A novel synthesis of two decades of microsatellite studies on European beech reveals decreasing genetic diversity from glacial refugia

Abstract

Genetic diversity influences the evolutionary potential of forest trees under changing environmental conditions, thus indirectly the ecosystem services that forests provide. European beech (Fagus sylvatica L.) is a dominant European forest tree species that increasingly suffers from climate change-related die-back. Here, we conducted a systematic literature review of neutral genetic diversity in European beech and created a meta-data set of expected heterozygosity (He) from all past studies providing nuclear microsatellite data. We propose a novel approach, based on population genetic theory and a min-max scaling to make past studies comparable. Using a new microsatellite data set with unprecedented geographic coverage and various re-sampling schemes to mimic common sampling biases, we show the potential and limitations of the scaling approach. The scaled meta-dataset reveals the expected trend of decreasing genetic diversity from glacial refugia across the species range and also supports the hypothesis that different lineages met and admixed north of the European mountain ranges. As a result, we present a map of genetic diversity across the range of European beech which could help to identify seed source populations harboring greater diversity and guide sampling strategies for future genome-wide and functional investigations of genetic variation. Our approach illustrates how to combine information from several nuclear microsatellite data sets to describe patterns of genetic diversity extending beyond the geographic scale or mean number of loci used in each individual study, and thus is a proof-of-concept for synthesizing knowledge from existing studies also in other species.

Supplementary information: The online version contains supplementary material available at 10.1007/s11295-022-01577-4.

Keywords: Chloroplast markers; Demography; Fagus sylvatica; Genetic diversity; Heterozygosity; Microsatellites; Min–max scaling; Nuclear markers; Single nucleotide polymorphisms (SNPs).

Fig. 1

PRISMA flow diagram representing the different phases of the systematic literature review of genetic diversity in Fagus sylvatica

Fig. 2

Geographic distribution of the studies of F. sylvatica genetic diversity included in this review. a Number of studies across the years coded by marker type; legend applies to a, b, and d. b Percentages and number of studies using each molecular marker system. c Number of studies per country. d Sample size and molecular marker for each analyzed population of F. sylvatica

Fig. 3

Spatial distribution of He across 227 F. sylvatica populations from nuclear microsatellite markers. Raw He retrieved from the studies (a), scaled He (b), and interpolation of scaled He (c, interpolation distance weighting, power = 5) across the distribution range of F. sylvatica. Legend and spatial scalebar annotation apply to all maps

Fig. 4

Correlation between non-scaled (a, c, and e) or scaled (b, d, and f) genetic diversity (He) from nuclear microsatellite markers; geographic variables, latitude (a and b) and longitude (c and d) with points color-coded by study; and distance from a putative origin area (Magri et al., 2006) corresponding to the Slovenian region. Blue and orange points denote populations in the French Alps and in the Iberian Peninsula, respectively, which are proposed to be additional refugia for genetic diversity (e and f)

Fig. 5

Correlation scatter plots (linear regression as blue line with confidence interval as gray area) between longitude and the raw and scaled He dataset from Ulaszewski et al., . Populations were splitted into north (blue) versus south group (yellow), west (blue) versus east (yellow) group, and two random groups. Loci were divided either randomly or according to the polymorphism level, lower (blue) or higher (yellow). Spearman’s R_s and p-values are reported inside the correlation plot