Phenotypic plasticity in the invasive pest Drosophila suzukii: activity rhythms and gene expression in response to temperature

Phenotypic plasticity may contribute to the invasive success of an alien species in a new environment. A higher plastic species may survive and reproduce in more diverse environments, thereby supporting establishment and colonization. We focused on plasticity in the circadian rhythm of activity, which can favour species coexistence in invasion, for the invasive species Drosophila suzukii, which is expected to be a weaker direct competitor than other Drosophila species of the resident community. We compared between the invasive D. suzukii and the resident D. melanogaster the circadian rhythms of the locomotor activity in adults and the expression of clock genes in response to temperature. We showed that D. suzukii is active in a narrower range of temperatures than D. melanogaster and that the activities of both species overlap during the day, regardless of the temperature. Both species are diurnal and exhibit rhythmic activity at dawn and dusk, with a much lower activity at dawn for D. suzukii females. Our results showed that the timeless and clock genes are good candidates to explain the plastic response that is observed in relation to temperature. Overall, our results suggest that thermal phenotypic plasticity in D. suzukii activity is not sufficient to explain the invasive success of D. suzukii and calls for testing other hypotheses, such as the release of competitors and/or predators.

The invasive pest Drosophila suzukii uses trans-generational medication to resist parasitoid attack

Animal medication is a behavioral strategy to resist enemies based on the use of substances from the environment. While it has been observed in several animals, whether invasive species can use medication to resist new enemies during its expansion is unknown. Here, we show that the worldwide invasive pest Drosophila suzukii performs trans-generational prophylactic medication by adapting its oviposition behavior in the presence of enemies. We find that flies preferentially lay their eggs on media containing atropine – an entomotoxic alkaloid – in the presence of parasitoids. We further show that flies developing on atropine more efficiently resist parasitization by parasitoids. Finally, we find that developing in hosts reared on atropine strongly impacts the life-history traits of parasitoids. This protective behavior is reported for the first time in a pest and invasive species, and suggests that animal medication may be an important driver of population dynamics during invasions.

Influence of oxidative homeostasis on bacterial density and cost of infection in Drosophila-Wolbachia symbioses

The evolution of symbioses along the continuum between parasitism and mutualism can be influenced by the oxidative homeostasis, that is the balance between reactive oxygen species (ROS) and antioxidant molecules. Indeed, ROS can contribute to the host immune defence to regulate symbiont populations, but are also toxic. This interplay between ROS and symbiosis is notably exemplified by recent results in arthropod-Wolbachia interactions. Wolbachia are symbiotic bacteria involved in a wide range of interactions with their arthropods hosts, from facultative, parasitic associations to obligatory, mutualistic ones. In this study, we used Drosophila-Wolbachia associations to determine whether the oxidative homeostasis plays a role in explaining the differences between phenotypically distinct arthropod-Wolbachia symbioses. We used Drosophila lines with different Wolbachia infections and measured the effects of pro-oxidant (paraquat) and antioxidant (glutathione) treatments on the Wolbachia density and the host survival. We show that experimental manipulations of the oxidative homeostasis can reduce the cost of the infection through its effect on Wolbachia density. We discuss the implication of this result from an evolutionary perspective and argue that the oxidative homeostasis could underlie the evolution of tolerance and dependence on Wolbachia.