Defensive symbiosis in the wild: seasonal dynamics of parasitism risk and symbiont-conferred resistance

<p class="MsoNormal"><span>Parasite-mediated selection can rapidly drive up resistance levels in host populations, but fixation of resistance traits may be prevented by costs of resistance. Black bean aphids (<em>Aphis fabae</em>) benefit from increased resistance to parasitoids when carrying the defensive bacterial endosymbiont <em>Hamiltonella defensa</em>. However, due to fitness costs that come with symbiont infection, symbiont-conferred resistance may result in either a net benefit or a net cost to the aphid host, depending on parasitoid presence as well as on the general ecological context. Balancing selection may therefore explain why in natural aphid populations, <em>H. defensa</em> is often found at intermediate frequencies.<strong> </strong>Here we present a two-year field study where we set out to look for signatures of balancing selection in natural aphid populations. We collected temporally well-resolved data on the prevalence of <em>H. defensa</em> in <em>A.f. fabae</em> and estimated the risk imposed by parasitoids using sentinel hosts.<strong> </strong>Despite a marked and consistent early-summer peak in parasitism risk and significant changes in symbiont prevalence over time, we found just a weak correlation between parasitism risk and <em>H. defensa </em>frequency dynamics. <em>H. defensa </em>prevalence in the populations under study was, in fact, better explained by the number of heat days that previous aphid generations were exposed to.<strong> </strong>Our study grants an unprecedentedly well-resolved insight into the dynamics of endosymbiont and parasitoid communities of <em>A.f. fabae </em>populations, and it adds to a growing body of empirical evidence suggesting that not only parasitism risk but rather multifarious selection is shaping <em>H. defensa</em> prevalence in the wild.</span></p>