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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). By contrast, natural hybridization between divergent populations has been considered an evolutionary dead end because it can lead to unfit or unviable hybrids (Barton & Hewitt, 1985). With climates changing rapidly as a result of anthropogenic warming, the effects of hybridization and introgression on the rate of adaptation are important for both natural and managed populations, particularly for species with long generation lengths such as trees (Aitken et al., 2008).
Although definitive evidence of whether hybridization more often promotes or hinders adaptation is lacking, recent studies have supported the former for many plant species (Choler et al., 2004; Campbell & Waser, 2007; Stift et al., 2008; De Carvalho et al., 2010; Thomasset et al., 2011). 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).
The maintenance of hybrid zones has been the subject of considerable debate, mainly because of different points of view about the relative powers of natural selection as a divergent force and gene flow as a homogenizing force (Harrison, 1993; Barton, 2001). Environment-independent models describe hybrid zones maintained by a balance between dispersal and selection against hybrids, with selection being independent of the environment (Mayr, 1942; Barton, 1979). The tension zone model assumes that hybrids are less fit than their parents regardless of location (endogenous selection; Gay et al., 2008; Carling & Zuckerberg, 2011). Hybrid inferiority is thought to result from the break-up of epistatic co-adapted gene complexes that affect fitness traits (Barton & Hewitt, 1985; Hewitt, 1988). Environment-dependent models involve genotype-by-environment interactions, where hybrid zones are maintained through selection gradients resulting from environmental heterogeneity (Endler, 1973, 1977; Slatkin, 1973; Harrison, 1986). In these models, hybrid fitness varies with the environment (exogenous selection). The bounded hybrid superiority model (Moore, 1977) is an important environment-dependent model, which postulates that hybrid individuals are fitter than either parental species in environments that are intermediate to parental habitats, but are less fit than parental species in their respective native habitats (Wang et al., 1997; Milne et al., 2003; Miglia et al., 2005; Goulson, 2009).
Picea glauca (Moench) Voss (white spruce) and Picea engelmannii Parry ex Engelm. (Engelmann spruce) are closely related, wind-dispersed, long-lived tree species that hybridize extensively in areas where their ranges overlap, mostly in British Columbia and the western part of Alberta, Canada. Picea glauca and P. engelmannii inhabit different ecological niches separated primarily by elevation. Picea engelmannii is a subalpine species that has relatively low tolerance of high temperatures and drought, but can withstand short growing seasons and high snowfall (Alexander & Shepperd, 1990). Picea glauca is primarily a boreal species that grows under highly variable conditions and can tolerate both low winter temperatures and summer drought, but is restricted to low elevations. Hybrids occupy ecological and elevational niches intermediate to those of the parental species. Hybridization followed by backcrossing has produced hybrids with intermediate characteristics in morphology and phenotypic traits, and clinal variation along elevational gradients (Roche, 1969; Daubenmire, 1974; Rajora & Dancik, 2000; Ledig et al., 2006). Neutral molecular marker studies have found clines in admixture along elevational and latitudinal gradients, with hybrids on average having a higher contribution from P. engelmannii than from P. glauca (A. R. De La Torre & S. Aitken, unpublished data). Although it has been suggested that hybrids are fitter than parentals in intermediate environments (Roche, 1969; Daubenmire, 1974; Xie et al., 1998; Ledig et al., 2006), this has never been tested.
Picea glauca and P. engelmannii are economically important tree species in Canada. The complex of these two species and their hybrids (collectively managed as ‘interior spruce’) represents the second most planted tree in British Columbia. Tens of millions of trees are planted annually for reforestation in the province, the majority of which are products of a large breeding programme that was initiated in the early 1960s. With British Columbia getting warmer and in some areas drier as a result of climate change (Mbogga et al., 2009), there is a need to develop climate-informed strategies to manage the spruce genetic resource. In order to develop these management strategies, it is important to have a thorough understanding of the hybrid zone complex and the effects of artificial selection on the genomic architecture of this zone.
Our study is the first to assess the evolutionary dynamics responsible for the maintenance of the P. glauca × P. engelmannii hybrid zone by combining genome-wide estimates of admixture, quantitative data from long-term common garden experiments, and environmental data. By doing so, we expect to contribute valuable information for current and future management of this species complex. Our objectives are: to test the bounded hybrid superiority model of hybrid zone maintenance by classifying individuals using a single nucleotide polymorphism (SNP)-based hybrid index, comparing the fitnesses of hybrids and parental species in extensive common garden experiments, and elucidating the effects of climate on the distribution of genetic variation within the hybrid zone; and to assess the effects of ongoing selective breeding and projected climate change on genetic composition within the hybrid zone.