Tracing the footprints of a moving hybrid zone under a demographic history of speciation with gene flow

Abstract

A lack of optimal gene combinations, as well as low levels of genetic diversity, is often associated with the formation of species range margins. Conservation efforts rely on predictive modelling using abiotic variables and assessments of genetic diversity to determine target species and populations for controlled breeding, germplasm conservation and assisted migration. Biotic factors such as interspecific competition and hybridization, however, are largely ignored, despite their prevalence across diverse taxa and their role as key evolutionary forces. Hybridization between species with well‐developed barriers to reproductive isolation often results in the production of offspring with lower fitness. Generation of novel allelic combinations through hybridization, however, can also generate positive fitness consequences. Despite this possibility, hybridization‐mediated introgression is often considered a threat to biodiversity as it can blur species boundaries. The contribution of hybridization towards increasing genetic diversity of populations at range margins has only recently gathered attention in conservation studies. We assessed the extent to which hybridization contributes towards range dynamics by tracking spatio‐temporal changes in the central location of a hybrid zone between two recently diverged species of pines: Pinus strobiformis and P. flexilis. By comparing geographic cline centre estimates for global admixture coefficient with morphological traits associated with reproductive output, we demonstrate a northward shift in the hybrid zone. Using a combination of spatially explicit, individual‐based simulations and linkage disequilibrium variance partitioning, we note a significant contribution of adaptive introgression towards this northward movement, despite the potential for differences in regional population size to aid hybrid zone movement. Overall, our study demonstrates that hybridization between recently diverged species can increase genetic diversity and generate novel allelic combinations. These novel combinations may allow range margin populations to track favourable climatic conditions or facilitate adaptive evolution to ongoing and future climate change.

Adaptive evolution in a conifer hybrid zone is driven by a mosaic of recently introgressed and background genetic variants

“…Using the full multivariate regression model containing the total effect of environment and geography, we identified solar radiation and water availability-related gradients to be the strongest predictors of D ST , while freeze-related gradients had the highest R 2 for D IS (Table 2 ). However, elevated LD (specifically D IS ) could arise in the absence of selection when populations experience recent and ongoing introgression 45 , 46 . To account for this, we implemented partial regression models that identify the primary components driving the elevated R 2 values for D ST and D IS along each environmental gradient.…”

Primary hybrid zone formation inTephroseris helenitis(Asteraceae), following postglacial range expansion along the central Northern Alps

“…Second, rather than being a stable tension zone, it could be a dynamic and moving one (Buggs, 2007;Currat et al, 2008;Gay et al, 2008;Menon et al, 2019). This would be supported by the high in-…”

Morphological Differences in Pinus strobiformis Across Latitudinal and Elevational Gradients

“…The ability of P. strobiformis to colonize its expected future range (Shirk et al, 2018) will be influenced by numerous factors, including colonization at the leading edge of the range shift, seed dispersal dynamics, resistance to white pine blister rust, competition with other species, introgression and hybridization (Menon et al, 2020), and genetic adaptation to local climate (Goodrich et al, 2016;Bucholz et al, 2020). Through our results, we have a better understanding of the environmental controls on cone and seed morphology, and we can more adequately evaluate seed provenances and transfer zones and provide better information for assisted migration strategies.…”