Abstract:
In order to reduce parasite-induced mortality, hosts may be involved in mutualistic interactions in
which the partner contributes to resistance against the parasite. The pea aphid, Acyrthosiphon pisum
Harris (Hemiptera: Aphididae), harbours secondary bacterial endosymbionts, some of which have
been reported to confer resistance against aphid parasitoids. Although this resistance often results in
death of the developing parasitoid larvae, some parasitoid individuals succeed in developing into
adults. Whether these individuals suffer from fitness reduction compared to parasitoids developing
in pea aphid clones without symbionts has not been tested so far. Using 30 pea aphid clones that differed
in their endosymbiont complement, we studied the effects of these endosymbionts on aphid
resistance against the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae: Aphidiinae),
host–parasitoid physiological interactions, and fitness of emerging adult parasitoids. The number of
symbiont species in an aphid clone was positively correlated with a number of resistance measurements
but there were also clear symbiont-specific effects on the host–parasitoid interaction. As in
previous studies, pea aphid clones infected with Hamiltonella defensa Moran et al. showed resistance
against the parasitoid. In addition, pea aphid clones infected with Regiella insecticolaMoran et al. and
co-infections of H. defensa–Spiroplasma, R. insecticola–Spiroplasma, and R. insecticola–H. defensa
showed reduced levels of parasitism and mummification. Parasitoids emerging from symbiontinfected
aphid clones often had a longer developmental time and reduced mass. The number of
teratocytes was generally lower when parasitoids oviposited in aphid clones with a symbiont complement.
Interestingly, unparasitized aphids infected with Serratia symbiotica Moran et al. and
R. insecticola had a higher fecundity than unparasitized aphids of uninfected pea aphid clones. We
conclude that in addition to conferring resistance, pea aphid symbionts also negatively affect parasitoids
that successfully hatch from aphid mummies. Because of the link between aphid resistance
and the number of teratocytes, the mechanism underlying resistance by symbiont infection may
involve interference with teratocyte development.
