Data from: Cheaper isn’t always worse: more protective isolates of a defensive symbiont are less costly to the aphid host

Defenses against parasites are typically associated with costs to the host that contribute to the maintenance of variation in resistance. This also applies to the defense provided by the facultative bacterial endosymbiont Hamiltonella defensa, which protects its aphid hosts against parasitoid wasps, while imposing life history costs in the absence of parasitoids. To investigate the cost-benefit relationship within protected hosts we introduced multiple isolates of H. defensa to the same genetic backgrounds of black bean aphids, Aphis fabae, and we quantified the protection against their parasitoid Lysiphlebus fabarum as well as the costs to the host (reduced lifespan and reproduction). Surprisingly, we observed the opposite of a trade-off. More protective isolates were significantly less costly to the host. This finding has important implications for the evolution of defensive symbiosis and highlights the need for a better understanding of how strain variation in protective symbionts is maintained.