Does Drosophila sechellia escape parasitoid attack by feeding on a toxic resource?

Abstract

Host shifts can drastically change the selective pressures that animals experience from their environment. Drosophila sechellia is a species restricted to the Seychelles islands, where it specialized on the fruit Morinda citrifolia (noni). This fruit is known to be toxic to closely related Drosophila species, including D. melanogaster and D. simulans, releasing D. sechellia from interspecific competition when breeding on this substrate. Previously, we showed that D. sechellia is unable to mount an effective immunological response against wasp attack, while the closely-related species can defend themselves from parasitoid attack by melanotic encapsulation. We hypothesized that this inability constitutes a trait loss due to a reduced risk of parasitoid attack in noni. Here we present a field study aimed to test the hypothesis that specialization on noni has released D. sechellia from the antagonistic interaction with its larval parasitoids. Our results from the field survey indicate that D. sechellia was found in ripe noni, whereas another Drosophila species, D. malerkotliana, was present in unripe and rotting stages. Parasitic wasps of the species Leptopilina boulardi emerged from rotten noni, where D. malerkotliana was the most abundant host. These results indicate that the specialization of D. sechellia on noni has indeed drastically altered its ecological interactions, leading to a relaxation in the selection pressure to maintain parasitoid resistance.