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.

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Published in Applied and Environmental Microbiology, v. 80, no. 18, p. 5818-5827.

Copyright © 2014, American Society for Microbiology. All Rights Reserved.

The copyright holder has granted permission for posting the article here.

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This work was funded by USDA-AFRI grant 2009-65104-05983.

Appl. Environ. Microbiol. 2014 Sep 80(18) 5818-27, FIG 1.ppt (147 kB)
FIG 1: Frequencies of H. defensa infection over time in population cages held at 20 (dashed line) and 25°C (solid line). At time zero, all cages started with 50% infected aphids. For each time point, mean infection frequencies (with standard error) are presented.

Appl. Environ. Microbiol. 2014 Sep 80(18) 5818-27, FIG 2.ppt (199 kB)
FIG 2: H. defensa and APSE4 titers over time. Mean genomic copies (y axis) of H. defensa (dnaK) and APSE (P28) for AC1 (A and B) and SV1 (C and D) at 20 and 25°C are shown. Values represent whole aphid (n = 8). Bars represent ranges.

Appl. Environ. Microbiol. 2014 Sep 80(18) 5818-27, FIG 3.ppt (142 kB)
FIG 3: Mean ratio of APSE copies to H. defensa copies. For each column, the mean ratio is shown (n = 8); bars represent 95% confidence intervals.

zam999105659so1.pdf (160 kB)
SUPPLEMENTAL FILE 1: H. defensa from hemolymph samples (Fig. S1) and estimates of selection from linear regression of logit over time (Table S1).

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