Sex makes them sleepy; The impact of aphid sexuality on parasitoid diapause
Résumé
Aphids and their parasitoids typically enter diapause before the onset of unfavorable winter conditions, using mainly photoperiod and temperature as diapause-inducing cues. At the end of the growing season, aphids produce sexual individuals that reproduce and lay diapausing eggs, while aphid parasitoids enter diapause as prepupae inside their mummified aphid host. In addition to relying on environmental cues, parasitoids may use host phenotype as another indirect signal of upcoming winter conditions. We thus predicted that diapause incidence would be higher for parasitoids developing in sexual aphid individuals than in parthenogenetic individuals under a given climatic condition.
To test this prediction, we used a clone of the pea aphid (Acyrthosiphon pisum) producing both parthenogenetic and oviparous females. These two aphid morphs were parasitized by females of the parasitoid Aphidius ervi (Québec population) and next assigned to one of two temperature/photoperiod regimes: (a) fall-like conditions [17°C; L10:D14] known to induce parasitoid diapause, and (b) summer-like conditions [20°C; L16:D08] in which low levels of diapause induction were expected on parthenogenetic aphids.
In line with our prediction, diapause incidence was five times higher for parasitoids that parasitized sexual (20%) rather than parthenogenetic aphids (4%) under summer-like conditions. However, under fall-like conditions, 80% of the parasitoids entered diapause regardless of the aphid morph, suggesting that the effect of the host phenotype cue is overridden by abiotic cues.
Our results could have implications for the impacts of climate change on the evolution of host-parasitoid interactions and parasitoids seasonal phenology.