Evidence of local adaptation driving differential introgression between the extents of the mosaic lodgepole X jack pine hybrid zone in western Canada

Background/Question/Methods

Hybrid zones present opportunities for novel genetic combinations that may provide parental species with new adaptive potential, which is essential for species experiencing new selective pressures. Lodgepole pine (Pinus contorta var. latifolia) and jack pine (Pinus banksiana) are both ecologically and economically important forest species in Canada that are experiencing increased threats, including climate change. These species form a mosaic hybrid zone in western Canada. Our study aimed to examine whether there is evidence of differential introgression of putatively adaptive loci between the northern and southern extents of the hybrid zone. We hypothesized that given the environmental influence on these species and the differences in climate across the hybrid zone, there will be evidence of differential introgression between the northern and southern extents. To examine this, we analyzed over 10,000 single nucleotide polymorphic (SNP) loci in 1378 individuals (669 jack pine, 540 lodgepole pine, 169 hybrids) and identified environmentally associated SNPs. We examined these SNPs to identify significantly different patterns of introgression between individuals in the northern and southern-most extents of the lodgepole pine X jack pine hybrid zone. Finally, we determined the functional gene annotations of statistically different SNP loci to investigate adaptive functions they may possess.

Results/Conclusions

We identified 356 environmentally associated loci; among these, 82 loci are associated with elevation in lodgepole pine, while 96 loci are associated with climate-moisture index in jack pine. Of these putatively adaptive SNPs, 28 have significantly different patterns of introgression between the northern and southern extents of the hybrid zone. Based on gene annotations, we found that the genes showing differential introgression are associated with essential functions such as phytohormone action, RNA biosynthesis, protein homeostasis and modification. The findings from this study highlight the important relationship these environmental variables have in shaping the genetic structure of these pine species. Given the evidence that alleles are being introgressed at different frequencies between the two hybrid extents, indicates that individuals within the hybrid zone, at different geographical locations, are undergoing local adaptation. As these pine species are faced with increased pressures, we aimed to characterize the genetic structure of this dynamic hybrid zone to gain a better understanding of local adaptation. This work has potential applications in modelling the future, seed-stock selection and forestry management practices of these trees.