Insights into the triggering factors and determination mechanisms of host alternation in aphids

The bird cherry-oat aphid Rhopalosiphum padi, is one of the ten percent of aphids that are host alternating. The annual life cycle of R. padi entails an obligatory migration between a primary woody host, Prunus padus (Rosaceae), where sexual reproduction occurs, and many secondary hosts of the Poaceae family, where successive generations reproduce parthenogenetically. Migration between the two types of host plants is achieved by specialized migrant morphs. Very little is known about the cues that trigger migrant development, host alternation establishment and the underlying mechanisms. In this study, we aimed to elucidate the key steps involved in migrant induction and determination of host alternation, and to shed light on these intricate processes. We showed that a specific stage of leaf maturity triggers the development into a migrant morph when presented to R. padi at the first or second larval instar stages. We also demonstrated the gradual establishment of acclimation to the secondary hosts that precedes the migration step. Finally, we showed that the switch to feeding on the alternative hosts is irreversible once emigrants have reached adulthood. These results not only allowed us to define the critical periods of the leaf stage associated with morph induction and determination, but also to pinpoint the emigrants developmental stages when acclimation to secondary hosts begins. These novel insights of these critical periods will facilitate the investigation of molecular changes associated with host alternation in aphids.