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.