Population dynamics of defensive symbionts in aphids

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Abstract
Vertically transmitted micro-organisms can increase in frequency in host populations by providing net benefits to hosts. While laboratory studies have identified diverse beneficial effects conferred by inherited symbionts of insects, they have not explicitly examined the population dynamics of mutualist symbiont infection within populations. In the pea aphid,Acyrthosiphon pisum, the inherited facultative symbiont,Hamiltonella defensa, provides protection against parasitism by the wasp,Aphidius ervi. Despite a high fidelity of vertical transmission and direct benefits of infection accruing to parasitized aphids,Hamiltonellaremains only at intermediate frequencies in natural populations. Here, we conducted population cage experiments to monitor the dynamics ofHamiltonellaand of another commonA. pisumsymbiont,Serratia symbiotica, in the presence and absence of parasitism. We also conducted fitness assays ofHamiltonella-infected aphids to search for costs to infection in the absence of parasitism. In the population cages, we found that the frequency ofA. pisuminfected withHamiltonellaincreased dramatically after repeated exposure to parasitism byA. ervi, indicating that selection pressures from natural enemies can lead to the increase of particular inherited symbionts in insect populations. In our laboratory fitness assays, we did not detect a cost to infection withHamiltonella, but in the population cages not exposed to parasitism, we found a significant decline in the frequency of bothHamiltonellaandSerratia. The declining frequencies ofHamiltonella-infected aphids in population cages in the absence of parasitism indicate a probable cost to infection and may explain whyHamiltonellaremains at intermediate frequencies in natural populations.