Do facultative symbionts affect fitness of pea aphids in the sexual generation?

Many aphids carry one or more facultative symbiotic bacteria which can provide a variety of fitness benefits for their hosts. They have chiefly been investigated in asexually reproducing aphids, with studies of the sexual generation limited to investigation of transmission rates and the potential for sex ratio manipulation. The effects of two facultative symbionts on the mating success of pea aphids, A cyrthosiphon pisum H arris ( H emiptera: A phididae, M acrosiphini), were investigated in a no‐choice experiment. We compared the fitness of aphids with natural infections of either R egiella insecticola Moran et al. or H amiltonella defensa Moran et al. with that of aphids from genetically identical cured lines. Female fecundity was unaffected by the presence of facultative symbionts. However, females mated to males cured of H . defensa laid fewer eggs on average than females mated to males carrying H . defensa ; in one case the percentage of melanized eggs (eggs that were either not fertilized or in which early death occurred) was also smaller with cured compared to naturally infected males. In addition, males with H . defensa suffered higher mortality during the experiment than cured males. Four of the aphid lines used also hosted S piroplasma infections, a symbiont previously reported to cause male‐killing in pea aphids. Despite this, three of the S piroplasma ‐infected lines produced males, two at high numbers. We conclude that removing a natural symbiont infection may have a negative fitness effect on male aphids in some aphid clones, whereas sexual females from the same clones are largely unaffected.