Abstract
Hybrid zones contain extensive standing genetic variation that facilitates rapid responses to selection. The Picea glauca × Picea engelmannii hybrid zone in western Canada is the focus of tree breeding programs that annually produce ~90 million reforestation seedlings. Understanding the direct and indirect effects of selective breeding on adaptive variation is necessary to implement assisted gene flow (AGF) polices in Alberta and British Columbia that match these seedlings with future climates. We decomposed relationships among hybrid ancestry, adaptive traits, and climate to understand the implications of selective breeding for climate adaptations and AGF strategies. The effects of selection on associations among hybrid index estimated from ~6,500 SNPs, adaptive traits, and provenance climates were assessed for ~2,400 common garden seedlings. Hybrid index differences between natural and selected seedlings within breeding zones were small in Alberta (average +2%), but larger and more variable in BC (average −7%, range −24% to +1%), slightly favoring P. glauca ancestry. The average height growth gain of selected seedlings over natural seedlings within breeding zones was 36% (range 12%–86%). Clines in growth with temperature‐related variables were strong, but differed little between selected and natural populations. Seedling hybrid index and growth trait associations with evapotranspiration‐related climate variables were stronger in selected than in natural seedlings, indicating possible preadaptation to drier future climates. Associations among cold hardiness, hybrid ancestry, and cold‐related climate variables dominated signals of local adaptation and were preserved in breeding populations. Strong hybrid ancestry–phenotype–climate associations suggest that AGF will be necessary to match interior spruce breeding populations with shifting future climates. The absence of antagonistic selection responses among traits and maintenance of cold adaptation in selected seedlings suggests breeding populations can be safely redeployed using AGF prescriptions similar to those of natural populations.
Highlights
Natural hybrid zones contain large amounts of standing genetic variation that provides the raw material for rapid selection responses and transgressive adaptive phenotypes, even if the selection pressure is relatively weak (Barrett & Schluter, 2008; Rieseberg, Archer, & Wayne, 1999)
We address four primary questions. (i) How does selection modify hybrid ancestry composition within and among breeding zones? (ii) What are the effects of selective breeding on adaptive phenotypic traits and the relationships among traits? (iii) How does selection change the relationships between growth gains, adaptive traits, and climate? (iv) How do changes in the hybrid index of selected seedlots modify relationships with adaptive traits, or with climate? By answering these questions, we can inform how assisted gene flow (AGF) policies should redeploy selectively bred interior spruce reforestation seedlots
Growth gains result from both increased growth rate and delayed bud set of selected seedlings, but have not compromised the adaptive synchrony of autumn cold hardiness development relative to their natural counterparts
Summary
Natural hybrid zones contain large amounts of standing genetic variation that provides the raw material for rapid selection responses and transgressive adaptive phenotypes, even if the selection pressure is relatively weak (Barrett & Schluter, 2008; Rieseberg, Archer, & Wayne, 1999). Implementing AGF requires a nuanced understanding of how selective breeding of interior spruce modifies adaptive traits, their trade-offs with growth within and among breeding populations, and their climatic associations across a hybrid zone that contains high levels of adaptive genetic variation. Decomposing the phenotypic, climatic, and hybrid ancestry components of adaptive variation allows us to describe the changes that underlie differences in ancestry within and among interior spruce breeding zones, relate these findings to future AGF needs. We achieve this by evaluating seedlings from selectively bred seedlots relative to their natural stand counterparts using a large common garden established in benign conditions. Our study compliments long-term replicated field trials established across AB and BC
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