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