Aphid Clones Research Articles

'Drifting' Buchnera genomes track the microevolutionary trajectories of their aphid hosts.

Evolution of Buchnera-aphid host symbioses is often studied among species at macroevolutionary scales. Investigations within species offer a different perspective about how eco-evolutionary processes shape patterns of genetic variation at microevolutionary scales. Our study leverages new and publicly available whole-genome sequencing data to study Buchnera-aphid host evolution in Myzus persicae, the peach potato aphid, a globally invasive and polyphagous pest. Across 43 different asexual, clonally reproducing isofemale strains, we examined patterns of genomic covariation between Buchnera and their aphid host and considered the distribution of mutations in protein-coding regions of the Buchnera genome. We found Buchnera polymorphisms within aphid strains, suggesting the presence of genetically different Buchnera strains within the same clonal lineage. Genetic distance between pairs of Buchnera samples was positively correlated to genetic distance between their aphid hosts, indicating shared evolutionary histories. However, there was no segregation of genetic variation for both M. persicae and Buchnera with plant host (Brassicaceae and non-tobacco Solanaceae) and no associations between genetic and geographic distance at global or regional spatial scales. Abundance patterns of non-synonymous mutations were similar to synonymous mutations in the Buchnera genome, and both mutation classes had similar site frequency spectra. We hypothesize that a predominance of neutral processes results in the Buchnera of M. persicae to simply 'drift' with the evolutionary trajectory of their aphid hosts. Our study presents a unique microevolutionary characterization of Buchnera-aphid host genomic covariation across multiple aphid clones. This provides a new perspective on the eco-evolutionary processes generating and maintaining polymorphisms in a major pest aphid species and its obligate primary endosymbiont.

Novel transinfections of Rickettsiella do not affect insecticide tolerance in Myzus persicae, Rhopalosiphum padi, or Diuraphis noxia (Hemiptera: Aphididae).

Aphids (Hemiptera: Aphidoidea) are economically important crop pests worldwide. Because of growing issues with insecticide resistance and environmental contamination by insecticides, alternate methods are being explored to provide aphid control. Aphids contain endosymbiotic bacteria that affect host fitness and could be targeted as potential biocontrol agents, but such novel strategies should not impact the effectiveness of traditional chemical control. In this work, we used a novel endosymbiont transinfection to examine the impact of the endosymbiont Rickettsiella viridis on chemical tolerance in 3 important agricultural pest species of aphid: Myzus persicae (Sulzer) (Hemiptera: Aphididae), Rhopalosiphum padi (Linnaeus) (Hemiptera: Aphididae), and Diuraphis noxia (Mordvilko ex Kurdjumov) (Hemiptera: Aphididae). We tested tolerance to the commonly used insecticides alpha-cypermethrin, bifenthrin, and pirimicarb using a leaf-dip bioassay. We found no observed effect of this novel endosymbiont transinfection on chemical tolerance, suggesting that the strain of Rickettsiella tested here could be used as a biocontrol agent without affecting sensitivity to insecticides. This may allow Rickettsiella transinfections to be used in combination with chemical applications for pest control. The impacts of other endosymbionts on insecticide tolerance should be considered, along with tests on multiple aphid clones with different inherent levels of chemical tolerance.

Genetic Diversity of Barley Accessions from East Asia for Greenbug Resistance.

The greenbug, Schizaphis graminum, is a dangerous pest of barley and other grain crops in the south of Russia. An effective and environmentally friendly way to control this insect is to cultivate resistant varieties. The differential interaction between the phytophage and host plants necessitates the search for new donors of resistance. Seven hundred and seventy-eight accessions of barley from East Asian countries (313 from China, 450 from Japan, and 15 from Nepal) were evaluated for greenbug resistance. The Krasnodar population of the insect and clones isolated from it were used in the experiments. Forty heterogeneous accessions were identified, in which plants with a high level of resistance to the aphid were found. As a result of damage assessment by the 108 S. graminum clones of 11 lines selected from heterogeneous accessions, 52 insect virulence phenotypes were identified. Experiments with aphid test clones showed that all 11 lines possess diverse greenbug resistance alleles, which differ from the previously identified Rsg1, but their efficiency is low. The frequency of clones virulent to ten lines and the cultivar Post (a carrier of the Rsg1 gene) varies from 60.4% to 98.0%. The exception is line 15903, which is resistant to the aphid population and protected by one dominant gene. The high resistance of other lines against a part of the natural population of S. graminum is also under oligogenic control. Lines 15600 and 16190 each have one dominant resistance gene, and line 28129 is protected by two genes, the dominant and recessive ones. A recessive resistance gene is presumably present in line 15600. Lines 16237/1 and 16237/2, isolated from the same collection accession, each have one dominant gene effective against individual aphid clones. The loss of effectiveness of distinctly manifested resistance genes causes the expression of previously masked genes with a weak phenotypic manifestation, which differentially interact with insect genotypes.

Aphid adaptation to cucurbits: sugars, cucurbitacin and phloem structure in resistant and susceptible melons

BackgroundAphis gossypii, a strictly phloemophagaous aphid, colonize hundreds of plant families, and a group of clones formed a cucurbit-specialised host-race. Cucurbits are unique in having evolved a specific extra-fascicular phloem (EFP), which carries defence-related metabolites such as cucurbitacin, whereas the fascicular phloem (FP) is common to all higher plants and carries primary metabolites, such as raffinose-family oligosaccharides (RFOs). Both cucurbitacins (in the EFP) and galactinol (in the FP) have been suggested to be toxic to aphids. We investigated these hypotheses in cucurbit-specialized A. gossypii fed on melon plants with or without aphid-resistance conferred by the NLR gene Vat. We selected a plant-aphid system with (i) Vat-mediated resistance not triggered, (ii) Vat-mediated resistance triggered by an aphid clone adapted to the presence of Vat resistant alleles and (iii) Vat-mediated resistance triggered by a non-adapted aphid clone.ResultsWe quantified cucurbitacin B, its glycosylated derivative, and sugars, in melon plants and aphids that fed on. The level of cucurbitacin in plants was unrelated to both aphid infestation and aphid resistance. Galactinol was present at higher quantities in plants when Vat-mediated resistance was triggered, but its presence did not correlate with aphid performance. Finally, we showed that cucurbit-specialized A. gossypii fed from the FP but could also occasionally access the EFP without sustainably feeding from it. However, the clone not adapted to Vat-mediated resistance were less able to access the FP when the Vat resistance was triggered.ConclusionWe concluded that galactinol accumulation in resistant plants does not affect aphids, but may play a role in aphid adaptation to fasting and that Cucurbitacin in planta is not a real threat to Aphis gossypii. Moreover, the specific phloem of Cucurbits is involved neither in A. gossypii cucurbit specialisation nor in adaptation to Vat-dependent resistance.

Influence of Bacterial Secondary Symbionts in Sitobion avenae on Its Survival Fitness against Entomopathogenic Fungi, Beauveria bassiana and Metarhizium brunneum.

The research was focused on the ability of wheat aphids Sitobion avenae, harboring bacterial secondary symbionts (BSS) Hamiltonella defensa or Regiella insecticola, to withstand exposure to fungal isolates of Beauveria bassiana and Metarhizium brunneum. In comparison to aphids lacking bacterial secondary symbionts, BSS considerably increased the lifespan of wheat aphids exposed to B. bassiana strains (Bb1022, EABb04/01-Tip) and M. brunneum strains (ART 2825 and BIPESCO 5) and also reduced the aphids' mortality. The wheat aphid clones lacking bacterial secondary symbionts were shown to be particularly vulnerable to M. brunneum strain BIPESCO 5. As opposed to wheat aphids carrying bacterial symbionts, fungal pathogens infected the wheat aphids lacking H. defensa and R. insecticola more quickly. When treated with fungal pathogens, bacterial endosymbionts had a favorable effect on the fecundity of their host aphids compared to the aphids lacking these symbionts, but there was no change in fungal sporulation on the deceased aphids. By defending their insect hosts against natural enemies, BSS increase the population of their host society and may have a significant impact on the development of their hosts.

Genetic diversity of barley accessions from Mongolia for greenbug resistance

BACKGROUND: Significant damage to barley crops in the southern regions of the Russian Federation is caused by the greenbug Schizaphis graminum Rondani. Cultivation of resistant varieties is a cheap, efficient and environmentally friendly way to combat the phytophage. The specificity of relations between S. graminum and barley is shown, that determines the need to broad the genetic diversity of cultivated varieties.
 AIM: The study of effectiveness and genetic control of the greenbug resistance in barley accessions from Mongolia.
 MATERIALS AND METHODS: We studied 175 barley landraces from Mongolia for resistance to the Krasnodar aphid population. We assessed damage to resistant lines selected from heterogeneous accessions k-3885, k-3904, and k-4080, as well as the variety Post (a carrier of the previously identified Rsg1 gene) by 86 aphid clones isolated from the population. Under laboratory conditions, the aphid resistance segregation of F2 hybrids was analyzed from crossing three accessions from Mongolia with a susceptible tester.
 RESULTS: Five heterogeneous accessions were identified, in which plants with high resistance to the pest were found; in 28 accessions, damage to the leaf surface of the resistant component varied from 31% to 60%. As a result of the assessment of the resistance of four barley accessions to S. graminum clones, 15 virulence phenotypes were identified. Accessions k-3885, k-3904 and k-4080 each have one dominant resistance allele, which differ from each other and differ from Rsg1.
 CONCLUSIONS: The genetic diversity of barley accessions from Mongolia in terms of greenbug resistance is low. The genotypes of S. graminum differentially interact not only with the major, but also with weakly manifested barley resistance genes.

Yellow–green color polymorphism in males of a pea aphid clone and its genetic pattern

AbstractAlthough most aphid species living on leaves have a green body color, little is known regarding the biosynthetic pathways of green pigments. We found that a clone of the pea aphid, Acyrthosiphon pisum (Harris) produced both green‐ and yellow‐colored males. The females of this clone were green in color, while 8.4% of the males produced were yellow. To date, yellow body color has been reported only in a single mutant clone in A. pisum. To explore the genetic pattern of yellow body color, green or yellow males were mated with green females of the same clone. The hatchability of the eggs sired by yellow males (26.2%) was less than half that of the eggs sired by green males (79.0%). The hatched foundresses of both groups were all green, with no yellow foundresses. Because aphids have an XX‐XO sex determination system, color polymorphism in males suggests that male body color may be governed by an X‐linked locus. If females possess heterozygosity at the putative locus, they can produce alternative phenotypes in males. The small proportion of yellow males and absence of yellow foundresses imply that the allele responsible for yellow body color has a deleterious effect. The present study suggests that this clone could be used to elucidate the biosynthetic pathways and underlying genetics of green pigments in aphids.

Bacterial Symbionts Confer Thermal Tolerance to Cereal Aphids Rhopalosiphum padi and Sitobion avenae

Simple SummaryThis study assesses the putative association between the chronic and acute thermal tolerance of cereal aphids Rhopalosiphum padi (L.) and Sitobion avenae (F.) and the abundance of their bacterial symbionts. Thermal tolerance indices were determined for 5-day-old apterous aphid individuals and were associated with the aphid-specific and total bacterial symbionts’ gene abundance (copy numbers). The results show a significantly higher bacterial symbionts’ gene abundance in temperature-tolerant aphid individuals than the susceptible ones for both aphid species. Moreover, the gene abundance of total (16S rRNA) bacteria and most of the aphid-specific bacterial symbionts for both cereal aphid species were significantly and positively associated with their critical thermal maxima values. Overall, the findings of the study suggest the potential role of the bacterial symbionts of aphids in conferring thermal tolerance to their hosts.High-temperature events are evidenced to exert significant influence on the population performance and thermal biology of insects, such as aphids. However, it is not yet clear whether the bacterial symbionts of insects mediate the thermal tolerance traits of their hosts. This study is intended to assess the putative association among the chronic and acute thermal tolerance of two cereal aphid species, Rhopalosiphum padi (L.) and Sitobion avenae (F.), and the abundance of their bacterial symbionts. The clones of aphids were collected randomly from different fields of wheat crops and were maintained under laboratory conditions. Basal and acclimated CTmax and chronic thermal tolerance indices were measured for 5-day-old apterous aphid individuals and the abundance (gene copy numbers) of aphid-specific and total (16S rRNA) bacterial symbionts were determined using real-time RT-qPCR. The results reveal that R. padi individuals were more temperature tolerant under chronic exposure to 31 °C and also exhibited about 1.0 °C higher acclimated and basal CTmax values than those of S. avenae. Moreover, a significantly higher bacterial symbionts’ gene abundance was recorded in temperature-tolerant aphid individuals than the susceptible ones for both aphid species. Although total bacterial (16S rRNA) abundance per aphid was higher in S. avenae than R. padi, the gene abundance of aphid-specific bacterial symbionts was nearly alike for both of the aphid species. Nevertheless, basal and acclimated CTmax values were positively and significantly associated with the gene abundance of total symbiont density, Buchnera aphidicola, Serratia symbiotica, Hamilton defensa, Regiella insecticola and Spiroplasma spp. for R. padi, and with the total symbiont density, total bacteria (16S rRNA) and with all aphid-specific bacterial symbionts (except Spiroplasma spp.) for S. avenae. The overall study results corroborate the potential role of the bacterial symbionts of aphids in conferring thermal tolerance to their hosts.

Similar cost of Hamiltonella defensa in experimental and natural aphid-endosymbiont associations.

Endosymbiont‐conferred resistance to parasitoids is common in aphids, but comes at a cost to the host in the absence of parasitoids. In black bean aphids (Aphis fabae), costs in terms of reduced lifespan and lifetime reproduction were demonstrated by introducing 11 isolates of the protective symbiont Hamiltonella defensa into previously uninfected aphid clones. Transfection of H. defensa isolates into a common genetic background allows to compare the costs of different endosymbiont isolates unconfounded by host genetic variation, but has been suggested to overestimate the realized costs of the endosymbiont in natural populations, because transfection creates new and potentially maladapted host–symbiont combinations that would be eliminated by natural selection in the field. In this experiment, we show that removing H. defensa isolates from their natural host clones with antibiotics results in a fitness gain that is comparable to the fitness loss from their introduction into two new clones. This suggests that estimating cost by transfecting endosymbiont isolates into a shared host genotype does not lead to gross overestimates of their realized costs, at least not in the two recipient genotypes used here. By comparing our data with data reported in previous publications using the same lines, we show that symbiont‐induced costs may fluctuate over time. Thus, costs estimated after extended culture in the laboratory may not always be representative of the costs at the time of collection in the field. Finally, we report the accidental observation that two isolates from a distinct haplotype of H. defensa could not be removed by cefotaxime treatment, while all isolates from two other haplotypes were readily eliminated, which is suggestive of variation in susceptibility to this antibiotic in H. defensa.

Evaluating the impact of pyrethroid insecticide resistance on reproductive fitness inSitobion avenae

AbstractResistance to insecticides used to control pests is an issue of increasing concern for agriculture. The grain aphid,Sitobion avenae, is a pest of cereals and grasses worldwide, and one of growing concern due to the evolution of resistance to certain insecticides. Resistance confers benefits to insects by enabling them to survive exposure to insecticide compounds; however, the mutations conferring resistance may also penalise the insect in pesticide‐free environments due to fitness costs associated with the new phenotype. Here we tested the hypothesis of a reproductive penalty linked to the knockdown resistance mutation (kdr) to pyrethroid insecticides. The mutation occurs predominantly in a single SA3 clone. To date, only heterozygous‐resistant forms (kdr‐SR) have been detected in populations in Ireland and the UK, and this suggests that a fitness penalty may preclude the formation of both male and female heterozygous‐resistant sexual forms. By designing an experiment which included a resistant and a non‐resistant clone, we were able to simulate reduced daylight and temperature conditions which, in nature, trigger sexual reproduction and therefore study the responses of each clone. This allowed us to detect the switch from asexual females to sexual females and males and report on the conditions associated with the production of sexual forms. The results showed that both aphid clones were able to produce sexual forms with no difference in the onset of sexual reproduction, although reproductive strategies differed between clones. The later onset of male forms in the SA3 clone may decrease the likelihood of mating interactions to create fully resistant (kdr‐RR) genotypes and this may constitute a fitness penalty due to pyrethroid resistance.

Flupyradifurone resistance in Myzus persicae populations from peach and tobacco in Greece.

Myzus persicae has evolved resistance to various insecticides in Greece. Here we examine the effectiveness of the insecticide flupyradifurone against aphid clones collected from tobacco and peach in Greece during 2017-2020. Furthermore, we monitored the frequency of the neonicotinoid resistance mutation R81T in the sampled clones, and the association between the responses to flupyradifurone and acetamiprid. Of 43 clones tested with flupyradifurone, 6.977%, 60.465% and 32.558% showed low (10-14), moderate (19-89) and high (104-1914) resistance factor (RF) values, respectively. Resistance was higher in clones from peach than from tobacco with 42.308% and 17.647% of clones (respectively) failing into the high RF category (median RF values 67.5 and 36.4 for clones from peach and tobacco, respectively). Acetamiprid resistance was detected in clones collected in 2019-2020, in line with our previous study in Greece. The analysis of the whole dataset (54 clones collected during 2017-2020) revealed that all tobacco clones had RF < 7.5, whereas 55.263%, 18.421% and 26.316% of the peach clones exhibited low (<12), moderate (20-48) and high (100-145) RF values, respectively. A significant but moderate association between flupyradifurone and acetamiprid responses was detected (r =0.513, P < 0.001). The R81T mutation was detected in aphids from peach (5.6% and 32.6% as homozygotes and heterozygotes, respectively) and in one aphid specimen (heterozygote) from tobacco. R81T was partially associated with the resistance to both insecticides, but many highly resistant clones did not possess the mutation, indicating the possible operation of one or more alternative underlying resistance mechanisms. The use of flupyradifurone and acetamiprid in IPM/IRM should be based on further ongoing susceptibility monitoring. © 2021 Society of Chemical Industry.

Controlling insecticide resistant clones of the aphid, Myzus persicae, using the entomopathogenic fungus Akanthomyces muscarius: fitness cost of resistance under pathogen challenge.

Biological control is a cornerstone of integrated pest management and could also play a key role in managing the evolution of insecticide resistance. Ecological theory predicts that the fitness cost of insecticide resistance can be increased under exposure to invertebrate natural enemies or pathogens, and can therefore increase the value of integrating biological control into pest management. In this study of the peach potato aphid, Myzus persicae, we aimed to identify whether insecticide resistance affected fitness and vulnerability of different aphid clones to the entomopathogenic fungus Akanthomyces muscarius. Insecticide resistant clones were found to be slightly less susceptible to the pathogen than susceptible clones. However, this pattern could also be explained by the influence of length of laboratory culture, which was longer in susceptible clones and was positively correlated with susceptibility to fungi. Furthermore, resistance status did not affect aphid development time or intrinsic rate of increase of aphids. Finally, in a cage trial the application of fungus did not increase the competitive fitness of insecticide resistant clone 'O'. We found no fitness cost in reproductive rate or pathogen susceptibility associated with chemical resistance in M. persicae. In contrast, some susceptible clones, particularly those subject to decades of laboratory rearing, showed enhanced susceptibility to a fungal pathogen, but not reduced reproductive fitness, an observation consistent with down-regulation of costly immune functions in culture. Overall, fungal pathogen control is compatible with insecticides and should not increase the selection pressure for resistance of M. persicae to chemical insecticides.

Identification of novel aphid-killing bacteria to protect plants.

SummaryAphids, including the peach‐potato aphid, Myzus persicae, are major insect pests of agriculture and horticulture, and aphid control measures are limited. There is therefore an urgent need to develop alternative and more sustainable means of control. Recent studies have shown that environmental microbes have varying abilities to kill insects. We screened a range of environmental bacteria isolates for their abilities to kill target aphid species. Tests demonstrated the killing aptitude of these bacteria against six aphid genera (including Myzus persicae). No single bacterial strain was identified that was consistently toxic to insecticide‐resistant aphid clones than susceptible clones, suggesting resistance to chemicals is not strongly correlated with bacterial challenge. Pseudomonas fluorescens PpR24 proved the most toxic to almost all aphid clones whilst exhibiting the ability to survive for over three weeks on three plant species at populations of 5–6 log CFU cm−2 leaf. Application of PpR24 to plants immediately prior to introducing aphids onto the plants led to a 68%, 57% and 69% reduction in aphid populations, after 21 days, on Capsicum annuum, Arabidopsis thaliana and Beta vulgaris respectively. Together, these findings provide new insights into aphid susceptibility to bacterial infection with the aim of utilizing bacteria as effective biocontrol agents.

Self and non-self recognition affects clonal reproduction and competition in the pea aphid.

The spatial interaction of clonal organisms is an unsolved but crucial topic in evolutionary biology. We evaluated the interactions between aphid clones using a colour mutant (yellow) and an original (green) clone. Colonies founded by two aphids of the same clone and mixed colonies, founded by a green aphid and a yellow aphid, were set up to observe population growth for 15 days. We confirmed positive competition effects, with mixed colonies increasing in size more rapidly than clonal colonies. In mixed colonies where reproduction started simultaneously, green aphids overwhelmed yellow aphids in number, and yellow aphids restrained reproduction. However, when yellow aphids started to reproduce earlier, they outnumbered the green aphids. To test whether aphids have the ability to control reproduction according to the densities of self and non-self clones, one yellow aphid or one antennae-excised yellow aphid was transferred into a highly dense green clone colony. Intact yellow aphids produced fewer nymphs in crowded green colonies, whereas the fecundity of antennae-excised aphids did not change. Thus, we conclude that aphid clones can discriminate between self and non-self clones, and can regulate their reproduction, depending on whether they are superior or inferior in number to their competitors.

Infection of susceptible/tolerant barley genotypes with Barley yellow dwarf virus alters the host plant preference of Rhopalosiphum padi clones depending upon their ability to transmit BYDV

Discovering mechanisms of plant–virus–vector interactions is fundamental to understand their ecology and evolution and to apply this knowledge in plant protection. To study the influence of varying Barley yellow dwarf virus (BYDV) transmission efficiencies on host plant preference of Rhopalosiphum padi (L.) clones, we performed host choice experiments with two barley cultivars (BYDV susceptible cv. ‘Rubina’ and BYDV tolerant cv. ‘Vixen’) including healthy and virus-infected plants. For the susceptible barley cultivar ‘Rubina’, aphid clone R07 (high transmission efficiency) preferred BYDV-infected over healthy host plants after 24 h, while clones D10 (medium transmission efficiency) and R09 (low transmission efficiency) preferred neither host. In contrast, BYDV infection of ‘Vixen’ did not affect the plant’s appeal for aphid clone R07. Host plant access, indicated by ingestion and observed by electrical penetration graph technique for a period of 2 h, was facilitated on BYDV-infected cv. ‘Rubina’ for the clones R07 and D10, whereas an opposite effect was observed for the clone R09. For R07 and R09, the difference was not visible after a period of 5 h. As observed earlier for BYDV-infected wheat, enhanced emission of volatile organic compounds associated with virus-induced attraction was detected for BYDV-infected cv. ‘Rubina.’ It is concluded that host plant preference is possibly linked with a high BYDV transmission efficiency as observed for the clone R07, leading to a fitness advantage of this clone as indicated by early increased ingestion. This advantage is not present on BYDV-tolerant genotypes most likely due to the absence of infection symptoms.

Effect of the Genotypic Variation of an Aphid Host on the Endosymbiont Associations in Natural Host Populations.

Simple SummaryThe host–endosymbiont complex could be a key determinant in spread and maintenance of the infection polymorphism of endosymbionts. Variation among host–endosymbiont complexes can contribute to genetic variation of a host species and then provide the necessary material for the operating coevolutionary dynamics. We studied the seasonal dynamic of facultative endosymbiont infections among different host clones of the grain aphid Sitobion avenae and whether their presence affects the total hymenopteran parasitism of aphid hosts at the field level. We observed that aphid infections in the field with endosymbionts increase over time, by favoring particular aphid clones closely associated with endosymbionts, but without an effect of endosymbionts on parasitism rate in the host populations. Our results highlight the importance of host–endosymbiont couples in shaping the prevalence and distributions of symbionts throughout nature and the success of their hosts as pests.Understanding the role of facultative endosymbionts on the host’s ecology has been the main aim of the research in symbiont–host systems. However, current research on host–endosymbiont dynamics has failed to examine the genetic background of the hosts and its effect on host–endosymbiont associations in real populations. We have addressed the seasonal dynamic of facultative endosymbiont infections among different host clones of the grain aphid Sitobion avenae, on two cereal crops (wheat and oat) and whether their presence affects the total hymenopteran parasitism of aphid hosts at the field level. We present evidence of rapid seasonal shifts in the endosymbiont frequency, suggesting a positive selection of endosymbionts at the host-level (aphids) through an agricultural growing season, by two mechanisms; (1) an increase of aphid infections with endosymbionts over time, and (2) the seasonal replacement of host clones within natural populations by increasing the prevalence of aphid clones closely associated to endosymbionts. Our results highlight how genotypic variation of hosts can affect the endosymbiont prevalence in the field, being an important factor for understanding the magnitude and direction of the adaptive and/or maladaptive responses of hosts to the environment.

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

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. 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. 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.

Virulence of Soybean Aphid, Aphis glycines (Hemiptera: Aphididae) Clones on Detached Leaves and Whole Plants

Since the discovery of the invasive soybean aphid, Aphis glycines Matsumura, in the USA in 2000, populations have expanded throughout the Midwestern USA and Canada. Its negative economic impact on soybean [Glycine max (L.) Merr.] yield provides the impetus to continue investigating the effectiveness of host plant resistance which has been affected by soybean aphid biotypes that overcome soybean plants containing Rag (resistance to A. glycines) genes. In this study, 14 aphid clones collected on soybean and buckthorn (Rhamnus cathartica L.) plants along with four known aphid biotypes (from our stock collection) were evaluated in no-choice assays by quantifying the number of aphids on soybean genotypes with Rag genes and the susceptible cultivar Williams 82. No-choice assays were conducted using detached leaves and whole plants. Inconsistent responses of the biotypes from the stock collection by using detached leaves confirm the efficacy of whole plant assays to accurately differentiate biotypes. None of the biotypes or field clones from Illinois, Indiana and South Dakota overcame the resistance of soybean genotypes LD14-8004 (Rag1) and PI437696. A soybean aphid clone from Wooster, Ohio readily colonized LD14-8002 (Rag2), but did not overcome any of the other resistance genes indicating that it is different from biotypes 3 and 4, which also overcome LD14-8002 (Rag2) and, respectively, LD14-8004 (Rag1) and LD14-8006 (Rag3).

Induced Systemic Resistance by a Plant Growth-Promoting Rhizobacterium Impacts Development and Feeding Behavior of Aphids.

The effects of microorganisms on plant-insect interactions have usually been underestimated. While plant growth-promoting rhizobacteria (PGPR) are known to induce plant defenses, endosymbiotic bacteria hosted by herbivorous insects are often beneficial to the host. Here, we aimed to assess whether PGPR-induced defenses in broad bean plants impact the pea aphid, depending on its genotype and the presence of endosymbionts. We estimated aphid reproduction, quantified defense- and growth-related phytohormones by GC-MS, and measured different plant growth and physiology parameters, after PGPR treatment. In addition, we recorded the feeding behavior of aphids by electropenetrography. We found that the PGPR treatment of broad bean plants reduced the reproduction of one of the pea aphid clones. We highlighted a phenomenon of PGPR-induced plant defense priming, but no noticeable plant growth promotion. The main changes in aphid probing behavior were related to salivation events into phloem sieve elements. We suggest that the endosymbiont Hamiltonella defensa played a key role in plant-insect interactions, possibly helping aphids to counteract plant-induced resistance and allowing them to develop normally on PGPR-treated plants. Our results imply that plant- and aphid-associated microorganisms add greater complexity to the outcomes of aphid-plant interactions.

Self-perpetuating ecological–evolutionary dynamics in an agricultural host–parasite system

Ecological and evolutionary processes may become intertwined when they operate on similar time scales. Here we show ecological-evolutionary dynamics between parasitoids and aphids containing heritable symbionts that confer resistance against parasitism. In a large-scale field experiment, we manipulated the aphid's host plant to create ecological conditions that either favoured or disfavoured the parasitoid. The result was rapid evolutionary divergence of aphid resistance between treatment populations. Consistent with ecological-evolutionary dynamics, the resistant aphid populations then had reduced parasitism and increased population growth rates. We fit a model to quantify costs (reduced intrinsic rates of increase) and benefits of resistance. We also performed genetic assays on 5 years of field samples that showed persistent but highly variable frequencies of aphid clones containing protective symbionts; these patterns were consistent with simulations from the model. Our results show (1) rapid evolution that is intertwined with ecological dynamics and (2) variation in selection that prevents traits from becoming fixed, which together generate self-perpetuating ecological-evolutionary dynamics.