Silencing the expression of the salivary sheath protein causes transgenerational feeding suppression in the aphid Sitobion avenae

Aphids produce gel saliva during feeding which forms a sheath around the stylet as it penetrates through the apoplast. The sheath is required for the sustained ingestion of phloem sap from sieve elements and is thought to form when the structural sheath protein (SHP) is cross‐linked by intermolecular disulphide bridges. We investigated the possibility of controlling aphid infestation by host‐induced gene silencing (HIGS) targeting shp expression in the grain aphid Sitobion avenae. When aphids were fed on transgenic barley expressing shp double‐stranded RNA (shp‐dsRNA), they produced significantly lower levels of shp mRNA compared to aphids feeding on wild‐type plants, suggesting that the transfer of inhibitory RNA from the plant to the insect was successful. shp expression remained low when aphids were transferred from transgenic plants and fed for 1 or 2 weeks, respectively, on wild‐type plants, confirming that silencing had a prolonged impact. Reduced shp expression correlated with a decline in growth, reproduction and survival rates. Remarkably, morphological and physiological aberrations such as winged adults and delayed maturation were maintained over seven aphid generations feeding on wild‐type plants. Targeting shp expression therefore appears to cause strong transgenerational effects on feeding, development and survival in S. avenae, suggesting that the HIGS technology has a realistic potential for the control of aphid pests in agriculture.     

Quantitative trait loci mapping of phenotypic plasticity and genotype-environment interactions in plant and insect performance

Community genetic studies generally ignore the plasticity of the functional traits through which the effect is passed from individuals to the associated community. However, the ability of organisms to be phenotypically plastic allows them to rapidly adapt to changing environments and plasticity is commonly observed across all taxa. Owing to the fitness benefits of phenotypic plasticity, evolutionary biologists are interested in its genetic basis, which could explain how phenotypic plasticity is involved in the evolution of species interactions. Two current ideas exist: (i) phenotypic plasticity is caused by environmentally sensitive loci associated with a phenotype; (ii) phenotypic plasticity is caused by regulatory genes that simply influence the plasticity of a phenotype. Here, we designed a quantitative trait loci (QTL) mapping experiment to locate QTL on the barley genome associated with barley performance when the environment varies in the presence of aphids, and the composition of the rhizosphere. We simultaneously mapped aphid performance across variable rhizosphere environments. We mapped main effects, QTL × environment interaction (QTL×E), and phenotypic plasticity (measured as the difference in mean trait values) for barley and aphid performance onto the barley genome using an interval mapping procedure. We found that QTL associated with phenotypic plasticity were co-located with main effect QTL and QTL×E. We also located phenotypic plasticity QTL that were located separately from main effect QTL. These results support both of the current ideas of how phenotypic plasticity is genetically based and provide an initial insight into the functional genetic basis of how phenotypically plastic traits may still be important sources of community genetic effects.     

How the insect immune system interacts with an obligate symbiotic bacterium

The animal immune system provides defence against microbial infection, and the evolution of certain animal–microbial symbioses is predicted to involve adaptive changes in the host immune system to accommodate the microbial partner. For example, the reduced humoral immune system in the pea aphid Acyrthosiphon pisum, including an apparently non-functional immune deficiency (IMD) signalling pathway and absence of peptidoglycan recognition proteins (PGRPs), has been suggested to be an adaptation for the symbiosis with the bacterium Buchnera aphidicola. To investigate this hypothesis, the interaction between Buchnera and non-host cells, specifically cultured Drosophila S2 cells, was investigated. Microarray analysis of the gene expression pattern in S2 cells indicated that Buchnera triggered an immune response, including upregulated expression of genes for antimicrobial peptides via the IMD pathway with the PGRP-LC as receptor. Buchnera cells were readily taken up by S2 cells, but were subsequently eliminated over 1–2 days. These data suggest that Buchnera induces in non-host cells a defensive immune response that is deficient in its host. They support the proposed contribution of the Buchnera symbiosis to the evolution of the apparently reduced immune function in the aphid host.     

Aphis (Hemiptera: Aphididae) species groups found in the Midwestern United States and their contribution to the phylogenetic knowledge of the genus

A phylogeny of the genus Aphis Linnaeus, 1 758 was built primarily from specimens collected in the Midwest of the United States. A data matrix was constructed with 68 species and 41 morphological characters with respective character states of alate and apterous viviparous females. Dendrogram topologies of analyses performed using UPGMA (Unweighted Pair Group Method with Arithmetic Mean), Maximum Parsimony and Bayesian analysis of Cytochrome Oxidase I, Elongation Factor 1‐α and primary endosymbiont Buchnera aphidicola 16S sequences were not congruent. Bayesian analysis strongly supported most terminal nodes of the phylogenetic trees. The phylogeny was strongly supported by EF1‐α, and analysis of COI and EF1‐α molecular data combined with morphological characters. It was not supported by single analysis of COI or Buchnera aphidicola 16S. Results from the Bayesian phylogeny show 4 main species groups: asclepiadis, fabae, gossypii, and middletonii. Results place Aphis and species of the genera Protaphis Börner, 1952, Toxoptera Koch, 1856 and Xerobion Nevsky, 1928 in a monophyletic clade. Morphological characters support this monophyly as well. The phylogeny shows that the monophyletic clade of the North American middletonii species group belong to the genus Protaphis: P. debilicornis (Gillette & Palmer, 1929 ), comb. nov., P. echinaceae (Lagos and Voegtlin, 2009 ), comb. nov., and P. middletonii (Thomas, 1879 ). The genus Toxoptera should be considered a subgenus of Aphis (stat. nov.). The analysis also indicates that the current genus Iowana Frison, 1954 should be considered a subgenus of Aphis (stat. nov.).     

Facultative 'secondary' bacterial symbionts and the nutrition of the pea aphid, Acyrthosiphon pisum

Abstract.  Facultative 'secondary' bacterial symbionts influence various traits of aphids, including plant utilization patterns and resistance to parasitoids. The present study is designed to test the hypothesis that these multiple effects are underlain by symbiont-mediated changes to the aphid requirement for the dominant dietary nutrients, sucrose and amino acids. The performance of pea aphids ( Acyrthosiphon pisum ) on chemically defined diets of systematically altered sucrose and amino acid content varies among eight parthenogenetic clones, with a pattern that does not match the aphid complements of secondary symbionts, Hamiltonella defensa , Regiella insecticola and Serratia symbiotica . Aphid performance is reduced, increased and unaffected by elimination of S. symbiotica , R. insecticola and H. defensa , respectively, but with no significant effect on the range of diets on which aphids performed well. It is concluded that the impact of secondary symbionts on aphid traits is most unlikely to have a purely nutritional basis.     

Hymenopteran parasitoids and dipteran predators of <Emphasis Type="Italic">Diuraphis noxia</Emphasis> in the west-central Great Plains of North America: Species records and geographic range

Parasitoids and predatory flies were sampled in the wheat production region of the west-central Great Plains (southeastern Wyoming, western Nebraska, and north-central Colorado) of North America using plant material infested with the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae). Samples were taken April through October in 2001 and 2002, which was 15–16 years after first detection of D. noxia and 5–6 years after the last release of natural enemies for its control in this region. The natural enemies detected were (in order of high to low detection frequencies across three states and 2 years): Aphelinus albipodus Hayat and Fatima (Hymenoptera: Aphelinidae), Eupeodes volucris Osten Sacken (Diptera: Syrphidae), Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae, Aphidiinae), Leucopis gaimarii Tanasijtshuk (Diptera: Chamaemyiidae), Aphidius avenaphis (Fitch), Aphidius matricariae Haliday, Diaeretiella rapae (MIntosh), Aphidius ervi Haliday, Praon yakimanum Pike and Starý (Hymenoptera: Braconidae, Aphidiinae), and Aphelinus asychis Walker (Hymenoptera: Aphelinidae). The results confirmed establishment of one of the 10 exotic parasitoid species released for D. noxia control (A. albipodus) in the west-central Great Plains. It is unknown whether detection of A. asychis, A. matricariae, and D. rapae can be attributed to exotic introductions or preexisting populations. Other species detected in this study have been previously documented from the western US, although the recognized distributions have expanded for A. avenaphis, L. gaimarii, and P. yakimanum compared to the first few years after initial detection of D. noxia. Thus, there is definitive establishment of one exotic introduced for D. noxia and considerable range expansion of preexisting species that prey upon D. noxia.     

Density-dependent induction and suppression of soldier differentiation in an aphid social system

We investigated the mechanism underlying the control of soldier production in colonies of a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the second instar. High aphid density was shown to induce soldier production in T. styraci. Analysis of natural colonies revealed that the soldier proportion tended to increase with aphid density but reached a plateau. Artificial diet experiments identified a similar plateau of soldier proportion under high-density conditions. In order to gain insights into the controlling mechanism of soldier production, the effect of soldiers on reproducing adult aphids was examined using the artificial diet system. It was experimentally demonstrated that soldier production was suppressed by coexisting soldiers, whereas coexisting non-soldiers facilitated soldier production. These results suggested that caste ratio in the colony of T. styraci is controlled by positive and negative feedbacks consisting of density-dependent induction and suppression of soldier differentiation.     

Development of PCR-based chloroplast DNA markers that characterize domesticated cowpea (Vigna unguiculata ssp. unguiculata var. unguiculata) and highlight its crop-weed complex

In 1992, Vaillancourt and Weeden discovered a very important mutation for studying cowpea evolution and domestication. A loss of a BamHI restriction site in chloroplast DNA characterized all domesticated accessions and a few wild (Vigna unguiculata ssp. unguiculata var. spontanea) accessions. In order to screen a larger number of accessions, primers were designed to check this mutation using PCR RFLP or direct PCR methods. Using these new primers, 54 domesticated cowpea accessions and 130 accessions from the wild progenitor were screened. The absence of haplotype 0 was confirmed within domesticated accessions, including primitive landraces from cultivar-groups Biflora and Textilis, suggesting that this mutation occurred prior to domestication. However, 40 var. spontanea accessions distributed from Senegal to Tanzania and South Africa showed haplotype 1. Whereas this marker could not be used to identify a precise center of origin, it did highlight the widely distributed cowpea crop-weed complex. Its very high frequency in West Africa could be interpreted as a result of either genetic swamping of the wild/weedy gene pool by the domesticated cowpea gene pool or as the result of domestication by ethnic groups focusing primarily on cowpea as fodder.     

Effect of low and high-saponin lines of alfalfa on pea aphid

Pea aphid fed on a high-saponin line of alfalfa showed reduction of reproduction and survival, and disturbance in development of its population. This line negatively influenced aphid probing behaviour, particularly prolonging the non-probing period and probing of the peripheral tissues (epidermis and mesophyll) and shortening the period of phloem sap ingestion. The high-saponin line of alfalfa differed from the low-saponin one by the presence of zanhic acid tridesmoside and a higher level of 3-GlcA,28-AraRhaXyl medicagenic acid glycoside. The saponins incorporated into sucrose-agarose gels significantly reduced number of the aphid probes into the gels and extended their duration in comparison to the control gels (without tested compounds). Role of zanhic acid tridesmoside and 3-GlcA,28-AraRhaXyl medicagenic acid glycoside as potential factors for partial resistance of alfalfa towards the pea aphid is discussed.