Trade-off associated with selection for increased ability to resist parasitoid attack in Drosophila melanogaster.

Costs of resistance are widely assumed to be important in the evolution of parasite and pathogen defence in animals, but they have been demonstrated experimentally on very few occasions. Endoparasitoids are insects whose larvae develop inside the bodies of other insects where they defend themselves from attack by their hosts'' immune systems (especially cellular encapsulation). Working with Drosophila melanogaster and its endoparasitoid Leptopilina boulardi, we selected for increased resistance in four replicate populations of flies. The percentage of flies surviving attack increased from about 0.5% to between 40% and 50% in five generations, revealing substantial additive genetic variation in resistance in the field population from which our culture was established. In comparison with four control lines, flies from selected lines suffered from lower larval survival under conditions of moderate to severe intraspecific competition.     

Absence of direct sexual selection for parasitoid encapsulation in Drosophila melanogaster

Resistance against parasites may play a role in female mate choice, especially if males that have actually survived parasitism can be discriminated from males that have not been parasitised. Larvae of several Drosophila species are subject to attack by hymenopteran parasitoids, but have the ability to kill the parasitoid egg through the process of encapsulation. Because an encapsulated egg remains visible in the abdomen of the adult fly throughout its life, its presence in a male signals to a female that the male has the genes to survive parasitism. The hypothesis that females preferentially mate with males bearing an encapsulated egg in their abdomen was tested using D. melanogaster. No indication was found for this female preference. The absence of preference for males with “good genes” could result from sensory constraints in the female or a negative correlation between encapsulation ability and some other fitness component. Alternatively, it is hypothesised that the black abdominal ends of the males of many species in the melanogaster-group evolved to mimic encapsulated eggs, leading to the breakdown of capsule recognition by the female.