Development, specificity and sublethal effects of symbiont-conferred resistance to parasitoids in aphids

Authors

  • Maike Schmid,

    1. Institute of Evolutionary Biology and Environmental Studies, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
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  • Raphael Sieber,

    1. Institute of Evolutionary Biology and Environmental Studies, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
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  • Yannick-Serge Zimmermann,

    1. Institute of Evolutionary Biology and Environmental Studies, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
    2. Institute for Ecopreneurship, School of Life Sciences, University of Applied Sciences Northwestern Switzerland, Gründenstrasse 40, 4132 Muttenz, Switzerland
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  • Christoph Vorburger

    Corresponding author
    1. Institute of Integrative Biology, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland
    2. EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland
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Correspondence author. E-mail: christoph.vorburger@eawag.ch

Summary

1. One of the most exciting recent discoveries in the field of ecological immunology has been that insects employ the help of heritable symbionts as a defence against parasitoids and pathogens. Aphids commonly harbour the facultative bacterial endosymbiont Hamiltonella defensa, which is known to increase their resistance to parasitoids. It is unknown how this resistance develops during the aphids’ ontogeny, following the transmission bottleneck between mother and offspring, and how specific symbiont-conferred defences are.

2. We addressed these issues in the black bean aphid, Aphis fabae, by exposing aphids of different age classes to the parasitoid Lysiphlebus fabarum. The susceptibility of aphids that were either naturally or experimentally infected with H. defensa was compared with that of uninfected aphids.

3. Susceptibility to parasitoids decreased with aphid age, but aphids harbouring H. defensa showed an earlier and/or steeper decline to lower levels of susceptibility than aphids without this symbiont. This is consistent with the hypothesis that during aphid development, symbiont-conferred resistance builds up with bacterial population growth, which we documented using quantitative polymerase chain reaction (qPCR).

4. Parasitoids that successfully overcame the symbiont-conferred resistance still suffered from sublethal effects of H. defensa. They exhibited lower emergence, delayed development and reduced size compared with parasitoids developing in aphids without H. defensa.

5. The most striking result was a strong interaction on the rates of parasitism between aphid sublines infected with different isolates of H. defensa and the parasitoid lines they were exposed to, suggesting a high specificity of symbiont-conferred resistance.

6. Based on these results, we conclude that when faced with hosts possessing H. defensa, aphid parasitoids are under selection to preferentially attack the youngest host stages and/or to discriminate against symbiont-protected aphids. Furthermore, the specificity induced by H. defensa in the interaction between host and parasitoid is likely to have important consequences for co-evolution. It may result in negative frequency-dependent selection and thus promote genotypic variation.

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