Abstract
Norway spruce (Picea abies) is a dominant conifer species of major economic importance in northern Europe. Extensive breeding programs were established to improve phenotypic traits of economic interest. In southern Sweden, seeds used to create progeny tests were collected on about 3,000 trees of outstanding phenotype (‘plus’ trees) across the region. In a companion paper, we showed that some were of local origin but many were recent introductions from the rest of the natural range. The mixed origin of the trees together with partial sequencing of the exome of >1,500 of these trees and phenotypic data retrieved from the Swedish breeding program offered a unique opportunity to dissect the genetic basis of local adaptation of three quantitative traits (height, diameter and bud‐burst) and assess the potential of assisted gene flow. Through a combination of multivariate analyses and genome‐wide association studies, we showed that there was a very strong effect of geographical origin on growth (height and diameter) and phenology (bud‐burst) with trees from southern origins outperforming local provenances. Association studies revealed that growth traits were highly polygenic and bud‐burst somewhat less. Hence, our results suggest that assisted gene flow and genomic selection approaches could help to alleviate the effect of climate change on P. abies breeding programs in Sweden.
Highlights
Local adaptation is pervasive and phenotypes often match their environments as demonstrated by the clinal distribution of many phenotypic traits along environmental gradients (Savolainen, Lascoux, & Merilä, 2013)
The sequencing of >1,500 Norway spruce trees coming from the Swedish breeding program allowed us to analyse the influence of tree origins on phenotypic traits and to investigate their genetic basis
From a practical point of view, our study lends support on strategies based on assisted gene flow to alleviate the impact of climate change in central and southern Sweden breeding program
Summary
Local adaptation is pervasive and phenotypes often match their environments as demonstrated by the clinal distribution of many phenotypic traits along environmental gradients (Savolainen, Lascoux, & Merilä, 2013). Because of the continuous introduction of material from the rest of the natural range of Norway spruce, the Norway spruce breeding program today includes individuals from the seven P. abies genetic clusters (Chen et al, 2019): Alpine, Fennoscandian and Carpathian, and central Europe (resulting from hybridization between Alpine and Carpathian clusters), central and southern Sweden (hybridization between Alpine and Fennoscandian clusters), northern Poland (hybridization between Fennoscandian and Carpathian clusters) and Russia-Baltics (hybridization between Fennoscandian and Carpathian clusters with a strong introgression of Picea obovata) The aim of these introductions was twofold: (a) to obtain a large amount of seeds and (b) to take advantage of the fact that trees from lower latitudes have a longer growth period and thereby a higher yearly growth rate than local provenances when moved northwards (Clapham et al, 1998; Dormling, Gustafsson, & Wettstein, 1968; Ekberg, Eriksson, & Dormling, 1979). We argue that while data from breeding programs might sometimes be incomplete or suboptimal, they are readily available and contain a lot of valuable information for evolutionary biologists
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