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The cotton aphid, Aphis gossypii Glover, is a species of polyphagous aphid with many biotypes, and its host transfer has always been the focus of research on the control of cotton aphid. An important factor affecting aphid specialization is the nutritional association with microbial symbionts that provide the host with nutrients lacking in the diet. We analyzed the microbial composition and biodiversity of reared on zucchini for 10 generations (T1–T10) and cotton as a control (CK), by high-throughput Illumina sequencing of 16S ribosomal RNA genes. The findings showed that the change in plant hosts decreased the richness and variety of microbial species. Regardless of whether the plant host is altered or not, Proteobacteria and Firmicutes are the predominate phyla in cotton-specialized aphid. Additionally, cotton-specialized aphids that live in zucchini had considerably lower relative abundances of non-dominant phyla (Bacteroidetes) than cotton hosts. At the genus level the dominant communities were Buchnera, Acinetobacter, and Arsenophonus. The relative abundance of Buchnera was significantly higher in aphids reared on zucchini than those on cotton, whereas the opposite was observed for Acinetobacter, as well as for some non-dominant communities (Stenotrophomonas, Pseudomons, Flavobacterium, Novosphingobium). Collectively, this study clarifies the dynamic changes of symbiotic bacteria in cotton-specialized aphids reared on zucchini for multiple generations. Among them, Buchnera is crucial for the cotton-specialized aphid to get nutrients during the transfer of the host and has a favorable impact on the colonization of cotton-specialized aphid populations on zucchini hosts. It not only enriches our understanding of the relationship between the bacterial microbiota of aphids and their adaptability to new hosts, zucchini, but also expands the current body of research on the mechanisms underlying the host shifting ability of cotton-specialized aphids.

IntroductionReproductive polymorphism and symbiotic bacteria are commonly observed in aphids, but their interaction remains largely unclear. In polymorphic aphid species (Aphis gossypii), offspring of parthenogenetic females (PFs) develops into sexuparae which produces gynoparae and males successively. Gynoparae further produces sexual females (SFs), and these sexual females mate with males to produce offspring.MethodsIn this study, we investigated the dynamic changes of symbiotic bacteria during the above-mentioned five reproductive morph switch in A. gossypii via 16S rRNA sequencing technology.ResultsThe results showed that species richness and community diversity of symbiotic bacteria in males were the highest. Proteobacteria was absolutely dominant bacterial phylum (with relative abundance of more than 90%) in the five reproductive morphs of A. gossypii, and Buchnera was absolutely dominant genus (with relative abundance of >90%), followed by Rhodococcus, Pseudomonas, and Pantoea. Male-killing symbiont Arsenophonus presented the highest relative abundance in gynoparae, a specific morph whose offsprings were exclusively sexual females. Both principal component analysis (PCA) and clustering analysis showed trans-generation similarity in microbial community structure between sexuparae and sexual females, between PFs and gynoparae. PICRUSt 2 analysis showed that symbiotic bacteria in the five reproductive morphs were mainly enriched in metabolic pathways.DiscussionReproductive morph switch induced by environmental changes might be associated with bacterial community variation and sexual polymorphism of aphids. This study provides a new perspective for further deciphering the interactions between microbes and reproductive polymorphism in host aphids.

Fipronil is widely used in the agricultural world as an efficient phenylpyrazole insecticide to control pests. Binodoxys communis is a key parasitic natural enemy of major homopteran pests and can successfully control the population of pests such as cotton aphids. It has not yet been studied what effects would sublethal doses of fipronil have on Binodoxys communis larvae. Here, this study evaluated the effect of fipronil on Binodoxys communis larvae and analyze the transcriptome results. The results showed that LC10 (1.19 mg/L) and LC25 (1.73 mg/L) had significant negative effects on the survival rate and parasitism rate of F0 generation. Moreover, exposure to high concentrations (LC25) of fipronil still had obvious passive effect on the F1 generation of Binodoxys communis. These results indicated that sublethal doses of fipronil have malignant effects on the biological functions of parasitoids and their offspring. The results of transcriptome analysis showed that differentially expressed genes (DEGs) of Binodoxys communis after LC10 treatment are mainly related to immunity and detoxification. LC25 treatment instead resulted in changes in the expression of genes related to nutrition, energy and metabolism reactions. Seven of the identified DEGs were selected for real-time fluorescence quantitative PCR analysis. To the best of our knowledge, this is the first report to evaluate the sublethal, intergenerational, and transcriptomic side effects of fipronil on larvae of parasitic natural pest enemies. Our findings provide data to accurately assess the risk of fipronil usage on Binodoxys communis larvae, and provide important theoretical support for the comprehensive prevention and control of natural enemies and pesticides.

IntroductionBacteria play critical roles in the reproduction, metabolism, physiology, and detoxification of their insect hosts. The ladybird beetle (Harmonia axyridis) harbors a myriad of endosymbiotic microbes. However, to date, little is known about how the microbial composition of H. axyridis varies throughout its life cycle.MethodsIn this study, 16S rRNA amplicon sequencing and quantitative PCR were employed to investigate the diversity and dynamics of bacterial symbionts across the egg, larval, pupae, and adults stages of H. axyridis.ResultsHigher bacterial community richness and diversity were observed in eggs, followed by those in adults and pupae. The community richness index differed significantly between second-instar larvae and other developmental stages. Proteobacteria, Firmicutes, and Actinobacteria were the dominant phyla. Staphylococcus, Enterobacter, Glutamicibacter, and Acinetobacter were the dominant bacteria genera; however, their relative abundances fluctuated across host developmental stages. Interestingly, the larval stage harbored high proportions of Firmicutes, whereas the adult microbial community largely consisted of Proteobacteria.DiscussionThis study is the first to determine the symbiotic bacterial composition across key life stages of H. axyridis. These outcomes can foster the development of environmental risk assessments and novel biological control strategies.

The vast majority of parasitoids are capable of precise and meticulous regulation of nutrition and metabolism within the host. An important building block of life, amino acids are critical to the development of parasitoids. To date, research on how parasitoids regulate host amino acid metabolism remains limited. In this study, Aphis gossypii and its dominant parasitoid Binodoxys communis were used as a study system to explore how parasitism may change the regulation of amino acids in A. gossypii with UHPLC-MS/MS and RT-qPCR techniques. Here, for the first 8 h of parasitism the abundance of almost all amino acids in cotton aphids increased, and after 16 h most of the amino acids decreased. An amino acid of parasitic syndrome, the content of Tyr increased gradually after being parasitized. The expression of genes related to amino acid metabolism increased significantly in early stages of parasitism and then significantly decreased gradually. At the same time, the abundance of Buchnera, a cotton aphid specific symbiont increased significantly. Our comprehensive analyses reveal impacts of B. communis on the amino acid regulatory network in cotton aphid from three aspects: amino acid metabolism, gene expression, and bacterial symbionts. Therefore, this research provides an important theoretical basis for parasitoid nutritional regulation in host, which is highly significant as it may inform the artificial reproduction of parasitoids and the biological control of insect pests.

Given the ever-increasing commercial planting of transgenic plants across the world, an evaluation of their impacts on non-target organisms is as an important part of the risk assessment process. Propylea japonica is a dominant non-target predator and pollen feeder insect that is prevalent in Bt cotton fields, and it is thus in direct contact with Bt proteins. However, the effect of Bt proteins on P. japonica has not received much attention. In this study, the effects of Cry1Ac and/or Cry2Ab proteins on P. japonica were investigated from three aspects. First, no significant differences in the diversity of the microbiota nor change in species composition and community structure were observed among Cry protein treatments. Firmicutes are the most abundant bacterial phylum present in P. japonica, followed by Proteobacteria and Actinobacteria. The most abundant genus was Staphylococcus. Second, the expression levels of the detoxification and digestion-related genes did not change significantly in any Cry protein treatment. Third, none of the Cry proteins affected the population fitness of P. japonica. These results indicated that P. japonica was not sensitive to Bt proteins, suggesting that growing Bt cotton expressing Cry1Ac and/or Cry2Ab will pose negligible risks to P. japonica.