Abstract
The nature of selection responsible for the maintenance of the economically and ecologically important Picea glauca × Picea engelmannii hybrid zone was investigated. Genomic, phenotypic and climatic data were used to test assumptions of hybrid zone maintenance and to model future scenarios under climate change. Genome-wide estimates of admixture based on a panel of 86 candidate gene single nucleotide polymorphisms were combined with long-term quantitative data on growth and survival (over 20 yr), as well as one-time assessments of bud burst and bud set phenology, and cold hardiness traits. A total of 15 498 individuals were phenotyped for growth and survival. Our results suggest that the P. glauca × P. engelmannii hybrid zone is maintained by local adaptation to growing season length and snowpack (exogenous selection). Hybrids appeared to be fitter than pure species in intermediate environments, which fits expectations of the bounded hybrid superiority model of hybrid zone maintenance. Adaptive introgression from parental species has probably contributed to increased hybrid fitness in intermediate habitats. While P. engelmannii ancestry is higher than P. glauca ancestry in hybrid populations, on average, selective breeding in managed hybrid populations is shifting genomic composition towards P. glauca, potentially pre-adapting managed populations to warmer climates.
Highlights
The role of hybridization in adaptive evolution has been a contentious issue in evolutionary biology
22 putatively pure P. glauca individuals from Fort Nelson (FN) and 40 putatively pure P. engelmannii from southwestern USA were obtained for genotyping from grafts of mature trees sampled from natural populations and archived in clone banks
Hybrids have a combination of P. engelmannii and P. glauca genomes; they have inherited a combination of genes that may provide them with an adaptive advantage in intermediate environments
Summary
The role of hybridization in adaptive evolution has been a contentious issue in evolutionary biology. Some researchers have argued that hybridization is a potent evolutionary force that facilitates adaptive evolution and can lead to new species (Anderson, 1949; Arnold, 1997; Abbott et al, 2013). According to this perspective, new gene combinations resulting from hybridization may promote the development of adaptations to novel or changing environments (Rieseberg et al, 2003). In sunflower (Helianthus spp.), new gene combinations generated by introgression have contributed to ecological divergence (Rieseberg et al, 2003), and adaptation in several abiotic tolerance traits (Whitney et al, 2010). Adaptive introgression has increased flooding tolerance in Iris (Martin et al, 2005, 2006) and Rorippa (Stift et al, 2008), drought tolerance in Pinus (Ma et al, 2010) and light tolerance in Silene (Goulson, 2009)
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