Data for: Does parasitoid species diversity promote protective symbiont diversity?

How does diversity in nature come about? One factor contributing to this diversity are species interactions; diversity on one trophic level can shape diversity on lower or higher trophic levels. For example, parasite diversity enhances host immune diversity. Protective symbionts mediate host resistance and are therefore also engaged in reciprocal selection with their host’s parasites. Here, we applied experimental evolution in a well-known symbiont-aphid-parasitoid system to study whether parasitoid diversity contributes to maintaining symbiont diversity. We used caged populations of black bean aphids (Aphis fabae), containing uninfected individuals and individuals infected with different strains of the bacterial endosymbiont Hamiltonella defensa, which protects aphids against parasitoids. Over multiple generations, these populations were exposed to three different species of parasitoid wasps (Aphidius colemani, Binodoxys acalephae, or Lysiphlebus fabarum), simultaneous or sequential mixtures of these species, or no wasps. Surprisingly, we observed little selection for H. defensa in most treatments, even when it clearly provided protection against a fatal parasitoid infection. This seemed to be caused by high induced costs of resistance: aphids surviving parasitoid attacks suffered an extreme reduction in fitness. In marked contrast to previous studies looking at the effect of different genotypes of a single parasitoid species, we found little evidence for a diversifying effect of multiple parasitoid species on symbiont diversity in hosts.

Methods
Data on aphid demograph from the experimental evolution experiments ("basic") was collected by counting aphids (roughly estimated by counting aphids in groups of ca. 10 individuals) and mummies (exact counts) and measuring plant size (i.e. total stem length of all plants) for each pot in each cage. Prior to data analysis, data for each cage was combined to obtain a single value for each cage in each generation (see associated paper for details).

To analyse H. Defensa composition ("mol") we first collected aphids at the end of generations 3 and 6, extracted their DNA, used diagnostic PCRs to amplify part of the bacterial 16S rRNA and part of H. defensa's murE gene for sequencing to distinguish H. defensa haplotypes. Data was recorded as either H. defensa absent or present and in the later case the identity of the haplotype. From this we calculate the proportion of H. defensa negative aphids and each H. defensa haplotype. This proportion was used in subsequent analysis (see associated paper for details).

Data on the experimental infection test ("test") was collected by counting aphids and mummies on each plant at different time points (see associated paper for details).