High specificity of symbiont-conferred resistance in an aphid-parasitoid field community

<p class="MsoNormal"><span>Host-parasite coevolution is mediated by genetic interactions between the antagonists and may lead to reciprocal adaptation. In the black bean aphid, <em>Aphis fabae fabae</em>, resistance to parasitoids can be conferred by the heritable bacterial endosymbiont <em>Hamiltonella defensa</em>. <em>H. defensa</em> has been shown to be variably protective against different parasitoid species, and different genotypes of the black bean aphid's main parasitoid <em>Lysiphlebus fabarum</em>.<em> </em>However, these results were obtained using haphazard combinations of laboratory-reared insect lines with different origins, making it unclear how representative they are of natural, locally (co)adapted communities. We therefore comprehensively sampled the parasitoids of a natural <em>A. f. fabae</em> population and measured the ability of the five most abundant species to parasitize aphids carrying the locally prevalent <em>H. defensa</em> haplotypes. <em>H. defensa</em> provided resistance only against the dominant parasitoid <em>L. fabarum</em> (70% of all parasitoids), but not against less abundant parasitoids, and resistance to <em>L. fabarum</em> acted in a genotype-specific manner (G × G interactions between <em>H. defensa</em> and <em>L. fabarum</em>). These results confirm that strong species- and genotype-specificity of symbiont-conferred resistance is indeed a hallmark of wild <em>A. f. fabae</em> populations, and they are consistent with symbiont-mediated local adaptation of aphids to parasitoids.</span></p>