Evolutionary costs and benefits of infection with diverse strains of Spiroplasma in pea aphids*

The heritable endosymbiont Spiroplasma infects many insects and has repeatedly evolved the ability to protect its hosts against different parasites. Defenses do not come for free to the host, and theory predicts that more costly symbionts need to provide stronger benefits to persist in host populations. We investigated the costs and benefits of Spiroplasma infections in pea aphids (Acyrthosiphon pisum), testing 12 bacterial strains from three...