arachidic acid, dihomolinoleic acid, and behenic acid were identified. Lignoceric acid was detected only in nymphs of Bl. orientalis. Heneicosylic acid and docosahexaenoic acid were identified in adults of Ba. dubia.

Exposure to natural pathogens reveals costly aphid response to fungi but not bacteria

Immune responses are costly, causing trade-offs between defense and other host life history traits. Aphids present a special system to explore the costs associated with immune activation since they are missing several humoral and cellular mechanisms thought important for microbial resistance, and it is unknown whether they have alternative, novel immune responses to deal with microbial threat. Here we expose pea aphids to an array of heat-killed natural pathogens, which should stimulate immune responses without pathogen virulence, and measure changes in life-history traits. We find significant reduction in lifetime fecundity upon exposure to two fungal pathogens, but not to two bacterial pathogens. This finding complements recent genomic and immunological studies indicating that pea aphids are missing mechanisms important for bacterial resistance, which may have important implications for how aphids interact with their beneficial bacterial symbionts. In general, recent exploration of the immune systems of non-model invertebrates has called into question the generality of our current picture of insect immunity. Our data highlight that taking an ecological approach and measuring life-history traits to a broad array of pathogens provides valuable information that can complement traditional approaches.

Expansion of the antimicrobial peptide repertoire in the invasive ladybird Harmonia axyridis

The harlequin ladybird beetle Harmonia axyridis has emerged as a model species in invasion biology because of its strong resistance against pathogens and remarkable capacity to outcompete native ladybirds. The invasive success of the species may reflect its well-adapted immune system, a hypothesis we tested by analysing the transcriptome and characterizing the immune gene repertoire of untreated beetles and those challenged with bacteria and fungi. We found that most H. axyridis immunity-related genes were similar in diversity to their counterparts in the reference beetle Tribolium castaneum, but there was an unprecedented expansion among genes encoding antimicrobial peptides and proteins (AMPs). We identified more than 50 putative AMPs belonging to seven different gene families, and many of the corresponding genes were shown by quantitative real-time RT–PCR to be induced in the immune-stimulated beetles. AMPs with the highest induction ratio in the challenged beetles were shown to demonstrate broad and potent activity against Gram-negative bacteria and entomopathogenic fungi. The invasive success of H. axyridis can therefore be attributed at least in part to the greater efficiency of its immune system, particularly the expansion of AMP gene families and their induction in response to pathogens.

Route of infection and hematological effect of Metarhizium anisopliae (Metsch.) Sorokin on Dysdercus cingulatus (Fab.) adult

The primary objective of this work was to identify, under laboratory conditions, the route of infection and hemogram of Dysdercus cingulatus (Fab.) adults by Metarhizium anisopliae. The infection process in D. cingulatus by M. anisopliae involved the conidia adherence to the host cuticle and germination after 24 h post-infection, accompanied by falling of bristles. The subsequent step, within 24-48 h post-infection, comprised penetration of fungus through spiracles, root of bristles, hemolymph, and the three dorsal sacs. Subsequently, within 72-96 h post-infection, the fungus penetrated into trachea and sacs, then emerged on cuticular surface and was found to be maximum in hemolymph. A great decrease in hemocytes count was observed within 96 h from infection. The hemosomic index (HSI) decreased gradually as the incubation period increased. As far as we know, this is the first study to know the mechanism of action of M. anisopliae to D. cingulatus.

A strong immune response in young adult honeybees masks their increased susceptibility to infection compared to older bees

Honeybees, Apis mellifera, show age-related division of labor in which young adults perform maintenance (“housekeeping”) tasks inside the colony before switching to outside foraging at approximately 23 days old. Disease resistance is an important feature of honeybee biology, but little is known about the interaction of pathogens and age-related division of labor. We tested a hypothesis that older forager bees and younger “house” bees differ in susceptibility to infection. We coupled an infection bioassay with a functional analysis of gene expression in individual bees using a whole genome microarray. Forager bees treated with the entomopathogenic fungus Metarhizium anisopliae s.l. survived for significantly longer than house bees. This was concomitant with substantial differences in gene expression including genes associated with immune function. In house bees, infection was associated with differential expression of 35 candidate immune genes contrasted with differential expression of only two candidate immune genes in forager bees. For control bees (i.e. not treated with M. anisopliae) the development from the house to the forager stage was associated with differential expression of 49 candidate immune genes, including up-regulation of the antimicrobial peptide gene abaecin, plus major components of the Toll pathway, serine proteases, and serpins. We infer that reduced pathogen susceptibility in forager bees was associated with age-related activation of specific immune system pathways. Our findings contrast with the view that the immunocompetence in social insects declines with the onset of foraging as a result of a trade-off in the allocation of resources for foraging. The up-regulation of immune-related genes in young adult bees in response to M. anisopliae infection was an indicator of disease susceptibility; this also challenges previous research in social insects, in which an elevated immune status has been used as a marker of increased disease resistance and fitness without considering the effects of age-related development.

Honeybees have a highly developed form of social biology in which tasks are distributed among workers according to their age, with younger bees performing housekeeping tasks (“house bees”) before switching to foraging duties when they grow older. This division of labor is vital to colony function and survival. Pathogens are known to be partly responsible for the current decline in honeybee populations around the world, but we understand little about the responses of different types of worker bee to infection. In this study, we infected house and forager bees with an insect pathogen. We measured bee survival rate and the expression of genes that regulate the immune system. More immune genes were up regulated in house bees than foragers in response to infection, but foragers were more resistant to the pathogen than house bees. We found that development from the house to forager stages resulted in increased expression of genes that regulate the production of antimicrobial proteins. The inference is that parts of the immune system are activated during development, resulting in greater resistance to infectious disease in forager bees. Our study provides new insights into the functioning of the honeybee immune system and its interaction with social organisation.

Comparison of free fatty acids composition of cuticular lipids of Calliphora vicina larvae and pupae

The chemical characterization of the free fatty acid (FFA) fractions of the cuticular lipids of Calliphora vicina larvae and pupae was performed by separating the FFA fraction using high-performance liquid chromatography with laser light scattering detection (HPLC-LLSD) and quantitatively analyzing the FFA using gas chromatography-electron impact mass spectrometry (GC-MS). Thirty-two saturated and unsaturated FFA were identified and quantified in the insect lipids. Cuticular FFA profiles of C. vicina larvae and pupae were compared. Cuticular FFA of larvae and pupae accounted for 70.8 and 77.8 % of the total lipids, respectively. The cuticular lipids of C. vicina larvae contained 24 FFA ranging from 8:0 to 24:0, whereas the cuticular lipids of pupae contained 32 FFA ranging from 6:0 to 26:0. The cuticular lipids of the larvae contained 16 saturated, five monounsaturated, one diunsaturated, and two polyunsaturated FFA. The cuticular lipids of the pupae contained 18 saturated, nine monounsaturated, two diunsaturated, and three polyunsaturated FFA. The major cuticular FFA in C. vicina larvae and pupae was 18:1 (47.6 and 41.7 %, respectively). The highest amounts of total cuticular FFA were detected in larvae of C. vicina (1.7 mg/g of the insect body). The quantities of total cuticular FFA in pupae were smaller (1.4 mg/g of the insect body).

Antimicrobial activity of alcohols from Musca domestica

Information on the stimulatory and inhibitory effects of cuticular alcohols on growth and virulence of insecticidal fungi is unavailable. Therefore, we set out to describe the content of cuticular and internal alcohols in the body of housefly larvae, pupae, males and females. The total cuticular alcohols in larvae, males and females of Musca domestica were detected in comparable amounts (4.59, 3.95 and 4.03 μg g(-1) insect body, respectively), but occurred in smaller quantities in pupae (2.16 μg g(-1)). The major free alcohol in M. domestica larvae was C(12:0) (70.4%). Internal alcohols of M. domestica larvae were not found. Among cuticular pupae alcohols, C(12:0) (31.0%) was the most abundant. In the internal lipids of pupae, only five alcohols were identified in trace amounts. The most abundant alcohol in males was C(24:0) (57.5%). The percentage content of cuticular C(24:0) in males and females (57.5 and 36.5%, respectively) was significantly higher than that of cuticular lipids in larvae and pupae (0.9 and 5.6%, respectively). Only two alcohols were present in the internal lipids of males in trace amounts (C(18:0) and C(20:0)). The most abundant cuticular alcohols in females were C(24:0) (36.5%) and C(12:0) (26.8%); only two alcohols (C(18:0) and C(20:0)) were detected in comparable amounts in internal lipids (3.61±0.32 and 5.01±0.42 μg g(-1), respectively). For isolated alcohols, antimicrobial activity against 10 reference strains of bacteria and fungi was determined. Individual alcohols showed approximately equal activity against fungal strains. C(14:0) was effective against gram-positive bacteria, whereas gram-negative bacteria were resistant to all tested alcohols. Mixtures of alcohols found in cuticular lipids of larvae, pupae, males and females of M. domestica generally presented higher antimicrobial activity than individual alcohols. In contrast, crude extracts containing both cuticular and internal lipids showed no antifungal activity against the entomopathogenic fungus Conidiobolus coronatus, which efficiently kills adult house flies.

The composition of the cuticular and internal free fatty acids and alcohols from Lucilia sericata males and females

GC, GC-MS, and HPLC-LLSD analyses were used to identify and quantify cuticular and internal lipids in males and females of the blow-fly (Lucilia sericata). Sixteen free fatty acids, seven alcohols and cholesterol were identified and quantitatively determined in the cuticular lipids of L. sericata. Cuticular fatty acids ranged from C(6) to C(20) and included unsaturated entities such as 16:1n-9, 18:1n-9, 20:4n-3 and 20:5n-3. Cuticular alcohols (only saturated and even-numbered) ranged from C(12) to C(20) in males and C(10) to C(22) in females. Only one sterol was found in the cuticular lipids of both males and females. 23 free fatty acids, five alcohols and cholesterol were identified in the internal lipids. Internal fatty acids were present in large amounts-7.4 mg/g (female) and 10.1 mg/g (male). Only traces of internal alcohols (from C(14) to C(26) in males, from C(14) to C(22) in females) were found in L. sericata. Large amounts of internal cholesterol were identified in L. sericata males and females (0.49 and 0.97 mg/g of the insect body, respectively).

Recombinant immunosuppressive protein from Pimpla hypochondrica venom (rVPr1) increases the susceptibility of Mamestra brassicae larvae to the fungal biological control agent, Beauveria bassiana

Although fungi are used to control a variety of insect pests, it is accepted that their usage could be increased if their efficacy was greater. The outcome of the interaction of a fungus and a pest insect may be influenced by a number of criteria, including the ability of the insect to mount effective immune responses against the pathogen. In view of this, we aimed to determine if a recombinant immunosuppressive wasp venom protein (rVPr1) can increase the susceptibility of larvae of the lepidopteran pest, Mamestra brassicae, to the fungal biological control agent, Beauveria bassiana. Bioassays indicated that when larvae were injected with 3.5 µl of rVPr1 and 100 B. bassiana conidia (combined injection assays), a significant reduction in survival of larvae occurred compared with each treatment on its own (P=0.006). Similar results were obtained when larvae were dipped in a solution containing 3 × 10(6) B. bassiana conidia per ml and then injected with 3.5 µl of rVPr1 2 days later (topical application assays), (P<0.001). These results indicate that rVPr1 can increase the efficacy of B. bassiana toward a lepidopteran pest, and are discussed within the context of insect immune responses and integrated pest management.

Antimicrobial peptides: modes of mechanism, modulation of defense responses

Complicated schemes of classical breeding and their drawbacks, environmental risks imposed by agrochemicals, decrease of arable land, and coincident escalating damages of pests and pathogens have accentuated the necessity for highly efficient measures to improve crop protection. During co-evolution of host-microbe interactions, antimicrobial peptides (AMPs) have exhibited a brilliant history in protecting host organisms against devastation by invading pathogens. Since the 1980s, a plethora of AMPs has been isolated from and characterized in different organisms. Nevertheless the AMPs expressed in plants render them more resistant to diverse pathogens, a more orchestrated approach based on knowledge of their mechanisms of action and cellular targets, structural toxic principle, and possible impact on immune system of corresponding transgenic plants will considerably improve crop protection strategies against harmful plant diseases. This review outlines the current knowledge on different modes of action of AMPs and then argues the waves of AMPs’ ectopic expression on transgenic plants’ immune system.

Coronatin-1 isolated from entomopathogenic fungus Conidiobolus coronatus kills Galleria mellonella hemocytes in vitro and forms potassium channels in planar lipid membrane

Entomopathogenic fungi are important natural regulatory factors of insect populations and have potential as biological control agents of insect pests. The cosmopolitan soil fungus Conidiobolus coronatus (Entomopthorales) easily attacks Galleria mellonella (Lepidoptera) larvae. Prompt death of invaded insects is attributed to the action of toxic metabolites released by the invader. Effect of fungal metabolites on hemocytes, insect blood cells involved in innate defense response, remains underexplored to date. C. coronatus isolate 3491 inducing 100% mortality of G. mellonella last instar larvae exposed to sporulating colonies, was cultivated at 20 °C in minimal medium. Post-incubation filtrates were used as a source of fungal metabolites. A two-step HPLC (1 step: Shodex KW-803 column eluted with 50 mM KH(2)PO(4) supplemented with 0.1 M KCl, pH 6.5; 2 step: ProteinPak™ CM 8HR column equilibrated with 5 mM KH(2)PO(4), pH 6.5, proteins eluted with a linear gradient of 0.5 M KCl) allowed the isolation of coronatin-1, an insecticidal 36 kDa protein showing both elastolytic and chitinolytic activities. Addition of coronatin-1 into primary in vitro cultures of G. mellonella hemocytes resulted in rapid disintegration of spherulocytes freely floating in culture medium and shrinkage of plasmatocytes adhering to the bottom of culture well. Coronatin-1 stimulated pseudopodia atrophy and, in consequence, disintegration of nets formed by cultured hemocytes. After incorporation of coronatin-1 into planar lipid membrane (PLM) ion channels selective for K(+) ions in 50/450 mM KCl solutions were observed. Potassium current flows were recorded in nearly 70% of experiments with conductance from 300 pS up to 1 nS. All observed channels were active at both positive and negative membrane potentials. Under experimental conditions incorporated coronatin-1 exhibited a zero current potential (E(rev)) of 47.7 mV, which indicates K(+)-selectivity of this protein. The success of the purification of coronatin-1 will allow further characterization of the mode of action of this molecule, including ability of coronatin-1 to form potassium channels in immunocompetent hemocytes.

A comprehensive transcriptome and immune-gene repertoire of the lepidopteran model host Galleria mellonella

Background

The larvae of the greater wax moth Galleria mellonella are increasingly used (i) as mini-hosts to study pathogenesis and virulence factors of prominent bacterial and fungal human pathogens, (ii) as a whole-animal high throughput infection system for testing pathogen mutant libraries, and (iii) as a reliable host model to evaluate the efficacy of antibiotics against human pathogens. In order to compensate for the lack of genomic information in Galleria, we subjected the transcriptome of different developmental stages and immune-challenged larvae to next generation sequencing.

Results

We performed a Galleria transcriptome characterization on the Roche 454-FLX platform combined with traditional Sanger sequencing to obtain a comprehensive transcriptome. To maximize sequence diversity, we pooled RNA extracted from different developmental stages, larval tissues including hemocytes, and from immune-challenged larvae and normalized the cDNA pool. We generated a total of 789,105 pyrosequencing and 12,032 high-quality Sanger EST sequences which clustered into 18,690 contigs with an average length of 1,132 bases. Approximately 40% of the ESTs were significantly similar (E ≤ e^-03) to proteins of other insects, of which 45% have a reported function. We identified a large number of genes encoding proteins with established functions in immunity related sensing of microbial signatures and signaling, as well as effector molecules such as antimicrobial peptides and inhibitors of microbial proteinases. In addition, we found genes known as mediators of melanization or contributing to stress responses. Using the transcriptomic data, we identified hemolymph peptides and proteins induced upon immune challenge by 2D-gelelectrophoresis combined with mass spectrometric analysis.

Conclusion

Here, we have developed extensive transcriptomic resources for Galleria. The data obtained is rich in gene transcripts related to immunity, expanding remarkably our knowledge about immune and stress-inducible genes in Galleria and providing the complete sequences of genes whose primary structure have only partially been characterized using proteomic methods. The generated data provide for the first time access to the genetic architecture of immunity in this model host, allowing us to elucidate the molecular mechanisms underlying pathogen and parasite response and detailed analyses of both its immune responses against human pathogens, and its coevolution with entomopathogens.

Immunity in lepidopteran insects

Lepidopteran insects provide important model systems for innate immunity of insects, particularly for cell biology of hemocytes and biochemical analyses of plasma proteins. Caterpillars are also among the most serious agricultural pests, and understanding of their immune systems has potential practical significance. An early response to infection in lepidopteran larvae is the activation of hemocyte adhesion, leading to phagocytosis, nodule formation, or encapsulation. Plasmatocytes and granular cells are the hemocyte types involved in these responses. Infectious microorganisms are recognized by binding of hemolymph plasma proteins to microbial surface components. This "pattern recognition" triggers phagocytosis and nodule formation, activation of prophenoloxidase and melanization and the synthesis of antimicrobial proteins that are secreted into the hemolymph. Many hemolymph proteins that function in such innate immune responses of insects were first discovered in lepidopterans. Microbial proteinases and nucleic acids released from lysed host cells may also activate lepidopteran immune responses. Hemolymph antimicrobial peptides and proteins can reach high concentrations and may have activity against a broad spectrum of microorganisms, contributing significantly to clearing of infections. Serine proteinase cascade pathways triggered by microbial components interacting with pattern recognition proteins stimulate activation of the cytokine Spätzle, which initiates the Toll pathway for expression of antimicrobial peptides. A proteinase cascade also results inproteolytic activation of phenoloxidase and production of melanin coatings that trap and kill parasites and pathogens. The proteinases in hemolymph are regulated by specific inhibitors, including members of the serpin superfamily. New developments in lepidopteran functional genomics should lead to much more complete understanding of the immune systems of this insect group.

Cuticular lipids of insects as potential biofungicides: methods of lipid composition analysis

The main function of cuticular lipids in insects is the restriction of water transpiration through the surface. Lipids are involved in various types of chemical communication between species and reduce the penetration of insecticides, chemicals, and toxins and they also provide protection from attack by microorganisms, parasitic insects, and predators. Hydrocarbons, which include straight-chain saturated, unsaturated, and methyl-branched hydrocarbons, predominate in the cuticular lipids of most insect species; fatty acids, alcohols, esters, ketones, aldehydes, as well as trace amounts of epoxides, ethers, oxoaldehydes, diols, and triacylglycerols have also been identified. Analyses of cuticular lipids are chemically relatively straightforward, and methods for their extraction should be simple. Classically, extraction has relied mainly on application of apolar solvents to the entire insect body. Recently, several alternative methods have been employed to overcome some of the shortcomings of solvent extraction. These include the use of solid-phase microextraction (SPME) fibers to extract hydrocarbons from the headspace of heated samples, SPME to sample live individuals, and a less expensive method (utilized for social wasps), which consists of the collection of cuticular lipids by means of small pieces of cotton rubbed on the body of the insect. Both classical and recently developed extraction methods are reviewed in this work. The separation and analysis of the insect cuticular lipids were performed by column chromatography, thin-layer chromatography (TLC), high performance liquid chromatography with a laser light scattering detector (HPLC-LLSD), gas chromatography (GC), and GC-mass spectrometry (MS). The strategy of lipid analysis with the use of chromatographic techniques was as follows: extraction of analytes from biological material, lipid class separation by TLC, column chromatography, HPLC-LLSD, derivatization, and final determination by GC, GC-MS, matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) MS, and liquid chromatography-mass spectrometry (LC-MS).

Fungal secondary metabolites as modulators of interactions with insects and other arthropods

Fungi share a diverse co-evolutionary history with animals, especially arthropods. In this review, we focus on the role of secondary metabolism in driving antagonistic arthropod-fungus interactions, i.e., where fungi serve as a food source to fungal grazers, compete with saprophagous insects, and attack insects as hosts for growth and reproduction. Although a wealth of studies on animal-fungus interactions point to a crucial role of secondary metabolites in deterring animal feeding and resisting immune defense strategies, causal evidence often remains to be provided. Moreover, it still remains an unresolved puzzle as to what extent the tight regulatory control of secondary metabolite formation in some model fungi represents an evolved chemical defense system favored by selective pressure through animal antagonists. Given these gaps in knowledge, we highlight some co-evolutionary aspects of secondary metabolism, such as induced response, volatile signaling, and experimental evolution, which may help in deciphering the ecological importance and evolutionary history of secondary metabolite production in fungi.

The composition of the free fatty acids from Dendrolimus pini exuviae

The pine moth Dendrolimus pini effectively resists many insecticides, but it can be controlled by the use of bioinsecticides such as entomopathogenic fungi. In the use of microbial agents for the biocontrol of D. pini, it is important to identify the cuticular lipids of this pest if we are to understand the factors responsible for the preferential adhesion or selective repulsion of entomopathogenic fungi that are potentially useful in biocontrol. In this work the qualitative and quantitative analyses of free fatty acids in two exuviae extracts (petroleum ether and dichloromethane) and two developmental stages (larval-larval and larval-pupal molts) were studied. The free fatty acid composition of the epicuticular lipids from exuviae of D. pini was characterized chemically using gas chromatography (GC) and gas chromatography-electron impact mass spectrometry (GC-MS). Structural analyses of the dichloromethane extracts from larval-larval exuviae (LLE) and larval-pupal exuviae (LPE) revealed that the carbon numbers for the major acid moieties ranged from C(8:0) to C(34:0). Only C(23:0) was not identified in the LPE extract. The relative contents of fatty acids in the extracts varied from trace amounts to 34%. The fatty acids extracted by dichloromethane were essentially the same as those in the petroleum ether extract. We also identified dehydroabietic acid in the exuviae of D. pini. The respective quantities of dehydroabietic acid obtained from D. pini LLE and LPE were 1763+/-103 microg/g exuviae and 11521+/-1198 microg/g of exuviae.

A recombinant immunosuppressive protein from Pimpla hypochondriaca (rVPr1) increases the susceptibility of Lacanobia oleracea and Mamestra brassicae larvae to Bacillus thuringiensis

The precise mechanisms underlying Bacillus thuringiensis-mediated killing of pest insects are not clear. In some cases, death may be due to septicaemia caused by Bt and/or gut bacteria gaining access to the insect haemocoel. Since insects protect themselves from microbes using an array of cellular and humoral immune defences, we aimed to determine if a recombinant immunosuppressive wasp venom protein (rVPr1) could increase the susceptibility of two pest Lepidoptera (Lacanobia oleracea and Mamestra brassicae) to Bt. Bio-assays indicated that injection of 6 microl of rVPr1 into the haemocoel of both larvae caused similar levels of mortality (less than 38%). On the other hand, the LD(30-40) of Bt for M. brassicae larvae was approximately 20 times higher than that for L. oleracea larvae. Furthermore, in bio-assays where larvae were injected with rVPr1, then fed Bt, a significant reduction in survival of larvae for both species occurred compared to each treatment on its own (P<0.001); and for L. oleracea larvae, this effect was more than additive. The results are discussed within the context of insect immunity and protection against Bt.

Galleria mellonella as a model system for studying Listeria pathogenesis

Essential aspects of the innate immune response to microbial infection are conserved between insects and mammals. This has generated interest in using insects as model organisms to study host-microbe interactions. We used the greater wax moth Galleria mellonella, which can be reared at 37 degrees C, as a model host for examining the virulence potential of Listeria spp. Here we report that Galleria is an excellent surrogate model of listerial septic infection, capable of clearly distinguishing between pathogenic and nonpathogenic Listeria strains and even between virulent and attenuated Listeria monocytogenes strains. Virulence required listerial genes hitherto implicated in the mouse infection model and was linked to strong antimicrobial activities in both hemolymph and hemocytes of infected larvae. Following Listeria infection, the expression of immune defense genes such as those for lysozyme, galiomycin, gallerimycin, and insect metalloproteinase inhibitor (IMPI) was sequentially induced. Preinduction of antimicrobial activity by treatment of larvae with lipopolysaccharide (LPS) significantly improved survival against subsequent L. monocytogenes challenge and strong antilisterial activity was detected in the hemolymph of LPS pretreated larvae. We conclude that the severity of septic infection with L. monocytogenes is modulated primarily by innate immune responses, and we suggest the use of Galleria as a relatively simple, nonmammalian model system that can be used to assess the virulence of strains of Listeria spp. isolated from a wide variety of settings from both the clinic and the environment.

Cloning and expression of the gene for an insect haemocyte anti-aggregation protein (VPr3), from the venom of the endoparasitic wasp, Pimpla hypochondriaca

A venom protein from the endoparasitic wasp, Pimpla hypochondriaca, was recently biochemically isolated. This protein possessed haemocyte anti-aggregation activity in vitro and shares the same N-terminal amino acid sequence as that deduced from a gene termed vpr3. The vpr3 gene was identified by sequence analysis of randomly isolated cDNAs from a P. hypochondriaca venom gland library. Presently, the gene for the full-length sequence of mature VPr3 protein was amplified from the P. hypochondriaca venom gland cDNA library by PCR. The amplicon was directionally cloned into a pET expression vector so that recombinant VPr3 (rVPr3) would have an N-terminal polyhistidine (His) tag. High levels of target protein expression were obtained following addition of IPTG (1 mM) and growth of the bacteria at 37 degrees C for 5 h, or at 24 degrees C for 20 h. Following lysis of bacteria grown at 37 degrees C, the target protein partitioned into the insoluble fraction. However, at 24 degrees C, a small amount of soluble protein was consistently detected. The amount of soluble rVPr3 was subsequently increased when the transformed bacteria were grown in Overnight Express Instant TB medium at 24 degrees C. Soluble rVPr3 was purified utilizing the MagneHis Protein Purification System. Recombinant VPr3 was determined to have adverse effects on the cytoskeleton of Lacanobia oleracea haemocytes and to inhibit the ability of these cells to form aggregates in vitro.

Cellular encapsulation in the eastern subterranean termite, Reticulitermes flavipes (Isoptera), against infection by the entomopathogenic fungus Metarhizium anisopliae

Reticulitermes flavipes workers were topically inoculated with approximately 10,000 conidia of the entomopathogenic fungus Metarhizium anisopliae. After being kept in groups of 20 individuals for 1-9 d, histopathological examination showed that termites had an individual immune reaction. The nodule formation at the point of entrance of the fungal hyphae was identified as a cellular encapsulation and the different steps in the nodule formation are described. The relative number of hemocytes per termite increased 24h after fungal exposure and remained high in the hemolymph for at least 3 d before decreasing back to pre-exposure levels. The role of an individual immune cellular reaction in social insects is discussed.

Antifungal activity of the termite alkaloid norharmane against the mycelial growth of Metarhizium anisopliae and Aspergillus nomius

Antifungal activity of norharmane, a beta-carboline alkaloid found in termites (Isoptera, Rhinotermitidae) was tested against two entomopathogenic fungi, Metarhizium anisopliae and Aspergillus nomius. It was determined that, at physiological concentration (10 microg ml(-1)), norharmane had no significant effect on A. nomius mycelial growth rate but reduced M. anisopliae growth rate by 11.9%. Contrary to previous findings, we suggest that norharmane has a limited role in disease resistance against fungal pathogens in individual subterranean termites, and we discuss the potential role of this chemical at a colony level.

The cuticular fatty acids of Calliphora vicina, Dendrolimus pini and Galleria mellonella larvae and their role in resistance to fungal infection

Epicuticular lipids in many terrestrial arthropods consist of vast numbers of polar and non-polar aliphatic compounds, which are mainly responsible for the water balance in these animals but can also affect conidia germination of entomopathogenic fungi. In this work the qualitative and quantitative profiles of cuticular fatty acids from three insect species differing in their susceptibility to fungal infection were studied. In an innovative approach, laser light scattering detection was coupled with HPLC in order to identify the non-chromophoric chemicals usually present in cuticular extracts. The acids identified contained from 5 to 20 carbon atoms in the alkyl chain and included unsaturated entities such as C(16:1), C(18:1), C(18:2), C(18:3) and C(20:1). There was a marked dominance of acids containing 16-18 carbon atoms. The relative contents of fatty acids in the extracted waxes varied from trace amounts to 44%. Cuticular fatty acids profile of Calliphora vicina (species resistant to fungal infection) significantly differs from profiles of Dendrolimus pini and Galleria mellonella (both species highly susceptible to fungal infection). The major difference is the presence of C(14:0), C(16:1) and C(20:0) in the cuticle of C. vicina. These three fatty acids are absent in the cuticle of D. pini while G. mellonella cuticle contains their traces. The concentrations of four fatty acids dominating in the G. mellonella larval cuticle (C(16:0), C(18:0), C(18:1) and C(18:2)) were found to fluctuate during the final larval instar and correlate with fluctuations in the susceptibility of larvae to fungal infection. The possible role of cuticular fatty acids in preventing fungal infection is discussed.

Different defense strategies of Dendrolimus pini, Galleria mellonella, and Calliphora vicina against fungal infection

The resistance of Galleria mellonella, Dendrolimus pini, and Calliphora vicina larvae against infection by the enthomopathogen Conidiobolus coronatus was shown to vary among the studied species. Exposure of both G. mellonella and D. pini larvae to the fungus resulted in rapid insect death, while all the C. vicina larvae remained unharmed. Microscopic studies revealed diverse responses of the three species to the fungal pathogen: (1) the body cavities of D. pini larvae were completely overgrown by fungal hyphae, with no signs of hemocyte response, (2) infected G. mellonella larvae formed melanotic capsules surrounding the fungal pathogen, and (3) the conidia of C. coronatus did not germinate on the cuticle of C. vicina larvae. The in vitro study on the degradation of the insect cuticle by proteases secreted by C. coronatus revealed that the G. mellonella cuticle degraded at the highest rate. The antiproteolytic capacities of insect hemolymph against fungal proteases correlated well with the insects' susceptibility to fungal infection. The antiproteolytic capacities of insect hemolymph against fungal proteases correlated well with the insects' susceptibility to fungal infection. Of all the tested species, only plasmatocytes exhibited phagocytic potential. Exposure to the fungal pathogen resulted in elevated phagocytic activity, found to be the highest in the infected G. mellonella. The incubation of insect hemolymph with fungal conidia and hyphae revealed diverse reactions of hemocytes of the studied insect species. The encapsulation potential of D. pini hemocytes was low. Hemocytes of G. mellonella showed a high ability to attach and encapsulate fungal structures. Incubation of C. vicina hemolymph with C. coronatus did not result in any hemocytic response. Phenoloxidase (PO) activity was found to be highest in D. pini hemolymph, moderate in G. mellonella, and lowest in the hemolymph of C. vicina. Fungal infection resulted in a significant decrease of PO activity in G. mellonela larvae, while that in the larvae of D. pini remained unchanged. PO activity in C. vicina exposed to fungus slightly increased. The lysozyme-like activity increased in the plasma of all three insect species after contact with the fungal pathogen. Anti E. coli activity was detected neither in control nor in infected D. pini larvae. No detectable anti E. coli activity was found in the control larvae of G. mellonella; however, its exposure to C. coronatus resulted in an increase in the activity to detectable level. In the case of C. vicina exposure to the fungus, the anti E. coli activity was significantly higher than in control larvae. The defense mechanisms of D. pini (species of economic importance in Europe) are presented for the first time.