Genetic characterization and predictive modelling of a pine hybrid zone in western Canada: Implications for range expansion of the mountain pine beetle

Background/Question/Methods

Understanding the physical and genetic structure of hybrid zones can illuminate factors affecting their formation and stability. In north central Alberta, the overlapping ranges of lodgepole pine (Pinus contorta) and jack pine (Pinus banksiana) form a spatially complex and poorly defined hybrid zone. This hybrid zone is of considerable interest as spatial variation in species boundaries and future hybrid dynamics will influence the availability of host trees for the mountain pine beetle. Such host-species variation may affect outbreak risk and the probability of further range expansion of this destructive forest pest. We characterized this hybrid zone through 1998 genotyped lodgepole, jack pine and hybrids in British Columbia and Alberta at 11 microsatellite loci. Using a Bayesian assignment method (STRUCTURE) we determined genetic ancestry and used these results to model the relationship between species occurrence and climatic and spatial covariates. Following contemporary characterization of the existing hybrid zone we predicted future spatial boundaries among species classes using projected future climatic conditions from a GCM using logistic mixed-effects regression.

Results/Conclusions

Our results suggest that the hybrid zone is dominated by the introgression of lodgepole pine but is maintained by environmental constraints. Further, we found that the distribution of the different pine classes is well explained by climatic and environmental covariates as indicated by ROC analysis and that current species distribution maps need to be redefined. Finally our results indicate that we should expect shifts in species distributions in the future which may facilitate the further spread of the mountain pine beetle into new geographic regions.