Abstract
Antagonistic relationships between parasitoids and their insect hosts involve multiple traits and are shaped by their ecological and evolutionary context. The parasitoid wasp Cotesia melitaearum and its host butterfly Melitaea cinxia occur in several locations around the Baltic sea, with differences in landscape structure, population sizes and the histories of the populations. We compared the virulence of the parasitoid and the susceptibility of the host from five populations in a reciprocal transplant-style experiment using the progeny of five independent host and parasitoid individuals from each population. The host populations showed significant differences in the rate of encapsulation and parasitoid development rate. The parasitoid populations differed in brood size, development rate, pupal size and adult longevity. Some trait differences depended on specific host-parasitoid combinations, but neither species performed systematically better or worse in experiments involving local versus non-local populations of the other species. Furthermore, individuals from host populations with the most recent common ancestry did not perform alike, and there was no negative effect due to a history of inbreeding in the parasitoid. The complex pattern of variation in the traits related to the vulnerability of the host and the ability of the parasitoid to exploit the host may reflect multiple functions of the traits that would hinder simple local adaptation.
Highlights
A parasitoid wasp is a parasite that develops as a larva in or on a host arthropod, eventually killing it
We first discuss specific results related to encapsulation rate and parasitoid life-history traits, and the broader patterns in terms of genetic differentiation among populations, the potential for local adaptation, and inbreeding
Population-level differences in the encapsulation ability of the host are not surprising as such differences have been found previously, even in Cotesia-Lepidoptera interactions [45], and encapsulation ability of M. cinxia is known to vary among families and with stress [8,56]
Summary
A parasitoid wasp is a parasite that develops as a larva in or on a host arthropod, eventually killing it. In parasitoid-arthropod interactions, like host-parasite interactions in general, hosts evolve to avoid or resist parasitism, and parasites evolve to become more virulent. This antagonistic coevolution, which contributes to biological diversity [1], is often expected to lead to systematic and predictable reciprocal changes or local adaptation of one or the other of the partners in the interaction [2,3]. An important measure of both host resistance and parasitoid virulence is the ability of a host to kill parasitoid eggs or larvae via mechanisms such as encapsulation. This ability may vary geographically [4,5]
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