Recent climate change and historical population structure predict spatial patterns of admixture between two host-specialized pine sawfly species

Human disturbance can have profound effects on biodiversity, including
increasing hybridization between reproductively isolated species. One
approach for understanding how human activity affects hybridization
dynamics is to evaluate correlations between disturbance (e.g.,
urbanization, temperature change) and hybridization. Because variation in
hybridization can also arise from historical factors unrelated to recent
human disturbance, it is essential to account for population structure to
avoid spurious correlations. Here, we combine environmental and
high-coverage whole-genome resequencing data to investigate how human
disturbance and population structure affect hybridization dynamics between
a pair of pine sawflies adapted to different pines, Neodiprion lecontei
and Neodiprion pinetum. We find that N. lecontei and N. pinetum exhibit
strikingly different patterns of population structure, which we
hypothesize stems from differences in host. We also find that recent
admixture is both asymmetric and geographically variable. Linear
regression analyses reveal that admixture proportion is predicted by
indirect human disturbance (i.e., climate change) and not direct human
disturbance (e.g., urbanization) in both N. lecontei and N. pinetum.
Lastly, in N. pinetum, we find evidence of a spurious association between
admixture and direct human disturbance that disappears when regression
models account for population structure via inclusion of genetic principal
component scores as covariates. Together, our data suggest that indirect
human disturbance and population structure both contribute to geographic
variation in admixture between N. lecontei and N. pinetum. Our study also
highlights the importance of adequately controlling for population
structure when attempting to identify environmental predictors (human
disturbance-related or not) of hybridization.