Maternally transmitted symbionts can spread in host populations if they\udprovide a fitness benefit to their hosts. Hamiltonella defensa, a bacterial\udendosymbiont of aphids, protects hosts against parasitoids but only occurs at\udmoderate frequencies in most aphid populations. This suggests that harbouring\udthis symbiont is also associated with costs, yet the nature of these costs has\udremained elusive. Here, we demonstrate an important and clearly defined\udcost: reduced longevity. Experimental infections with six different isolates of\udH. defensa caused strongly reduced lifespans in two different clones of the black\udbean aphid, Aphis fabae, resulting in a significantly lower lifetime reproduction.\udHowever, the two aphid clones were unequally affected by the presence\udof H. defensa, and the magnitude of the longevity cost was further determined\udby genotype · genotype interactions between host and symbiont, which has\udimportant consequences for their coevolution.
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