Conclusion
To summarize, our study species diverged recently (Hambäck et al., 2013)
but have during a limited time evolved key phenotypic differences in the
defence against parasitoid wasps and other parasites. These phenotypic
differences allowed us to pinpoint important links between natural
selection on immune genes and host-parasitoid interactions where the
species with the highest immunocompetence had the highest number of
positively selected immune genes when focused upon coding gene evolution
over a deeper time scale. In contrast, analyses of recent selection
dynamics across the genome detected no differential evolution of the
immune system among these species. When examining candidate
immune-related genes, our results are consistent with the variable
selection pattern of immune genes in other studies (Heger & Ponting,
2007; Waterhouse et al., 2007), but also show that selection acts on
multiple genes in the immune pathways (Kraaijeveld et al., 1998). While
the arms-race between host and their natural enemies is expected to
increase the rate of divergence between allopatric populations (Buckling
& Hodgson, 2007; Buckling & Rainey, 2002), we find a varying degree of
support evidence for this hypothesis varies when assessed via molecular
tests of selection, with tests covering older events being more
concordant with expectations. It is likely that the phenotypic
differences in encapsulation capacity initially evolved during periods
when the beetles were geographically isolated and thereby coincided with
an allopatric speciation process. As such, the expected selection
dynamics shaping the immune system of the beetles are likely old, where
the observed phenotypic and genotypic differences have arisen during or
after the speciation process in a coevolutionary process between hosts
and parasitoid wasps.