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Abstract
Insects and other animals commonly form symbioses with heritable bacteria. These symbioses often have profound impacts on their hosts including providing protection against natural enemies. The pea aphid, Acyrthosiphon pisum, is a model organism for studying heritable symbioses and is associated with seven symbionts known or suspected to provide resistance to abiotic and biotic stresses. However, effects of infection with a common symbiont called X-type remain unclear. Using genetically controlled experimental lines, I found that X-type does not confer any of the established benefits associated with aphid symbioses, including protection against parasitoids, pathogens and thermal stress, yet is costly to aphids. Interestingly, X-type rarely occurs as a single infection and instead typically co-infects aphids with Hamiltonella defensa, which protects against parasitoids. Moreover, observed costs are ameliorated in co-infected aphids. Together these findings suggest that X-type is maintained in nature by the novel strategy of hitchhiking with H. defensas benefits.