Abstract
Parasitoid wasps are major causes of mortality of many species,
resulting in host immune defences commonly being the target of adaptive
evolution, though such targets outside model species are poorly
understood. Here we compare the power of different molecular tests of
selection to provide such insights in novel species. We combined our
understanding of variation in immune defence capacity among three
closely related Galerucella leaf beetles with a shared parasitoid
wasp, with information on genomic targets of parasitoid attacks from
exemplar insect species. Based on this, we predicted that these genomic
targets would vary in their evolutionary history across three closely
related leaf beetle species, such that genomic targets would experience
stronger positive selection in the species with strongest immune
response to attack. Codon based tests revealed variation among species
in positive selection genome wide, and showed that parasitoid-relevant
immune genes experienced more positive selection in the species with the
greatest immunocompetence (G. pusilla ), while almost no immune
genes were under positive selection in the species with the least
immunocompetence (G. calmariensis ). Genome wide analyses of the
haplotype frequency spectrum also identified genes experiencing positive
selection across the species, though few were parasitoid-relevant immune
genes and no species was particularly enriched for them. Thus, our codon
based test, which summarizes all sweep events since the last common
ancestor, found results consistent with our a priori hypothesis,
providing a series of targets for future functional genomic study.
Key words: host-parasitoid systems, positive selection, immune genes,
beetles, Galerucella