Patterns of genetic variation in leading-edge populations of Quercus robur: genetic patchiness due to family clusters

Pekka Vakkari,Katri Kärkkäinen,Juha Heikkinen,Mari Rusanen,Tea Huotari

doi:10.1007/s11295-020-01465-9

Pekka Vakkari, Katri Kärkkäinen + Show 3 more

Open Access

https://doi.org/10.1007/s11295-020-01465-9

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Abstract

The genetic structure of populations at the edge of species distribution is important for species adaptation to environmental changes. Small populations may experience non-random mating and differentiation due to genetic drift but larger populations, too, may have low effective size, e.g., due to the within-population structure. We studied spatial population structure of pedunculate oak, Quercus robur, at the northern edge of the species’ global distribution, where oak populations are experiencing rapid climatic and anthropogenic changes. Using 12 microsatellite markers, we analyzed genetic differentiation of seven small to medium size populations (census sizes 57–305 reproducing trees) and four populations for within-population genetic structures. Genetic differentiation among seven populations was low (Fst = 0.07). We found a strong spatial genetic structure in each of the four populations. Spatial autocorrelation was significant in all populations and its intensity (Sp) was higher than those reported in more southern oak populations. Significant genetic patchiness was revealed by Bayesian structuring and a high amount of spatially aggregated full and half sibs was detected by sibship reconstruction. Meta-analysis of isoenzyme and SSR data extracted from the (GD)2 database suggested northwards decreasing trend in the expected heterozygosity and an effective number of alleles, thus supporting the central-marginal hypothesis in oak populations. We suggest that the fragmented distribution and location of Finnish pedunculate oak populations at the species’ northern margin facilitate the formation of within-population genetic structures. Information on the existence of spatial genetic structures can help conservation managers to design gene conservation activities and to avoid too strong family structures in the sampling of seeds and cuttings for afforestation and tree improvement purposes.

Highlights

Genetic variation within populations is crucial for their survival in the changing environment
Northern marginal populations in Europe are of special importance, as they will serve as potential gene pools for expanding populations in areas where the climate becomes more favorable to the species
Due to the different population sizes, the number of trees per distance class varies among populations, and the width of confidence limits varies among populations

Summary

Introduction

Genetic variation within populations is crucial for their survival in the changing environment. Climate change may lead to a situation where marginal populations of European trees experience intensified selection, at the southern margin (rear edge), due to higher temperatures and more severe drought (Hampe and Petit 2005) and at the northern margin (leading edge), due to the less predictable weather and rapidly rising mean temperatures. During a range shift, marginal populations at the rear edge and at the leading-edge face diverse challenges and play different roles in the evolution of populations and species (Hampe and Petit 2005). Northern marginal populations in Europe are of special importance, as they will serve as potential gene pools for expanding populations in areas where the climate becomes more favorable to the species. Understanding the genetic structure of leadingedge populations at different hierarchical levels (individualsubpopulation and subpopulation-population levels) can help to assess conservation priorities and allocation of resources within peripheral regions

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