Manipulation of host ecdysteroid hormone levels facilitates infection by the fungal insect pathogen, Metarhizium rileyi

The gut microbial community structure of the oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) affects the the virulence of the entomopathogenic fungus Metarhizium rileyi

Mythimna separata, the oriental armyworm, is a lepidopteran pest that threatens cereal crops. In the current study, two strains (XSBN200920 and JHML200710) of entomopathogenic fungus Metarhizium rileyi were tested for virulence against oriental armyworms. When treated with spore suspensions of both strains at a concentration of 1.0 × 108 spores/mL, the 3rd instar larvae's survival rate was considerably different (P < 0.01). The median lethal time of the insects exposed to XSBN200920 was about 3 d longer than that of JHML200710. The results of 16S ribosomal RNA sequencing showed that Chao1 richness in the JHML200710 treatment group was significantly decreased compared with the CK ( 0.02% Tween 80). The dominant gut bacteria species at the phylum level were Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidota in the three groups. The CK group had a much higher associated abundance of cyanobacteria than the other two fungal treatment groups. Sixteen genera revealed significant variations in the gut bacteria of the insects at the genus level. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional gene and enzyme analysis showed that when compared with the CK group, the XSBN200920 treatment group showed a significant reduction in six aspects, including betalain biosynthesis, spliceosome, and neuroactive ligand-receptor interaction. These findings suggested that healthy and fungus-infected insects' intestinal microbial community structure differed significantly. And the virulence of M. rileyi is closely linked to its ability to alter the structure of the intestinal microbiome of insects. The results offer a starting point for examining the relationship between the gut microbial diversity of oriental armyworms and variations in the virulence of pathogenic fungi.

Differential pathogenicity of heterologous nucleopolyhedroviruses on Spodoptera frugiperda corresponds to modulated expression of immune-response-related genes

The fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a notorious invasive pest wreaking havoc on various crops globally. Nucleopolyhedroviruses (NPVs) are viral pathogens that specially target lepidopteran pests. However, the homologous virus, Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV), has not been commercialized in China. Therefore, understanding the molecular mechanisms underlying heterologous virus-host interactions can inform the design of virus-based insecticides for controlling S. frugiperda. The pathogenicity of the four heterologous NPVs on S. frugiperda varied greatly. Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) exhibited the most potent virulence on larvae and induced the most robust sublethal effects on adults. Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) infection was characterized by more moderate pathogenicity, and larvae were relatively resistant to Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) and Spodoptera litura multiple nucleopolyhedrovirus (SlMNPV). Larval mortality was virus-concentration and larval stage dependent. Specifically, the corrected mortality rate of third instar larvae after treatment with 1 × 106, 1 × 107, and 1 × 108 OBs/mL MbMNPV was 88.9 %, 100.0 %, and 100.0 %, respectively. All four NPVs negatively affected the longevity and fecundity of S. frugiperda adults. Female adults surviving treatment with MbMNPV and SeMNPV were unable to lay eggs. Transcriptomic analysis revealed that MbMNPV infection might suppress the antiviral immune response, and dysregulate biological pathways of S. frugiperda larvae to facilitate systemic infection. However, the overall transcript profiles remain unchanged after SlMNPV infection. The results reinforce the potential of NPVs, specifically MbMNPV, as potent biocontrol agents for S. frugiperda. These findings yield valuable insights into the complex arms race between S. frugiperda and NPVs that may advance the development of virus-based strategies to mitigate the destructive impact of this pest.

Viral- and fungal-mediated behavioral manipulation of hosts: summit disease

Summit disease, in which infected hosts seek heights (gravitropism), first noted in modern times by nineteenth-century naturalists, has been shown to be induced by disparate pathogens ranging from viruses to fungi. Infection results in dramatic changes in normal activity patterns, and such parasite manipulation of host behaviors suggests a strong selection for convergent outcomes albeit evolved via widely divergent mechanisms. The two best-studied examples involve a subset of viral and fungal pathogens of insects that induce "summiting" in infected hosts. Summiting presumably functions as a means for increasing the dispersal of the pathogen, thus significantly increasing fitness. Here, we review current advances in our understanding of viral- and fungal-induced summit disease and the host behavioral manipulation involved. Viral genes implicated in this process include a host hormone-targeting ecdysteroid UDP-glucosyltransferase (apparently essential for mediating summit disease induced by some viruses but not all) and a protein tyrosine phosphatase, with light dependance implicated. For summit disease-causing fungi, though much remains obscure, targeting of molting, circadian rhythms, sleep, and responses to light patterns appear involved. Targeting of host neuronal pathways by summit-inducing fungi also appears to involve the production of effector molecules and secondary metabolites that affect host muscular, immune, and/or neurological processes. It is hypothesized that host brain structures, particularly Mushroom Bodies (no relation to the fungus itself), important for olfactory association learning and control of locomotor activity, are critical targets for mediating summiting during infection. This phenomenon expands the diversity of microbial pathogen-interactions and host dynamics. KEY POINTS: • Summit disease or height seeking (gravitropism) results from viral and fungal pathogens manipulating insect host behaviors presumably to increase pathogen dispersal. • Insect baculoviruses and select fungal pathogens exhibit convergent evolution in host behavioral manipulation but use disparate molecular mechanisms. • Targets for affecting host behavior include manipulation of host hormones, feeding, locomotion, and immune, circadian, and neurological pathways.

Expression of a viral ecdysteroid UDP-glucosyltransferase enhanced the insecticidal activity of the insect pathogenic fungus Beauveria bassiana

BACKGROUND

Entomopathogenic fungi, such as Beauveria bassiana, hold promise as biological control agents against insect pests. However, the efficacy of these fungi can be hindered by insect immune responses. One strategy to enhance fungal virulence is to manipulate host immune by targeting key regulatory molecules like 20-hydroxyecdysone (20E).

RESULTS

In this study, we engineered B. bassiana strains to constitutively express the enzyme ecdysteroid UDP-glucosyltransferase (EGT), which inactivates 20E, a crucial insect molting hormone. The engineered strain Bb::EGT-1 exhibited robust expression of EGT, leading to a significant reduction in insect 20E levels upon infection. Moreover, infection with Bb::EGT-1 resulted in accelerated larval mortality. Immune responses analysis revealed repression of insect immune response genes and decreased phenoloxidase (PO) activity in larvae infected with Bb::EGT-1. Microbiome analysis indicated alterations in bacterial composition within infected insects, with increased abundance observed during infection with Bb::EGT-1. Additionally, the presence of bacteria hindered hyphal emergence from insect cadavers, suggesting a role for microbial competition in fungal dissemination.

CONCLUSIONS

Constitutive expression of EGT in B. bassiana enhances fungal virulence by reducing insect 20E levels, suppressing immune responses, and altering the insect microbiome. These findings highlighted the potential of engineered fungi as effective biocontrol agents against insect pests and provide insights into the complex interactions between entomopathogenic fungi, their hosts, and associated microbes. © 2024 Society of Chemical Industry.

Transcriptome and proteome analyses reveal genes and signaling pathways involved in the response to two insect hormones in the insect-fungal pathogen Hirsutella satumaensis

The insect hormones ecdysone (20E) and juvenile hormone III (JH) have been demonstrated to stimulate the secretion of conidia mucilage and pigments in Hirsutella satumaensis. However, the underlying mechanisms remain elusive. Here, comparative transcriptome and proteome analyses were performed to identify the fungal genes and proteins of H. satumaensis that are up- or downregulated in response to insect hormones. A total of 17,407 unigenes and 1,016 proteins in conidia mucilage were identified. The genes involved in response to the hormones were classified into four functional groups: (1) stress response-related genes that are required for the removal of reactive oxygen species (glutathione synthetase, c7144) and genes involved in the response to osmotic stress in the hemocoel, such as those encoding proteins involved in the G, mTOR, and MAPK signaling pathways (2); insect hormone metabolic genes, including genes encoding ecdysteroid UDP-glucosyltransferase, ecdysteroid-22-kinase, and a key aldehyde dehydrogenase in a juvenile hormone synthesis pathway (3); secretory proteins that share homology with those of the host Bombyx mori, including fibrohexamerin, sericin 1, metalloprotease 1 protein, and silk gum protein, which were revealed by the omics data; and (4) proteins related to amino sugar metabolism and oxidative phosphorylation that were specifically expressed in mucilage in response to 20E and JH, respectively. These findings revealed that H. satumaensis can mount effective responses by modulating the expression of genes involved in the detoxification, adaptation, and evasion of insect hormone-mediated immune responses, providing fresh insights into fungal pathogen-host insect interactions.IMPORTANCEInsect hormones are highly important for the regulation of insect growth, development, and immune system function. Thus, the expansion of entomopathogenic fungi (EPF) could be affected by these hormones when they inhabit the host hemocoel. However, the molecular basis of EPF in response to insect hormones has yet to be determined. Our results revealed that EPF are impacted by 20E and JH, both of which act as signals, as these hormones lead to changes in metabolic pathways of the fungus, thus demonstrating a direct relationship between the fungus and the hormones. Furthermore, adaptive strategies, such as the use of ecdysone-inactivating enzymes and secreted filamentous proteins in H. satumaensis, which strongly resemble those of the host insect, have been discovered, thus illustrating the importance of adaptation to insect hormones for a better understanding of the interaction between insects and EPF.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

Virulence of Metarhizium rileyi Is Determined by Its Growth and Antioxidant Stress and the Protective and Detoxifying Enzymes of Spodoptera frugiperda

Spodoptera frugiperda is one of the most destructive crop pests in the world. Metarhizium rileyi is an entomopathogenic fungus specific for noctuid pests and is a very promising prospect in biological control against S. frugiperda. Two M. rileyi strains (XSBN200920 and HNQLZ200714) isolated from infected S. frugiperda were used to evaluate the virulence and biocontrol potential to different stages and instars of S. frugiperda. The results showed that XSBN200920 was significantly more virulent than HNQLZ200714 to eggs, larvae, pupae, and adults of S. frugiperda. In the larvae infected with the two M. rileyi strains, the activity of three protective enzymes (including peroxidase (POD), superoxide dismutase (SOD), catalase (CAT)) and two detoxifying enzymes (including glutathione-S transferase (GST) and carboxylesterase (CarE)) increased firstly and then decreased. The expression levels of protective enzymes and detoxification enzymes in larvae treated with XSBN200920 were greater than with HNQLZ200714. Furthermore, antioxidant stress-related gene (MrSOD and MrCAT family genes) expression in the two strains was measured by RT-qPCR (real-time quantitative PCR). The expression of these genes was significantly higher in the XSBN200920 strain compared to HNQLZ200714. There were also significant differences in the sensitivity of the two strains to the growth of different carbon and nitrogen sources and oxidative stress agents. In addition, the activity expression of antioxidant enzymes on the third day of culturing in XSBN200920 was significantly higher than with HNQLZ200714. In summary, the high virulence of M. rileyi XSBN200920 was not only determined by the expression levels of protective and detoxifying enzymes of the host but also regulated by the growth of entomogenic fungi and the resistance to the oxidative stress against S. frugiperda at different stages and instars. This study provides a theoretical fundament for the systematic control of Spodoptera frugiperda using Metarhizium rileyi.

Sensing of a spore surface protein by a Drosophila chemosensory protein induces behavioral defense against fungal parasitic infections

In addition to innate immunity in a physiological context, insects have evolved behavioral defenses against parasite attacks. Here, we report that Drosophila can sense the CFEM (common in fungal extracellular membrane) protein Mcdc9, which acts as a negative virulence factor of the entomopathogenic fungus Metarhizium robertsii. The individual deletions of 18 CFEM genes in Metarhizium followed by fly infection identified three null mutants that could kill the flies more quickly than the wild-type strain, among which Mcdc9 can coat fungal spores and interact with the fly chemosensory protein CheA75a. The deletion of Mcdc9 in the fungus or the knockdown of CheA75a in flies had a similar effect, in which a greater number of fungal spores were left on flies than on the respective controls after topical infection. Thus, similar to the accelerated death of the wild-type flies treated with ΔMcdc9, the CheA75a^RNAi flies succumbed more quickly than the control insects topically challenged with the wild-type strain. The CheA75a gene is highly transcribed in fly legs and wings, and positive electrophysiological responses were evidenced in tarsal sensilla after stimulation with the Mcdc9 protein. The results imply that this CFEM protein could be sensed as a contact elicitor inducing the hygienic behavior of flies against fungal parasitic infection, which reveals a previously unsuspected mechanism of fungus-insect interactions.