Abstract
Secondary endosymbionts of aphids have an important ecological and evolutionary impact on their host, as they provide resistance to natural enemies but also reduce the host’s lifespan and reproduction. While secondary symbionts of aphids are faithfully transmitted from mother to offspring, they also have some capacity to be transmitted horizontally between aphids. Here we explore whether 11 isolates from 3 haplotypes of the secondary endosymbiont Hamiltonella defensa differ in their capacity for horizontal transmission. These isolates vary in the protection they provide against parasitoid wasps as well as the costs they inflict on their host, Aphis fabae. We simulated natural horizontal transmission through parasitoid wasps by stabbing aphids with a thin needle and assessed horizontal transmission success of the isolates from one shared donor clone into three different recipient clones. Specifically, we asked whether potentially costly isolates reaching high cell densities in aphid hosts are more readily transmitted through this route. This hypothesis was only partially supported. While transmissibility increased with titre for isolates from two haplotypes, isolates of the H. defensa haplotype 1 were transmitted with greater frequency than isolates of other haplotypes with comparable titres. Thus, it is not sufficient to be merely frequent—endosymbionts might have to evolve specific adaptations to transmit effectively between hosts.
Highlights
We hypothesised that costly isolates of H. defensa may persist in host populations because they gain some fitness from horizontal transmission, for example via parasitoid wasps (Gehrer and Vorburger, 2012), aided by a high density in the host
Isolates that reach a higher titre in the host’s hemolymph might increase their horizontal transmission rate as the wasp’s ovipositor is more likely to be contaminated with enough bacteria to establish an infection in a new host
We confirmed that isolates differ in their titre, we demonstrated that simple stabs with symbiont-contaminated needles did result in horizontal transmission, and we found that—within H. defensa haplotypes—higher titre was connected to increased transmission rate
Summary
They often influence ecologically relevant traits and the ecological niche of their hosts. Aphid and endosymbiont produce all the essential amino acids lacking from the aphid’s diet (Hansen and Moran, 2011). On their own, neither of the two organisms can survive, making this symbiosis a primary endosymbiosis. In addition to B. aphidicola, aphids can harbour other bacterial endosymbionts. While these provide benefits to the aphid, such as protection against
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