Locust Bacterial Symbionts: An Update

Gut microorganisms of Locusta migratoria in various life stages and its possible influence on cellulose digestibility

Locusta migratoria is an important phytophagous pest, and its gut microbial communities play an important role in cellulose degradation. In this study, the gut microbial and cellulose digestibility dynamics of Locusta migratoria were jointly analyzed using high-throughput sequencing and anthrone colorimetry. The results showed that the gut microbial diversity and cellulose digestibility across life stages were dynamically changing. The species richness of gut bacteria was significantly higher in eggs than in larvae and imago, the species richness and cellulose digestibility of gut bacteria were significantly higher in early larvae (first and second instars) than in late larvae (third to fifth instars), and the diversity of gut bacteria and cellulose digestibility were significantly higher in imago than in late larvae. There is a correlation between the dynamics of gut bacterial communities and cellulose digestibility. Enterobacter, Lactococcus, and Pseudomonas are the most abundant genera throughout all life stages. Six strains of highly efficient cellulolytic bacteria were screened, which were dominant gut bacteria. Carboxymethyl cellulase activity (CMCA) and filter paper activity (FPA) experiments revealed that Pseudomonas had the highest cellulase enzyme activity. This study provides a new way for the screening of cellulolytic bacteria and lays the foundation for developing insects with significant biomass into cellulose-degrading bioreactors.

IMPORTANCE

Cellulose is the most abundant and cheapest renewable resource in nature, but its degradation is difficult, so finding efficient cellulose degradation methods is an urgent challenge. Locusta migratoria is a large group of agricultural pests, and the large number of microorganisms that inhabit their intestinal tracts play an important role in cellulose degradation. We analyzed the dynamics of Locusta migratoria gut microbial communities and cellulose digestibility using a combination of high-throughput sequencing technology and anthrone colorimetry. The results revealed that the gut microbial diversity and cellulose digestibility were dynamically changed at different life stages. In addition, we explored the intestinal bacterial community of Locusta migratoria across life stages and its correlation with cellulose digestibility. The dominant bacterial genera at different life stages of Locusta migratoria were uncovered and their carboxymethyl cellulase activity (CMCA) and filter paper activity (FPA) were determined. This study provides a new avenue for screening cellulolytic bacteria and lays the foundation for developing insects with significant biomass into cellulose-degrading bioreactors.

Microbial associates of the elm leaf beetle: uncovering the absence of resident bacteria and the influence of fungi on insect performance

Microbial symbionts play crucial roles in the biology of many insects. While bacteria have been the primary focus of research on insect-microbe symbiosis, recent studies suggest that fungal symbionts may be just as important. The elm leaf beetle (ELB, Xanthogaleruca luteola) is a serious pest species of field elm (Ulmus minor). Using culture-dependent and independent methods, we investigated the abundance and species richness of bacteria and fungi throughout various ELB life stages and generations, while concurrently analyzing microbial communities on elm leaves. No persistent bacterial community was found to be associated with the ELB or elm leaves. By contrast, fungi were persistently present in the beetle's feeding life stages and on elm leaves. Fungal community sequencing revealed a predominance of the genera Penicillium and Aspergillus in insects and on leaves. Culture-dependent surveys showed a high prevalence of two fungal colony morphotypes closely related to Penicillium lanosocoeruleum and Aspergillus flavus. Among these, the Penicillium morphotype was significantly more abundant on feeding-damaged compared with intact leaves, suggesting that the fungus thrives in the presence of the ELB. We assessed whether the detected prevalent fungal morphotypes influenced ELB's performance by rearing insects on (i) surface-sterilized leaves, (ii) leaves inoculated with Penicillium spores, and (iii) leaves inoculated with Aspergillus spores. Insects feeding on Penicillium-inoculated leaves gained more biomass and tended to lay larger egg clutches than those consuming surface-sterilized leaves or Aspergillus-inoculated leaves. Our results demonstrate that the ELB does not harbor resident bacteria and that it might benefit from associating with Penicillium fungi.IMPORTANCEOur study provides insights into the still understudied role of microbial symbionts in the biology of the elm leaf beetle (ELB), a major pest of elms. Contrary to expectations, we found no persistent bacterial symbionts associated with the ELB or elm leaves. Our research thus contributes to the growing body of knowledge that not all insects rely on bacterial symbionts. While no persistent bacterial symbionts were detectable in the ELB and elm leaf samples, our analyses revealed the persistent presence of fungi, particularly Penicillium and Aspergillus on both elm leaves and in the feeding ELB stages. Moreover, when ELB were fed with fungus-treated elm leaves, we detected a potentially beneficial effect of Penicillium on the ELB's development and fecundity. Our results highlight the significance of fungal symbionts in the biology of this insect.

Analysis of gut microbiota of ladybug beetle (Harmonia axyridis) after feeding on different artificial diets

BACKGROUND

Harmonia axyridis is an effective natural enemy insect to a variety of phloem-sucking pests and Lepidopteran larvae, such as aphids, scabies, and phylloxera, while its industrial production is limited due to unmature artificial diet. Insect intestinal microbiota affect host development and reproduction. The aim of this study is to understand intestinal microbiota composition of H. axyridis and screen effective probiotics on artificial diet. Considering the role of the components and composition of the diet on the structure and composition of the intestinal microbiome, four kinds of diets were set up: (1) aphid; (2) basic diet; (3) basic diet + glucose; (4) basic diet + trehalose. The gut microbiota of H. axyridis was detected after feeding on different diets.

RESULTS

Results showed that the gut microbiota between artificial diet group and aphid groups were far apart, while the basic and glucose groups were clearly clustered. Besides, the glucose group and trehalose group had one unique phylum, Cryptophyta and Candidatus Saccharibacteria, respectively. The highest abundance of Proteobacteria was found in the aphid diet. The highest abundance of Firmicutes was found in the basic diet. However, the addition of glucose or trehalose alleviated the change. In addition, the relative abundance of Enterobacter, Klebsiella, Enterobacteriaceae_unclassified, Enterobacteriales_unclassified and Serratia in the aphid group was higher than other groups. Moreover, the function of gut genes in each group also showed clear differences.

CONCLUSION

These results have offered a strong link between artificial diets and gut microbes, and also have provided a theoretical basis for the screening of synergistic probiotics in artificial diet.

Insect Freeze-Tolerance Downunder: The Microbial Connection

Insects that are freeze-tolerant start freezing at high sub-zero temperatures and produce small ice crystals. They do this using ice-nucleating agents that facilitate intercellular ice growth and prevent formation of large crystals where they can damage tissues. In Aotearoa/New Zealand the majority of cold adapted invertebrates studied survive freezing at any time of year, with ice formation beginning in the rich microbiome of the gut. Some freeze-tolerant insects are known to host symbiotic bacteria and/or fungi that produce ice-nucleating agents and we speculate that gut microbes of many New Zealand insects may provide ice-nucleating active compounds that moderate freezing. We consider too the possibility that evolutionary disparate freeze-tolerant insect species share gut microbes that are a source of ice-nucleating agents and so we describe potential transmission pathways of shared gut fauna. Despite more than 30 years of research into the freeze-tolerant mechanisms of Southern Hemisphere insects, the role of exogenous ice-nucleating agents has been neglected. Key traits of three New Zealand freeze-tolerant lineages are considered in light of the supercooling point (temperature of ice crystal formation) of microbial ice-nucleating particles, the initiation site of freezing, and the implications for invertebrate parasites. We outline approaches that could be used to investigate potential sources of ice-nucleating agents in freeze-tolerant insects and the tools employed to study insect microbiomes.

Microbiome-Driven Proline Biogenesis in Plants under Stress: Perspectives for Balanced Diet to Minimize Depression Disorders in Humans

According to the World Health Organization (WHO), depression is a leading cause of disability worldwide and a major contributor to the overall global burden of mental disorders. An increasing number of studies have revealed that among 20 different amino acids, high proline consumption is a dietary factor with the strongest impact on depression in humans and animals, including insects. Recent studies acknowledged that gut microbiota play a key role in proline-related pathophysiology of depression. In addition, the multi-omics approach has alleged that a high level of metabolite proline is directly linked to depression severity, while variations in levels of circulating proline are dependent on microbiome composition. The gut–brain axis proline analysis is a gut microbiome model of studying depression, highlighting the critical importance of diet, but nothing is known about the role of the plant microbiome–food axis in determining proline concentration in the diet and thus about preventing excessive proline intake through food consumption. In this paper, we discuss the protocooperative potential of a holistic study approach combining the microbiota–gut–brain axis with the microbiota–plant–food–diet axis, as both are involved in proline biogenesis and metabolism and thus on in its effect on mood and cognitive function. In preharvest agriculture, the main scientific focus must be directed towards plant symbiotic endophytes, as scavengers of abiotic stresses in plants and modulators of high proline concentration in crops/legumes/vegetables under climate change. It is also implied that postharvest agriculture—including industrial food processing—may be critical in designing a proline-balanced diet, especially if corroborated with microbiome-based preharvest agriculture, within a circular agrifood system. The microbiome is suggested as a target for selecting beneficial plant endophytes in aiming for a balanced dietary proline content, as it is involved in the physiology and energy metabolism of eukaryotic plant/human/animal/insect hosts, i.e., in core aspects of this amino acid network, while opening new venues for an efficient treatment of depression that can be adapted to vast groups of consumers and patients. In that regard, the use of artificial intelligence (AI) and molecular biomarkers combined with rapid and non-destructive imaging technologies were also discussed in the scope of enhancing integrative science outcomes, agricultural efficiencies, and diagnostic medical precisions.

Microbial gut diversity in four grasshopper species and its correlation with cellulose digestibility

Grasshoppers are common pests, and their intestinal microbes have coevolved with them. These microorganisms have varied community structures, and they participate in the nutritional absorption and metabolism of grasshoppers. Here, we describe the gut microbiota diversity of four species of grasshoppers, Oxya chinensis, Pararcyptera microptera meridionalis, Gastrimargus marmoratus, and Calliptamus abbreviatus. We constructed a 16S rDNA gene library and analyzed the digestibility of cellulose and hemicellulose in grasshoppers using moss black phenol and anthrone colorimetry. The grasshopper with the highest microbial diversity in the gut among the four species was Oxya chinensis, and there were no significant differences in gut microbial diversity between the two geographic collections of Oxya chinensis. The most dominant phyla of the four grasshopper gut microorganisms were Proteobacteria, Bacteroidetes, and Firmicutes, and the most dominant genus was Enterobacter. The gut microbiota features of the four grasshoppers were correlated with their cellulose and hemicellulose digestibility. There was a significant positive correlation with cellulose digestibility for Pantoea. A significant negative correlation was found with cellulose digestibility for Acinetobacter, Enterococcus, Citrobacter, Serratia. A significant negative correlation was found with hemicellulose digestibility for Pantoea. This study contributes to the understanding of the structural composition of different species of grasshoppers gut microbiota, which may be useful for developing grasshopper digestive tracts as bioreactors for cellulose decomposition, improving the decomposition and utilization of agricultural straw, producing clean biomass energy, and processing biologically derived products.

Analysis of Intestinal Microbial Diversity of Four Species of Grasshoppers and Determination of Cellulose Digestibility

Simple Summary

Grasshoppers are typical phytophagous pests, which prefer eating monocotyledons with more cellulose and hemicellulose. Due to its large appetite and high utilization rate, the intestinal contents of grasshoppers have the potential to be developed into a bioreactor, which can be applied to improve straw utilization efficiency in the future. The digestive tract of grasshoppers is a complex ecosystem, inhabited by a large number of microorganisms. The existence of these microorganisms enables grasshoppers to have high decomposition and utilization of plant fibers. However, there are few reports on the microflora structure and diversity of the digestive tract of grasshoppers. In this study, the diversity of symbiotic bacteria in the intestinal tract of four species of grasshoppers, namely Acrida cinerea, Trilophidia annulata, Atractomorpha sinensis and Sphingonotus mongolicus, was studied by using the method of constructing a 16S rRNA gene library and Illumina Miseq sequencing technology. At the same time, the digestibility of cellulose and hemicellulose of the four species of grasshoppers were determined and the relationship between digestibility and intestinal microbial diversity was analyzed. This study provided basic data for the development of the digestible bioreactor of cellulose and hemicellulose, which may provide a new idea for degrading straw.

Abstract

Grasshoppers (Insecta, Orthoptera, Acridoidea) are a large group of agricultural and animal husbandry pests. They have a large food intake with high utilization of plants fibers. However, the composition of the grasshopper gut microbial community, especially the relationship between gut microbial community and cellulose digestibility, remains unclear. In this research, 16S rRNA gene sequences were used to determine the intestinal microbial diversity of Acrida cinerea, Trilophidia annulata, Atractomorpha sinensis and Sphingonotus mongolicus, and Spearman correlation analysis was performed between the intestinal microbes of grasshoppers and the digestibility of cellulose and hemicellulose. The results showed that Proteobacteria was the dominant phylum and Klebsiella was the dominant genus in the guts of the four species of grasshoppers; there was no significant difference in the species composition of the gut microbes of the four species of grasshoppers. Spearman correlation analysis showed that Brevibacterium and Stenotrophomonas were significantly correlated with cellulose digestibility. Brevibacterium, Clavibacter, Microbacterium and Stenotrophomonas were significantly associated with hemicellulose digestibility. Our results confirmed that the gut microbes of grasshoppers were correlated with the digestibility of cellulose and hemicellulose, and indicated that grasshoppers may have the potential to develop into bioreactors, which can be applied to improve straw utilization efficiency in the future.

Variation of TNF modulates cellular immunity of gregarious and solitary locusts against fungal pathogen Metarhizium anisopliae

Ecological immunology addresses the interactions between host immunity and the environment. Locusts display density-dependent phase transitions between solitary and gregarious locusts. In control practices and laboratory bioassays, gregarious locusts always exhibit stronger resistance to fungal pathogens than solitary locusts. However, few studies have investigated the mechanism of altered immune switch in locusts. Here, we combined mathematical simulation and experimental studies to show that gregarious locusts inhibit tumor necrosis factor (TNF) to alter immune defense by enhancing humoral defense and reducing cellular defense, and high levels of TNF reduce the survival of solitary locusts. Our study provides an important cue for understanding cellular immunity variations in response to different population densities and for improving the control efficacy of locust plagues.

Changes in population density lead to phenotypic differentiation of solitary and gregarious locusts, which display different resistance to fungal pathogens; however, how to regulate their cellular immune strategies remains unknown. Here, our stochastic simulation of pathogen proliferation suggested that humoral defense always enhanced resistance to fungal pathogens, while phagocytosis sometimes reduced defense against pathogens. Further experimental data proved that gregarious locusts had significantly decreased phagocytosis of hemocytes compared to solitary locusts. Additionally, transcriptional analysis showed that gregarious locusts promoted immune effector expression (gnbp1 and dfp) and reduced phagocytic gene expression (eater) and the cytokine tumor necrosis factor (TNF). Interestingly, higher expression of the cytokine TNF in solitary locusts simultaneously promoted eater expression and inhibited gnbp1 and dfp expression. Moreover, inhibition of TNF increased the survival of solitary locusts, and injection of TNF decreased the survival of gregarious locusts after fungal infection. Therefore, our results indicate that the alerted expression of TNF regulated the immune strategy of locusts to adapt to environmental changes.

Chemical Ecology and Olfaction in Short-Horned Grasshoppers (Orthoptera: Acrididae)

Chemoreception plays a crucial role in the reproduction and survival of insects, which often rely on their sense of smell and taste to find partners, suitable habitats, and food sources, and to avoid predators and noxious substances. There is a substantial body of work investigating the chemoreception and chemical ecology of Diptera (flies) and Lepidoptera (moths and butterflies); but less is known about the Orthoptera (grasshoppers, locusts, crickets, and wēta). Within the Orthoptera, the family Acrididae contains about 6700 species of short-horned grasshoppers. Grasshoppers are fascinating organisms to study due to their significant taxonomic and ecological divergence, however, most chemoreception and chemical ecology studies have focused on locusts because they are agricultural pests (e.g., Schistocerca gregaria and Locusta migratoria). Here we review studies of chemosensory systems and chemical ecology of all short-horned grasshoppers. Applications of genome editing tools and entomopathogenic microorganism to control locusts in association with their chemical ecology are also discussed. Finally, we identify gaps in the current knowledge and suggest topics of interest for future studies.

An Overview of Antimicrobial Compounds from African Edible Insects and Their Associated Microbiota

The need for easily biodegradable and less toxic chemicals in drug development and pest control continues to fuel the exploration and discovery of new natural molecules. Like certain plants, some insects can also respond rapidly to microbial infections by producing a plethora of immune-induced molecules that include antibacterial and antifungal peptides/polypeptides (AMPs), among other structurally diverse small molecules. The recent recognition that new natural product-derived scaffolds are urgently needed to tackle life-threatening pathogenic infections has been prompted by the health threats posed by multidrug resistance. Although many researchers have concentrated on the discovery of AMPs, surprisingly, edible insect-produced AMPs/small molecules have received little attention. This review will discuss the recent advances in the identification and bioactivity analysis of insect AMPs, with a focus on small molecules associated with the microbiota of selected African edible insects. These molecules could be used as templates for developing next-generation drugs to combat multidrug-resistant pathogens.

The maternal foam plug constitutes a reservoir for the desert locust's bacterial symbionts

A hallmark of the desert locust's ancient and deserved reputation as a devastating agricultural pest is that of the long-distance, multi-generational migration of locust swarms to new habitats. The bacterial symbionts that reside within the locust gut comprise a key aspect of its biology, augmenting its immunity and having also been reported to be involved in the swarming phenomenon through the emission of attractant volatiles. However, it is still unclear whether and how these beneficial symbionts are transmitted vertically from parent to offspring. Using comparative 16S rRNA amplicon sequencing and direct experiments with engineered bacteria, we provide evidence for vertical transmission of locust gut bacteria. The females may perform this activity by way of inoculation of the egg-pod's foam plug, through which the larvae pass upon hatching. Furthermore, analysis of the composition of the foam revealed chitin to be its major component, along with immunity-related proteins such as lysozyme, which could be responsible for the inhibition of some bacteria in the foam while allowing other, more beneficial, strains to proliferate. Our findings reveal a potential vector for the transgenerational transmission of symbionts in locusts, which contributes to the locust swarm's ability to invade and survive in new territories.

First draft genome assembly of the desert locust, Schistocerca gregaria

Background: At the time of publication, the most devastating desert locust crisis in decades is affecting East Africa, the Arabian Peninsula and South-West Asia. The situation is extremely alarming in East Africa, where Kenya, Ethiopia and Somalia face an unprecedented threat to food security and livelihoods. Most of the time, however, locusts do not occur in swarms, but live as relatively harmless solitary insects. The phenotypically distinct solitarious and gregarious locust phases differ markedly in many aspects of behaviour, physiology and morphology, making them an excellent model to study how environmental factors shape behaviour and development. A better understanding of the extreme phenotypic plasticity in desert locusts will offer new, more environmentally sustainable ways of fighting devastating swarms. Methods: High molecular weight DNA derived from two adult males was used for Mate Pair and Paired End Illumina sequencing and PacBio sequencing. A reliable reference genome of Schistocerca gregaria was assembled using the ABySS pipeline, scaffolding was improved using LINKS. Results: In total, 1,316 Gb Illumina reads and 112 Gb PacBio reads were produced and assembled. The resulting draft genome consists of 8,817,834,205 bp organised in 955,015 scaffolds with an N50 of 157,705 bp, making the desert locust genome the largest insect genome sequenced and assembled to date. In total, 18,815 protein-encoding genes are predicted in the desert locust genome, of which 13,646 (72.53%) obtained at least one functional assignment based on similarity to known proteins. Conclusions: The desert locust genome data will contribute greatly to studies of phenotypic plasticity, physiology, neurobiology, molecular ecology, evolutionary genetics and comparative genomics, and will promote the desert locust's use as a model system. The data will also facilitate the development of novel, more sustainable strategies for preventing or combating swarms of these infamous insects.

First draft genome assembly of the desert locust, Schistocerca gregaria

Background: At the time of publication, the most devastating desert locust crisis in decades is affecting East Africa, the Arabian Peninsula and South-West Asia. The situation is extremely alarming in East Africa, where Kenya, Ethiopia and Somalia face an unprecedented threat to food security and livelihoods. Most of the time, however, locusts do not occur in swarms, but live as relatively harmless solitary insects. The phenotypically distinct solitarious and gregarious locust phases differ markedly in many aspects of behaviour, physiology and morphology, making them an excellent model to study how environmental factors shape behaviour and development. A better understanding of the extreme phenotypic plasticity in desert locusts will offer new, more environmentally sustainable ways of fighting devastating swarms. Methods: High molecular weight DNA derived from two adult males was used for Mate Pair and Paired End Illumina sequencing and PacBio sequencing. A reliable reference genome of Schistocerca gregaria was assembled using the ABySS pipeline, scaffolding was improved using LINKS. Results: In total, 1,316 Gb Illumina reads and 112 Gb PacBio reads were produced and assembled. The resulting draft genome consists of 8,817,834,205 bp organised in 955,015 scaffolds with an N50 of 157,705 bp, making the desert locust genome the largest insect genome sequenced and assembled to date. In total, 18,815 protein-encoding genes are predicted in the desert locust genome, of which 13,646 (72.53%) obtained at least one functional assignment based on similarity to known proteins. Conclusions: The desert locust genome data will contribute greatly to studies of phenotypic plasticity, physiology, neurobiology, molecular ecology, evolutionary genetics and comparative genomics, and will promote the desert locust's use as a model system. The data will also facilitate the development of novel, more sustainable strategies for preventing or combating swarms of these infamous insects.