Stress-induced changes in abundance differ among obligate and facultative endosymbionts of the soybean aphid

Effects of fungicides on fitness and Buchnera endosymbiont density in Aphis gossypii

BACKGROUND

Several agricultural fungicides are known to affect insect pests directly and these effects may be transgenerational and mediated through impacts on endosymbionts, providing opportunities for pest control. The cotton aphid Aphis gossypii is a polyphagous pest that can cause large crop yield losses. Here, we tested the effects of three fungicides, pyraclostrobin, trifloxystrobin and chlorothalonil, on the fitness and Buchnera endosymbiont of A. gossypii.

RESULTS

The formulations of trifloxystrobin and pyraclostrobin, and the active ingredient of pyraclostrobin produced dose-dependent mortality in A. gossypii, whereas there was no dose-dependent mortality for chlorothalonil. The formulations of trifloxystrobin and pyraclostrobin significantly reduced the lifespan and fecundity of A. gossypii, and increased the density of Buchnera in the parental generation but not the (unexposed) F1 . When the active ingredient of pyraclostrobin was tested, the lifespan of the F0 generation was also reduced, but the density of Buchnera was not, indicating that non-insecticidal chemicals in the fungicide formulation may affect the density of the endosymbiont of A. gossypii. There was no transgenerational effect of the active ingredient of pyraclostrobin on the lifespan and Buchnera of (unexposed) F1 .

CONCLUSIONS

Our results suggest that formulations of two strobilurin fungicides have immediate impacts on the fitness of A. gossypii, and chemicals in the formulation impact the density of the primary Buchnera endosymbiont. Our study highlights the potential effects of non-insecticidal chemicals of fungicides on aphid pests and their primary endosymbionts but direct connections between fitness and Buchnera densities remain unclear. © 2023 Society of Chemical Industry.

Sublethal phosphine fumigation induces transgenerational hormesis in a factitious host, Corcyra cephalonica

BACKGROUND

The rice moth, Corcyra cephalonica (Stainton) (Lepidoptera: Pyralidae) is a pest of stored grains and widely used as a factitious host during the mass rearing of several natural enemies of crop pests. Hormesis is well-documented in pest insects, to some extent in natural enemies of pests.

RESULTS

We report transgenerational stimulatory effects of the widely used fumigant, phosphine. The study reports the consequences of sublethal, low lethal and median lethal concentrations (LC5 , LC25 and LC50 ) and untreated control for two sequential generations of the species (G1 to G2 ). In this study, we investigated the life-history traits, nutrient reserves (protein, lipid and carbohydrate) and larval gut microbiome (using 16 s rRNA V3-V4 metagenomics sequencing) of C. cephalonica. Stimulatory effects were observed for various biological traits of C. cephalonica, notably adult longevity, emergence and increased egg hatchability when exposed to LC5 of phosphine. The total protein, lipid and carbohydrate contents of C. cephalonica also were found to be significantly increased by LC5 in both generations. The microbial diversity of LC5 treated larval gut was higher and found to be different from the rest of the treatments. This is the first report showing hormesis to a fumigant insecticide.

CONCLUSION

Our findings increase knowledge on the interaction between hormesis, nutrient reserves and gut bacteria in C. cephalonica exposed to insecticides. Overall, the present study establishes phosphine-induced hormesis at LC5 in the host C. cephalonica, which might help improve the quality of mass rearing of various natural enemies. © 2023 Society of Chemical Industry.

Arsenophonus Interacts with Buchnera to Improve Growth Performance of Aphids under Amino Acid Stress

Amino acids play a crucial role in the growth and development of insects. Aphids cannot ingest enough amino acids in plant phloem to meet their requirements, and therefore, they are mainly dependent on the obligate symbiont Buchnera aphidicola to synthesize essential amino acids. Besides Buchnera, aphids may harbor another facultative symbiont, Arsenophonus, which alters the requirement of the cotton-melon aphid Aphis gossypii for amino acid. However, it is unclear how Arsenophonus regulates the requirement. Here, we found that Arsenophonus ameliorated growth performance of A. gossypii on an amino acid-deficient diet. A deficiency in lysine (Lys) or methionine (Met) led to changes in the abundance of Arsenophonus. Arsenophonus suppressed the abundance of Buchnera when aphids were fed a normal amino acid diet, but this suppression was eliminated or reversed when aphids were on a Lys- or Met-deficient diet. The relative abundance of Arsenophonus was positively correlated with that of Buchnera, but neither of them was correlated with the body weight of aphids. The relative expression levels of Lys and Met synthase genes of Buchnera were affected by the interaction between Arsenophonus infections and Buchnera abundance, especially in aphids reared on a Lys- or Met-deficient diet. Arsenophonus coexisted with Buchnera in bacteriocytes, which strengthens the interaction. IMPORTANCE The obligate symbiont Buchnera can synthesize amino acids for aphids. In this study, we found that a facultative symbiont, Arsenophonus, can help improve aphids' growth performance under amino acid deficiency stress by changing the relative abundance of Buchnera and the expression levels of amino acid synthase genes. This study highlights the interaction between Arsenophonus and Buchnera to ameliorate aphid growth under amino acid stress.

Ampicillin enhanced the resistance of Myzus persicae to imidacloprid and cyantraniliprole

BACKGROUND

Recent studies have shown that symbionts are involved in regulating insecticide detoxification in insects. However, there are few studies on the relationship between the symbionts found in Myzus persicae and the mechanism underlying host detoxification of insecticides. In this study, antibiotic ampicillin treatment was used to investigate the possible relationship between symbiotic bacteria and the detoxification of insecticides in the host, M. persicae.

RESULTS

Bioassays showed that ampicillin significantly reduced the susceptibilities of M. persicae to imidacloprid and cyantraniliprole. Synergistic bioassays and RNAi assays showed that the susceptibilities of M. persicae to imidacloprid and cyantraniliprole were related to metabolic detoxification enzyme activities and the expression level of the cytochrome P450 gene, CYP6CY3. Also, treatment to a combination of ampicillin and enzyme inhibitors or dsCYP6CY3 showed that the negative effect of ampicillin on the susceptibility of M. persicae was effectively inhibited bydetoxification enzyme inhibitors and dsCYP6CY3. Additionally, ampicillin treatment resulted in significant increases in the activities of multifunctional oxidases and esterases, the expression level of CYP6CY3 and fitness of M. persicae. Further, ampicillin significantly reduced the total bacterial abundance and changed symbiont diversity in M. persicae. The abundance of Pseudomonadaceae decreased significantly, while the abundance of Rhodococcus and Buchnera increased significantly.

CONCLUSION

Our study showed that ampicillin enhanced the resistance levels to imidacloprid and cyantraniliprole of M. persicae, which might be related to the selective elimination of symbiotic bacteria, the upregulated activities of detoxification enzymes and the increased fitness. © 2022 Society of Chemical Industry.

Rickettsia Infection Benefits Its Whitefly Hosts by Manipulating Their Nutrition and Defense

Endosymbionts play an essential role in the biology, physiology and immunity of insects. Many insects, including the whitefly Bemisia tabaci, are infected with the facultative endosymbiont Rickettsia. However, the mutualism between Rickettsia and its whitefly host remains unclear. This study investigated the biological and physiological benefits of Rickettsia infection to B. tabaci. Results revealed that infection of Rickettsia increased the fertility, the survival rate from nymph to adult and the number of female whiteflies. In addition, this facilitation caused a significant reduction in nymphal developmental duration but did not affect percentage rate of egg hatching. Rickettsia infected B. tabaci had significantly higher glycogen, soluble sugar and trehalose contents than Rickettsia negative B. tabaci individuals. Rickettsia also improved the immunity of its whitefly hosts. Rickettsia infested B. tabaci had lower mortality rates and higher semi-lethal concentrations (LC50) when exposed to the fungus Akanthomyces attenuatus and the insecticides imidacloprid and spirotetramat. The percentage of parasitism by Encarsia formosa was also reduced by Rickettsia infection. Overall, Rickettsia infection benefits B. tabaci by improving the nutritional composition of its host, and also protects B. tabaci by enhancing its resistance towards insecticides (imidacloprid and spirotetramat), entomopathogenic fungi (A. attenuatus) and its main parasitoid (E. formosa); all of which could significantly impact on current management strategies.

Microbiome Analysis of the Bamboo Aphid Melanaphis bambusae Infected with the Aphid Obligate Pathogen Conidiobolus obscurus (Entomophthoromycotina)

Insect-associated microbes exert diverse effects on host fitness. This study provides insights into the microbiota of the bamboo aphid, Melanaphis bambusae, and their response to Conidiobolus obscurus infection. 16S rRNA and ITS sequencing data were used to analyze the bacterial and fungal samples associated with healthy, infected, and starved aphids. At ≥97% nucleotide similarity, the total reads were clustered into 79 bacteria and 97 fungi operational Taxonomic Units (OTUs). The phyla Proteobacteria and Ascomycota dominated the bacterial and fungal communities, respectively. The significant divergence in OTU distribution presented differential profiles of the microbiota in response to host conditions. Lower α-diversity indices were found in bacterial and fungal diversity when the aphids were experiencing fungal infection and starvation stresses, respectively. The β-diversity analyses of the communities showed significant differences among the three host conditions, demonstrating that aphid-associated microbiota could significantly shift in response to varying host conditions. Moreover, some OTUs increased under fungal infection, which potentially increased aphid susceptibility. Presumably, C. obscurus infection contributed to this increase by causing the disintegration of host tissues other than host starvation. In conclusion, understanding the differentiation of aphid microbiota caused by fungal entomopathogens helped facilitate the development of novel pest management strategies.

SymbiQuant: A Machine Learning Object Detection Tool for Polyploid Independent Estimates of Endosymbiont Population Size

Quantifying the size of endosymbiont populations is challenging because endosymbionts are typically difficult or impossible to culture and commonly polyploid. Current approaches to estimating endosymbiont population sizes include quantitative PCR (qPCR) targeting endosymbiont genomic DNA and flow-cytometry. While qPCR captures genome copy number data, it does not capture the number of bacterial cells in polyploid endosymbiont populations. In contrast, flow cytometry can capture accurate estimates of whole host-level endosymbiont population size, but it is not readily able to capture data at the level of endosymbiotic host cells. To complement these existing approaches for estimating endosymbiont population size, we designed and implemented an object detection/segmentation tool for counting the number of endosymbiont cells in micrographs of host tissues. The tool, called SymbiQuant, which makes use of recent advances in deep neural networks includes a graphic user interface that allows for human curation of tool output. We trained SymbiQuant for use in the model aphid/Buchnera endosymbiosis and studied Buchnera population dynamics and phenotype over aphid postembryonic development. We show that SymbiQuant returns accurate counts of endosymbionts, and readily captures Buchnera phenotype. By replacing our training data with data composed of annotated microscopy images from other models of endosymbiosis, SymbiQuant has the potential for broad application. Our tool, which is available on GitHub, adds to the repertoire of methods researchers can use to study endosymbiosis at the organismal, genome, and now endosymbiotic host tissue or cell levels.

Endosymbiotic Bacteria Aid to Overcome Temperature Induced Stress in the Oriental Fruit Fly, Bactrocera dorsalis

Endosymbiotic microbiota are known to have an enormous impact on their host, influencing its physiology, behavior, fitness, and various other aspects. The present study hypothesizes that certain bacterial symbionts aid the Oriental fruit fly, Bactrocera dorsalis in its adaptation to survive thermal stress encountered in the environment. Investigative studies on the change in gut and reproductive tract microbiota diversity of male and female B. dorsalis revealed that certain genera of Acinetobacter, Brevibacillus, Bacillus, Enterobacter, Enterococcus, Pseudomonas, and Staphylococcus were involved in the adaptation of B. dorsalis to temperature stresses. The intestinal and reproductive tract bacterial community of B. dorsalis varied depending on the temperature the insects were reared at. We hypothesized that the microbiota present in B. dorsalis' gut helped it endure temperature stresses over prolonged periods. Out of 54 bacterial isolates, 25, 15, and 14 isolates were obtained from flies reared at 27 °C, 18 °C, and 35 °C, respectively. A 16S rDNA analysis revealed that the bacterial isolates (reared at different temperatures) belonged to different genera. The flies were supplemented with antibiotics to suppress the existing gut microbiota and subsequently fed with bacterial isolates from flies reared at 18 °C, 27 °C (control) or 35 °C separately. When these flies were placed in incubators pre-set at the above temperatures, the survival rate exhibited by the flies differed significantly. The flies fed with bacterial isolates from 18 °C could survive only in incubators pre-set at 18 °C, while flies fed with bacterial isolates from 35 °C could survive only at 35 °C and not vice versa. The microbiota supplementation assay established that the presence of specific bacterial isolates aided the flies' survival under varied thermal stresses.

Adaptive Variation of Buchnera Endosymbiont Density in Aphid Host Acyrthosiphon pisum Controlled by Environmental Conditions

The scarcity of transcriptional regulatory genes in Buchnera aphidicola, an obligate endosymbiont in aphids, suggests the stability of expressed gene patterns and metabolic pathways. This observation argues in favor of the hypothesis that this endosymbiont bacteria might contribute little to the host adaptation when aphid hosts are facing challenging fluctuating environment. Finding evidence for the increased expression or silenced genes involved in metabolic pathways under the pressure of stress conditions and/or a given environment has been challenging for experimenters with this bacterial symbiotic model. Transcriptomic data have shown that Buchnera gene expression changes are confined to a narrow range when the aphids face brutal environmental variations. In this report, we demonstrate that instead of manipulating individual genes, the conditions may act on the relative mass of endosymbiont corresponding to the needs of the host. The control of the fluctuating number of endosymbiont cells per individual host appears to be an unexpected regulatory modality that contributes to the adaptation of aphids to their environment. This feature may account for the success of the symbiotic advantages in overcoming the drastic changes in temperature and food supplies during evolution.

Evaluation of sublethal and transgenerational effects of sulfoxaflor on Aphis gossypii via life table parameters and 16S rRNA sequencing

BACKGROUND

Aphis gossypii, a polyphagous and recurrent pest induced by pesticides, causes tremendous loss crop yields each year. Previous studies on the mechanism of pesticide-induced sublethal effects mainly focus on the gene level. The symbiotic bacteria are also important participants of this mechanism, but their roles in hormesis are still unclear.

RESULTS

In this study, life table parameters and 16S rRNA sequencing were applied to evaluate the sublethal and transgenerational effects of sulfoxaflor on adult A. gossypii after 24-h LC₂₀ (6.96 mg L^-1 ) concentration exposure. The results indicated that the LC₂₀ of sulfoxaflor significantly reduced the finite rate of increase (λ) and net reproductive rate (R₀ ) of parent generation (G0), and significantly increased mean generation time (T) of G1 and G2, but not of G3 and G4. Both reproductive period and fecundity of G1 and G2 were significantly higher than those of the control. Furthermore, our sequencing data revealed that more than 95% bacterial communities were dominated by the phylum Proteobacteria, in which the maximum proportion genus was the primary symbiont Buchnera and the facultative symbiont Arsenophonus. Compared to those of the control, the abundance and composition of symbiotic bacteria of A. gossypii for three successive generations (G0-G2) were changed after G0 A. gossypii was exposed to sulfoxaflor: the diversity of the bacterial community was decreased, but the abundance of Buchnera was increased (G0), while the abundance of Arsenophonus was decreased. Contrary to G0, G1 and G2 cotton aphid exhibited an increased relative abundance of Arsenophonus in the sublethal treatment group.

CONCLUSION

Taken together, our results provide an insight into the interactions among pesticide resistance, aphids, and symbionts, which will eventually help to better manage the resurgence of A. gossypii. © 2021 Society of Chemical Industry.

Effects of Plant Stress on Aphid-Parasitoid Interactions: Drought Improves Aphid Suppression

Drought events have the potential to mediate tri-trophic interactions. Changes in plant quality influence herbivore performance, which affects prey availability and quality for natural enemies. Parasitoid wasps are particularly vulnerable to these changes since their development is inextricably linked to that of their prey. However, we know little about the indirect effects of plant water limitation on parasitoid performance, especially the consequences of mild water limitation. This study investigated the ability of a parasitoid wasp (Aphidius colemani Viereck (Hymenoptera: Braconidae)) to suppress aphid (Rhopalosiphum padi L. (Hemiptera: Aphididae)) populations on well-watered, mildly stressed, or highly stressed wheat (Triticum aestivum L. (Poales: Poaceae)). We then investigated the role that aphid body size and behavioral interactions might play in wasp acceptance of aphid hosts. We found improved aphid suppression under mild and high-water limitation, but the underlying mechanisms were different. Mild-stress appeared to maximize parasitoid performance, as indicated by the highest mummy production. Aphids were larger when feeding on mildly stressed plants than on any other treatment, which suggests improved aphid quality for parasitoids. Improved aphid suppression under high-stress may have been driven by enhanced nonconsumptive effects. Despite improved aphid suppression, mummy formation was lowest on highly stressed plants. High-stress conditions have been shown to negatively affect aphid performance, so improved aphid suppression may be driven by poor aphid performance exacerbating the cost of interactions with parasitoids, such as stinging. No differences were observed in parasitoid foraging behaviors such as antennation or stinging across any treatments. This study highlights the importance of plant water stress intensity in affecting outcomes of parasitoid-host interactions.

Effects of different temperature regimes on survival of Diaphorina citri and its endosymbiotic bacterial communities

The Asian citrus psyllid, Diaphorina citri, is a major pest of citrus and vector of citrus greening (huanglongbing) in Asian. In our field-collected psyllid samples, we discovered that Fuzhou (China) and Faisalabad (Pakistan), populations harbored an obligate primary endosymbiont Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.) and a secondary endosymbiont, Wolbachia surface proteins (WSP) which are intracellular endosymbionts residing in the bacteriomes. Responses of these symbionts to different temperatures were examined and their host survival assessed. Diagnostic PCR assays showed that the endosymbionts infection rates were not significantly reduced in both D. citri populations after 24 h exposure to cold or heat treatments. Although quantitative PCR assays showed significant reduction of WSP relative densities at 40°C for 24 h, a substantial decrease occurred as the exposure duration increased beyond 3 days. Under the same temperature regimes, Ca. C. ruddii density was initially less affected during the first exposure day, but rapidly reduced at 3-5 days compared to WSP. However, the mortality of the psyllids increased rapidly as exposure time to heat treatment increased. The responses of the two symbionts to unfavorable temperature regimes highlight the complex host-symbionts interactions between D. citri and its associated endosymbionts.

Developmental and Ecological Benefits of the Maternally Transmitted Microbiota in a Dung Beetle

To complete their development, diverse animal species rely on the presence of communities of symbiotic microbiota that are vertically transmitted from mother to offspring. In the dung beetle genus Onthophagus, newly hatched larvae acquire maternal gut symbionts by the consumption of a maternal fecal secretion known as the pedestal. Here, we investigate the role of pedestal symbionts in mediating the normal development of Onthophagus gazella. Through the stepwise removal of environmental and maternal sources of microbial inoculation, we find that pedestal microbiota can enhance both overall growth and developmental rate in O. gazella. Further, we find that the beneficial effects of symbionts on developmental outcomes are amplified in the presence of ecologically relevant temperature and desiccation stressors. Collectively, our results suggest that the pedestal may provide an adaptive function by transmitting beneficial microbiota to developing dung beetle larvae and that the importance of microbiota for developmental and fitness outcomes may be context dependent.