Assessment of the dunnocks' introduction to New Zealand using innate immune-gene diversity.
Immunity genes are proposed to be informative about evolutionary processes acting upon introduced populations (e.g., showing signatures of selection). This is because immunity genes are expected to be under pathogen-mediated selection, and this type of selection can be more pronounced when individuals are exposed to new environmental conditions. Here we assessed innate immune genetic diversity, via Toll-like receptors (TLRs), to quantify genetic differentiation between a deliberately introduced population of dunnocks (Prunella modularis) in New Zealand and its source population in the United Kingdom. We also asked whether the introduced population shows signatures of intergenerational and current selection in TLR. We expect intergenerational and current selection patterns because New Zealand dunnocks have been exposed to new environmental conditions following their introduction around the late 1800s, which may have driven pathogen-mediated selection. Counter to our expectation, we found only weak and non-significant genetic differentiation between the introduced and source populations. Further, the levels of genetic differentiation (G'ST) found in TLRs were similar to those found in microsatellites across the populations. Dunnocks, in general, have been under strong purifying selection over evolutionary time, but we found little evidence to support signatures of contemporary selection in TLR in the introduced population. Notably, however, we found a statistically significant heterozygosity advantage for males in TLR3, which lends support to possible current selection acting upon the introduced population. Overall, it appears that New Zealand dunnocks have retained a high proportion of the immunogenetic diversity of the source population, and that such diversity has probably been sufficient to defend against the potential pathogens found in New Zealand. Our results may explain-at least in part-why the introduction of dunnocks has been so successful; dunnocks have become one of the most common birds in New Zealand.