No evidence for single-copy immune-gene specific signals of selection in termites

Selection pressures from pathogens appear to play an important role in
shaping social evolution. Social behavior, in particular brood care, is
associated with pathogen pressure in wood-dwelling “lower” termites. Yet,
generally pathogen pressure is predicted to be low in wood-dwelling
termite species that never leave the nest except for the mating flight. In
comparison, pathogen pressure is predicted to be higher in species that
leave the nest to forage, and thus constantly encounter a diversity of
microbes from their environment. We hypothesized that such differences in
predicted pathogen pressure are also reflected by differences in the
intensity of natural selection on immune genes. We tested this hypothesis
in a phylogenetic framework, analyzing rates of non-synonymous and
synonymous substitutions on single-copy immune genes. Therefore, we
leveraged recent genomic and transcriptomic data from eight termite
species, representing wood-dwelling and foraging species as well as 14
additional species spanning the winged insects (Pterygota). Our results
provide no evidence for a role of pathogen pressure in selection intensity
on single-copy immune genes. Instead, we found evidence for a genome-wide
pattern of relaxed selection in termites.