Differential Introgression Between the Northern and Southern Extents of the Lodgepole x Jack Pine Hybrid Zone Suggests Environmentally-driven Selection and Local Adaptation.

Abstract

Hybrid zones provide a unique opportunity to study evolutionary consequences and selective pressures maintaining species boundaries in the hybridizing species. Lodgepole and jack pine are two native-Canadian species that form a mosaic hybrid zone in western Canada. Introgression occurs between lodgepole and jack pine through this hybrid zone by repeated backcrossing with advanced generation hybrid progeny. Using environmentally associated SNPs identified by redundancy analyses, we examined patterns of introgression between the northern and southern extents of this hybrid zone to identify differential introgression. Through genomic cline analyses, we found significant introgression of these SNPs across the hybrid zone. Twenty-eight SNPs had significantly different patterns of introgression between the northern and southern extents. Fine-scale patterns revealed several SNPs that were introgressing more frequently than expected, suggesting adaptive introgression. We found that patterns of adaptive introgression are occurring more frequently in the northern extent compared to the southern extent, suggesting different environmental pressures. The genetic variation observed between the two hybrid extents has implications in climate change considerations as it may result the two regions responding differently to changing environmental conditions. Using gene annotations and major allele frequency maps, we identified evidence of differing environmental pressures resulting in putative local adaptation within this hybrid zone. Sharing adaptations through this hybrid zone could be an important resource of adaptive potential in these pines species as climate change continues to disrupt forest ecosystems.

Spatially heterogeneous selection and inter-varietal differentiation maintain population structure and local adaptation in a widespread conifer

“…Hybrids in both locations present a higher ancestry from the interior north variety, suggesting asymmetric introgression from the interior to the coastal variety. Asymmetric introgression has been previously found in other conifer species inhabiting the same geographic region in western Canada, such as Picea glauca x P.engelmannii [49] and Pinus contorta x P. banksiana [50].…”