The genetic diversity of SMLS (Sitobion miscanthi L type symbiont) and its effect on the fitness, mitochondrial DNA diversity and Buchnera aphidicola dynamic of wheat aphid, Sitobion miscanthi (Hemiptera: Aphididae)

Cereal Aphid Parasitoids in Europe (Hymenoptera: Braconidae: Aphidiinae): Taxonomy, Biodiversity, and Ecology

Cereals are very common and widespread crops in Europe. Aphids are a diverse group of herbivorous pests on cereals and one of the most important limiting factors of cereal production. Here, we present an overview of knowledge about the taxonomy, biodiversity, and ecology of cereal aphid parasitoids in Europe, an important group of natural enemies contributing to cereal aphid control. We review the knowledge obtained from the integrative taxonomy of 26 cereal aphid primary parasitoid species, including two allochthonous species (Lysiphlebus testaceipes and Trioxys sunnysidensis) and two recently described species (Lipolexis labialis and Paralipsis brachycaudi). We further review 28 hyperparasitoid species belonging to three hymenopteran superfamilies and four families (Ceraphronoidea: Megaspillidae; Chalcidoidea: Pteromalidae, Encyrtidae; Cynipoidea: Figitidae). We also compile knowledge on the presence of secondary endosymbionts in cereal aphids, as these are expected to influence the community composition and biocontrol efficiency of cereal aphid parasitoids. To study aphid-parasitoid-hyperparasitoid food webs more effectively, we present two kinds of DNA-based approach: (i) diagnostic PCR (mainly multiplex PCR), and (ii) DNA sequence-based methods. Finally, we also review the effects of landscape complexity on the different trophic levels in the food webs of cereal aphids and their associated parasitoids, as well as the impacts of agricultural practices and environmental variation.

Implications of environmentally shaped microbial communities for insecticide resistance in Sitobion miscanthi

Insect-associated bacteria play an important role in the resistance to pesticides, yet bacterial community compositions in wild insect host populations and the environmental factors that shape them are mostly elusive. In this study, Sitobion miscanthi (Takahashi) populations were collected from major wheat growing regions in China. Following high-throughput sequencing of 16S rRNA gene fragments, association analyses were performed within the bacterial community associated with S. miscanthi, as well as with population resistance levels to four commonly used pesticides and different environmental factors. We found that bacterial community structures differed in various regions, and that the abundances of dominant bacteria such as Buchnera, Candidatus Regiella, Candidatus Hamiltonella showed high variations. The resistance of S. miscanthi to avermectin and bifenthrin was shown to decline with increasing bacterial diversity. Meanwhile, with the increase of bacterial network modularity, the resistance of S. miscanthi populations to imidacloprid, avermectin and bifenthrin also increased correspondingly. In addition, correlation analysis indicated that altitude and air pressure had the strongest impact on bacterial community diversity and relative abundance, followed by humidity, rainfall and temperature. Overall, insights into such complex interactions between bacteria and their insect hosts offer new directions for biological pest control.

Activation of Wheat Defense Response by Buchnera aphidicola-Derived Small Chaperone Protein GroES in Wheat Aphid Saliva

Salivary proteins secreted by aphids during feeding play an important role in regulating the plant defense response. We used mass spectrometry to identify 155 proteins from the wheat aphid, Sitobion miscanthi, among which 44 proteins were derived from the primary symbiont, Buchnera aphidicola. GroES, which is a highly abundant molecular chaperone that binds to GroEL, was detected in saliva. In vitro injection of purified GroES protein and overexpression of GroES in wheat leaves verified that GroES induced hydrogen peroxide accumulation and callose deposition in wheat and further activated the plant salic acid and jasmonic acid defense pathways. Our findings indicate that plants may have evolved new strategies to detect aphid attack and trigger defense responses by recognizing proteins derived from B. aphidicola, which is present in almost all aphid species.

Reduced insecticide susceptibility of the wheat aphid Sitobion miscanthi after infection by the secondary bacterial symbiont Hamiltonella defensa

BACKGROUND

Bacterial symbionts in insects, especially aphids, have a major influence on host adaptation. We previously showed that infection with the secondary symbiont Hamiltonella defensa increases the fitness of the wheat aphid Sitobion miscanthi, yielding increases in fitness parameters such as adult weight and offspring number. However, whether H. defensa affects the sensitivity of host aphids to insecticides remains unknown.

RESULTS

We tested the effects of H. defensa on host aphid susceptibility to the insecticides chlorpyrifos methyl, imidacloprid, cyantraniliprole and acetamiprid. Our results showed that compared with Hamiltonella-free aphid clones, Hamiltonella-infected aphid clones exhibited lower sensitivity to most of the tested insecticides at low concentrations. Quantitative polymerase chain reaction showed that the density of H. defensa in the infected clones was slightly decreased at 24 h but then sharply increased until the late stage after treatment with the different insecticides. H. defensa in the host aphids was detected by fluorescence in situ hybridization and was localized to the aphid hindgut. Levels of the detoxification enzymes acetylcholinesterase, glutathione transferase and carboxylesterase were significantly higher in Hamiltonella-infected clones than in Hamiltonella-free clones.

CONCLUSIONS

The findings indicated that infection with H. defensa reduced aphid susceptibility to the investigated insecticides at low concentrations, potentially by increasing detoxification enzyme activity in the host. Therefore, symbiont-mediated insecticide resistance should be taken into account when performing resistance-monitoring studies. Studies of symbiont-mediated insecticide resistance may enhance our understanding of the emergence of insecticide resistance in agricultural systems. © 2020 Society of Chemical Industry.

The price of protection: a defensive endosymbiont impairs nymph growth in the bird cherry-oat aphid, Rhopalosiphum padi

Bacterial endosymbionts have enabled aphids to adapt to a range of stressors, but their effects in many aphid species remain to be established. The bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus), is an important pest of cereals worldwide and has been reported to form symbiotic associations with Serratia symbiotica and Sitobion miscanthi L-type symbiont endobacteria, although the resulting aphid phenotype has not been described. This study presents the first report of R. padi infection with the facultative bacterial endosymbiont Hamiltonella defensa. Individuals of R. padi were sampled from populations in Eastern Scotland, UK, and shown to represent seven R. padi genotypes based on the size of polymorphic microsatellite markers; two of these genotypes harbored H. defensa. In parasitism assays, survival of H. defensa-infected nymphs following attack by the parasitoid wasp Aphidius colemani (Viereck) was 5 fold higher than for uninfected nymphs. Aphid genotype was a major determinant of aphid performance on two Hordeum species, a modern cultivar of barley H. vulgare and a wild relative H. spontaneum, although aphids infected with H. defensa showed 16% lower nymph mass gain on the partially resistant wild relative compared with uninfected individuals. These findings suggest that deploying resistance traits in barley will favor the fittest R. padi genotypes, but symbiont-infected individuals will be favored when parasitoids are abundant, although these aphids will not achieve optimal performance on a poor quality host plant.

Plant-Mediated Horizontal Transmission of Hamiltonella defensa in the Wheat Aphid Sitobion miscanthi

Hamiltonella defensa is mainly vertically transmitted, but evidence suggests that horizontal transmission may occur. Here, the first case of plant-mediated horizontal transmission of H. defensa between wheat aphids, Sitobion miscanthi, was reported. H. defensa was harbored in sheath cells, secondary bacteriocytes, and hemolymph. After Hamiltonella-infected aphids fed on wheat leaves, H. defensa was observed in aphid stylets and plant phloem. H. defensa persisted in wheat leaves for at least 10 days. Most Hamiltonella-uninfected aphids became infected with H. defensa after sustained feeding on infected plant leaves and showed almost 100% stable vertical transmission over the next five generations. These horizontal transmission experiments were replicated using two other plants, rice and corn, and two different wheat aphid species, Rhopalosiphum padi and Schizaphis graminum. Surprisingly, aphid feeding induced plant infection only locally rather than systemically in leaves. Our findings indicate that plants may act as horizontal transmission intermediaries, contributing to the ubiquity of the otherwise maternally inherited H. defensa.

Bacterial communities in natural versus pesticide-treated Aphis gossypii populations in North China

The cotton‐melon aphid, Aphis gossypii Glover, is a worldwide‐spreading species, and pesticide‐resistant populations are increasing rapidly. In this study, investigations were performed based on Illumina HiSeq sequencing of the 16S rDNA V4 region for the bacterial communities embodied as intracellular symbionts under natural and in pesticide‐treated populations of A. gossypii. The results revealed that more than 82% of bacterial communities belonged to the phylum Proteobacteria in which the maximum proportion (53.24%) was of the genus Arsenophonus; Hamiltonella composed 22.31; and 1.37% was of the genus Acinetobacter. The relative abundance of Hamiltonella was obvious, vertically transmitted, divided into two groups, and its infection influenced the bacterial communities in A. gossypii. Symbiont density and composition were changed in samples tested on different days. Azadirachtin and phoxim influenced on the composition of bacterial communities. Different biomarkers were used for pesticide‐treated samples with LEfSe results. These findings will increase awareness regarding bacterial communities in naturally occurring populations of A. gossypii and pave the way to study the relationship between symbionts and pesticide resistance.

Effect of the Secondary Symbiont Hamiltonella defensa on Fitness and Relative Abundance of Buchnera aphidicola of Wheat Aphid, Sitobion miscanthi

Bacterial symbionts associated with insects are often involved in host development and ecological fitness. In aphids, the role of these symbionts is variable and not fully understood across different host species. Here, we investigated the symbiont diversity of the grain aphid, Sitobion miscanthi (Takahashi), from 17 different geographical areas. Of these, two strains with the same symbiont profile, except for the presence of Hamiltonella defensa, were selected using PCR. The Hamiltonella-infected strain, YX, was collected from a Yuxi wheat field in Yunnan Province, China. The Hamiltonella-free strain, DZ, was collected from a Dezhou wheat field in Shandong Province, China. Using artificial infection with H. defensa and antibiotic treatment, a Hamiltonella-re-infected strain (DZ-H) and Hamiltonella-significantly decreased strain (DZ-HT) were established and compared to the Hamiltonella-free DZ strain in terms of ecological fitness. Infection with the DZ-H strain increased the fitness of S. miscanthi, which led to increases in adult weight, percent of wingless individuals, and number of offspring. Meanwhile, decreased abundance of H. defensa (DZ-HT strain) resulted in a lower adult weight and wingless aphid rate compared to the DZ-H strain. However, the indices of longevity in both the DZ-H and DZ-HT strains decreased slightly, but were not significantly different, compared to the DZ strain. Furthermore, quantitative PCR showed that the relative abundance of the primary symbiont Buchnera aphidicola in the DZ-H strain was significantly higher than in the DZ strain in all but the first developmental stage. These results indicate that H. defensa may indirectly improve the fitness of S. miscanthi by stimulating the proliferation of B. aphidicola.

Genetic Structure of the Bacterial Endosymbiont Buchnera aphidicola from Its Host Aphid Schlechtendalia chinensis and Evolutionary Implications

Buchnera aphidicola is a primary symbiotic bacterium which provides essential amino acids to aphids. In this study, we sequenced nuclear 16s rDNA and atpAGD genes for 156 individuals of B. aphidicola from eight geographically distant populations to investigate the genetic diversity and structure of B. aphidicola associated to the sumac gall aphid Schlechtendalia chinensis in central and southern China. Our analyses of the combined sequences showed that B. aphidicola from S. chinensis had high haplotype and nucleotide diversity (h = 0.893; π = 0.00164). One of the 16 haplotypes detected had a wide geographic distribution across the central and southern China and was probably the ancestral haplotype of B. aphidicola from S. chinensis. A network and phylogenetic analysis revealed a geographic structure in which the 16 haplotypes of B. aphidicola were divided into the northern and southern clades separated by the Yangtze River. The two clades diverged from each other at 22.1 ± 3.7 Mya according to our divergence time estimations. Therefore, the modern genetic structure in B. aphidicola from S. chinensis has been probably impacted by historical geological events. Combined with the data from GenBank, we also reconstructed the phylogenetic relationships of three aphid subfamilies and their symbiont bacteria. The results indicated significant topological correlations between the aphid and bacterial phylogenies at interspecific levels.