A candidate gene-based association study to investigate potentially adaptive genetic variation in European beech (Fagus sylvatica L.)

Abstract

Climate change models predict higher annual mean temperatures as well as a decrease of precipitation during summer months for Germany. Possible consequences for trees are a prolonged growing season, a higher risk of late frost and higher drought stress during the summer. These changing environmental conditions may lead to shifts in tree species competition. European beech (Fagus sylvatica L.) is one of the most important deciduous tree species in Central Europe. Thus, the genetic adaptation potential of this species to climate change is of great interest. Both the neutral and adaptive genetic variation of beech were investigated in this study. A translocation experiment was established with progenies of beech populations growing under different environmental conditions in Northern Germany. Repeated observations of important phenotypic traits (height, bud burst, drought stress sensitivity, mortality) revealed significant differences among populations. Interestingly, populations with a greater geographic distance partly showed more similar phenotypes than neighboring stands. Neutral genetic variation of the investigated seedling populations was analyzed using nine different microsatellite markers. Only low genetic differentiation was detected among the investigated beech populations. Genetic diversity was high for all populations and statistically not different from the adult stands of origin. The high genetic diversity is a good basis for adaptation, albeit it may only facilitate a short-term adaptation to climate change. Therefore, it is important to gain insights into the genetic basis of climate change relevant traits. Thus, bud burst-related candidate genes were investigated in the present study. The analysis of fragments of ten different candidate genes revealed 20 indels and 116 SNPs. In total, 46 SNPs were successfully used for genotyping of more than 1,400 individuals which were selected based on their bud burst timing. Association analyses were conducted to identify potentially adaptive SNP markers. These revealed 23 significantly associated SNPs with bud burst under a “general linear model”. An additionally applied “mixed linear model” revealed similar results. The phenotypic variation explained by the significantly associated SNPs with bud burst was low (R2 < 2.2 %), though in accordance with other studies in forest tree species. In addition to the association analyses, FST outlier analyses were conducted revealing seven different SNPs, which are potentially under balancing or directional selection. In total, four potentially adaptive SNPs were simultaneously revealed by both outlier and association analyses. These might have the highest probability of being involved in the manifestation of bud burst behavior. However, several potential adaptive SNPs identified in this study are non-coding or synonymous SNPs, and hence are not thought to be the causative SNPs, but rather linked to them. Nevertheless, linkage disequilibrium was found to be low in this study suggesting that the causative SNPs might be in close vicinity. The potentially adaptive SNPs identified in this study, should be verified in further experiments with additional populations.