Jennifer A. White

Use of adoptive transfer of T-cell-antigen-receptor-transgenic T cells for the study of T-cell activation in vivo

Summary: Adoptive transfer of TCR‐transgenic T cells uniformly expressing an identifiable TCR of known peptide/MHC specificity can be used to monitor the in vivo behavior of antigen‐specific T cells. We have used this system to show that naive T cells are initially activated within the T‐cell zones of secondary lymphoid tissue to prohferate in a B7‐dependent manner. If adjuvants or inflammatory cytokines are present during this period, enhanced numbers of T cells accumulate, migrate into B‐cell‐rich follicles, and acquire the capacity to produce IFN‐7 and help B cells produce IgG2a. If inflammation is absent, most of the initially activated antigen‐specific T cells disappear without entering the follicles and the survivors are poor producers of IL‐2 and IFN‐γ Our results indicate that inflammatory mediators play a key role in regulating the anatomic location, clonal expansion, survival and lymphokine production potential of antigen‐stimulated T cells in vivo.

Cytoplasmic incompatibility in the parasitic wasp Encarsia inaron: disentangling the roles of Cardinium and Wolbachia symbionts.

Many bacterial endosymbionts of insects are capable of manipulating their hosts reproduction for their own benefit. The most common strategy of manipulation is cytoplasmic incompatibility (CI), in which embryonic mortality results from matings between uninfected females and infected males. In contrast, embryos develop normally in infected females, whether or not their mate is infected, and infected progeny are produced. In this way, the proportion of infected females increases in the insect population, thereby promoting the spread of the maternally inherited bacteria. However, what happens when multiple endosymbionts inhabit the same host? The parasitoid wasp Encarsia inaron is naturally infected with two unrelated endosymbionts, Cardinium and Wolbachia, both of which have been documented to cause CI in other insects. Doubly infected wasps show the CI phenotype. We differentially cured E. inaron of each endosymbiont, and crossed hosts of different infection status to determine whether either or both bacteria caused the observed CI phenotype in this parasitoid, and whether the two symbionts interacted within their common host. We found that Wolbachia caused CI in E. inaron, but Cardinium did not. We did not find evidence that Cardinium was able to modify or rescue Wolbachia-induced CI, nor did we find that Cardinium caused progeny sex ratio distortion, leaving the role of Cardinium in E. inaron a mystery.

Specialisation of bacterial endosymbionts that protect aphids from parasitoids

1. Infection by the bacterial endosymbiont Hamiltonella defensa is capable of protecting the pea aphid from parasitism by Aphidius ervi and the black bean aphid from parasitism by Lysiphlebus fabarum. Here we investigate protection of a third aphid species, the cowpea aphid, Aphis craccivora, from four parasitoid species: Binodoxys communis, B. koreanus, Lysiphlebus orientalis, and Aphidius colemani.

Facultative endosymbionts mediate dietary breadth in a polyphagous herbivore

Summary Intraspecific variation in dietary breadth can influence important ecological and evolutionary processes, yet the mechanisms generating this variation are usually unknown. Maternally transmitted bacterial symbionts frequently infect insect herbivores, and many have been shown to mediate key ecological interactions. For polyphagous herbivores, infection with particular symbionts is often strongly correlated with feeding on particular plant species, suggesting that facultative symbionts might directly determine herbivore food plant specificity. However, previous tests of this hypothesis have returned inconsistent results, providing little empirical support for a causal relationship between facultative symbiont infection and dietary breadth. Here, we investigate whether heritable facultative symbionts mediate dietary breadth in the polyphagous aphid, Aphis craccivora. We first determined that asexual clones of the aphid differ dramatically in performance across two leguminous food plants, locust and alfalfa, and could be considered biotypes with distinct ecological characteristics. The heritable symbiont Arsenophonus is strongly associated with locust-origin aphids. We created experimental lines that share aphid genotypes but differed with respect to Arsenophonus infection status, and compared performance across three food plant species. Naturally Arsenophonus-infected clones performed 2–4× better on locust and up to 75% worse on two alternate plant species than uninfected controls, clearly demonstrating that Arsenophonus promotes specialization on locust. In both laboratory and field experiments, uninfected locust- and alfalfa-origin clones exhibited similar and modest performance on locust, indicating that the ‘locust-associated biotype’ would not exist without Arsenophonus. We also hypothesized that moving Arsenophonus, via transinfection, to an alfalfa-origin lineage would improve performance on locust and serve to expand dietary breadth. Indeed, transinfection doubled aphid performance on locust and halved aphid performance on alfalfa. However, because this aphid lineage naturally performs better on alfalfa, the transinfected symbiont functionally equalized aphid performance between locust and alfalfa, making the alfalfa biotype more generalized. Thus, the same symbiont can either reduce or expand dietary breadth, depending on host genotype. Our results unequivocally demonstrate that symbiont gain or loss can instantaneously and substantially change the topology of food plant use in a polyphagous insect, modifying diet in ways that potentially influence the insects ecological niche, evolutionary trajectory and pest status.

Cowpea aphid (Aphis craccivora) associated with different host plants has different facultative endosymbionts

Maternally inherited facultative endosymbiotic bacteria are common among insects, including many polyphagous insect herbivores. To investigate whether symbiont infection is structured by host plant in the polyphagous aphid Aphis craccivora Koch, pyrosequencing and diagnostic PCR were performed on 26 populations from two different host plants, alfalfa (Medicago sativa) or black locust (Robinia pseudoacacia). Results indicated that Aphis craccivora harbours distinctly different microbial communities in alfalfa versus locust. The facultative symbiont Hamiltonella was found only in aphids collected from alfalfa, and the facultative symbiont Arsenophonus was found only in aphids from locust. Hamiltonella is known to protect aphids against hymenopteran parasitoids, whereas the phenotypic effects of Arsenophonus in aphids are unknown. Correspondingly, a screen of the aphid samples for hymenopteran DNA indicated that Hamiltonella‐bearing alfalfa populations of A. craccivora experienced lower parasitism than Arsenophonus‐bearing locust populations. This study contributes to the growing body of evidence that correlative associations between bacterial endosymbionts and host plants may be a common phenomenon in polyphagous herbivores, and suggests that microbial symbionts have the potential to act as drivers for observed ecological differences among host‐associated populations of polyphagous insects.

Habitat modification contributes to associational resistance between herbivores

Associational resistance, in which one species gains protection from its consumers by association with a competitor, is common among plants but has seldom been documented among insects. Here we show that parasitism of an aboveground herbivore, European corn borer, by its specialist parasitoid Macrocentrus grandii, is reduced 98% in the presence of a belowground herbivore, corn rootworm. We tested the hypothesis that this positive indirect interaction between corn rootworm and corn borer was mediated by corn rootworm’s influence on the habitat: plant height was diminished by 33% and plant density by 20%, resulting in a more open habitat. We found that M. grandii showed a two- to five-fold preference for dense versus open habitats, and that experimental reduction of plant density in the absence of corn rootworm reduced M. grandii parasitism of corn borer by 13%. This result supports the presence of a habitat modification effect as a contributing factor to associational resistance for corn borer. We argue that associational resistance may be more common among phytophagous insects than previously appreciated.

Endosymbiont costs and benefits in a parasitoid infected with both Wolbachia and Cardinium

Theory suggests that maternally inherited endosymbionts can promote their spread and persistence in host populations by enhancing the production of daughters by infected hosts, either by improving overall host fitness, or through reproductive manipulation. In the doubly infected parasitoid wasp Encarsia inaron, Wolbachia manipulates host reproduction through cytoplasmic incompatibility (CI), but Cardinium does not. We investigated the fitness costs and/or benefits of infection by each bacterium in differentially cured E. inaron as a potential explanation for persistence of Cardinium in this population. We introgressed lines infected with Wolbachia, Cardinium or both with the cured line to create a similar genetic background, and evaluated several parasitoid fitness parameters. We found that symbiont infection resulted in both fitness costs and benefits for E. inaron. The cost was lower initial egg load for all infected wasps. The benefit was increased survivorship, which in turn increased male production for wasps infected with only Cardinium. Female production was unaffected by symbiont infection; we therefore have not yet identified a causal fitness effect that can explain the persistence of Cardinium in the population. Interestingly, the Cardinium survivorship benefit was not evident when Wolbachia was also present in the host, and the reproduction of doubly infected individuals did not differ significantly from uninfected wasps. Therefore, the results of our study show that even when multiple infections seem to have no effect on a host, there may be a complex interaction of costs and benefits among symbionts.

Factors limiting the spread of the protective symbiont Hamiltonella defensa in Aphis craccivora aphids

ABSTRACT Many insects are associated with heritable symbionts that mediate ecological interactions, including host protection against natural enemies. The cowpea aphid, Aphis craccivora, is a polyphagous pest that harbors Hamiltonella defensa, which defends against parasitic wasps. Despite this protective benefit, this symbiont occurs only at intermediate frequencies in field populations. To identify factors constraining H. defensa invasion in Ap. craccivora, we estimated symbiont transmission rates, performed fitness assays, and measured infection dynamics in population cages to evaluate effects of infection. Similar to results with the pea aphid, Acyrthosiphon pisum, we found no consistent costs to infection using component fitness assays, but we did identify clear costs to infection in population cages when no enemies were present. Maternal transmission rates of H. defensa in Ap. craccivora were high (ca. 99%) but not perfect. Transmission failures and infection costs likely limit the spread of protective H. defensa in Ap. craccivora. We also characterized several parameters of H. defensa infection potentially relevant to the protective phenotype. We confirmed the presence of H. defensa in aphid hemolymph, where it potentially interacts with endoparasites, and performed real-time quantitative PCR (qPCR) to estimate symbiont and phage abundance during aphid development. We also examined strain variation of H. defensa and its bacteriophage at multiple loci, and despite our lines being collected in different regions of North America, they were infected with a nearly identical strains of H. defensa and APSE4 phage. The limited strain diversity observed for these defensive elements may result in relatively static protection profile for this defensive symbiosis.

Pyrosequencing reveals a shift in symbiotic bacteria populations across life stages of Bactrocera dorsalis

Bactrocera dorsalis is one of the most economically important fruit flies around the world. In this study, 454 pyrosequencing was used to identify the bacteria associated with different developmental stages of B. dorsalis. At ≥ 97% nucleotide similarity, total reads could be assigned to 172 Operational Taxonomic Units belonging to six phyla. Proteobacteria dominated in immature stages while Firmicutes dominated in adult stages. The most abundant families were Enterococcaceae and Comamondaceae. The genus Comamonas was most abundant in pupae whereas completely absent in adults. Some identified species had low sequence similarity to reported species indicating the possibility of novel taxa. However, a majority sequence reads were similar to sequences previously identified to be associated with Bactrocera correcta, suggesting a characteristic microbial fauna for this insect genus. The type and abundance of different bacterial groups varied across the life stages of B. dorsalis. Selection pressure exerted by the host insect as a result of its habitat and diet choices could be the reason for the observed shift in the bacteria groups. These findings increase our understanding of the intricate symbiotic relationships between bacteria and B. dorsalis and provide clues to develop potential biocontrol techniques against this fruit fly.

Assessments of Fitness Effects by the Facultative Symbiont Rickettsia in the Sweetpotato Whitefly (Hemiptera: Aleyrodidae)

ABSTRACT The sweet potato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), harbors several bacterial symbionts, including the obligate primary symbiont Portiera aleyrodidarum and the facultative secondary symbionts Arsenophonus, Cardinium, Fritschea, Hamiltonella, Rickettsia, and Wolbachia. The roles of these symbionts are yet unknown. In this study, we tested for possible effects of one symbiont, Rickettsia, on some fitness parameters of B. tabaci (biotype B) by comparing whiteflies that carry this symbiont to whiteflies that do not. Preadult development of Rickettsiacarrying whiteflies was faster, but all the other parameters that were measured: longevity, total number of progeny, sex ratio, and nymphal survivorship did not differ significantly. Estimates of the intrinsic growth rate (r) were almost identical for the two groups. Cross-mating between clefts-carrying and Rickettsia-free whiteflies provided no evidence for cytoplasmic incompatibility. Vertical transmission of Rickettsia was found to be nearly complete. Our results do not clearly identify a selective advantage that would explain the high prevalence of Rickettsia in B. tabaci populations, thus, other fitness parameters and horizontal transmission routes are suggested and discussed.