Fine‐scale spatial genetic structure across the species range reflects recent colonization of high elevation habitats in silver fir (Abies alba Mill.)

Enikő I Major,Bruno Fady,Camilla Avanzi,Flaviu Popescu,Katalin Csilléry,Katrin Heer,Giovanni G Vendramin,Dragos Postolache,Mária Höhn,Andrea Piotti,Lars Opgenoorth

doi:10.1111/mec.16107

Abstract

Variation in genetic diversity across species ranges has long been recognized as highly informative for assessing populations’ resilience and adaptive potential. The spatial distribution of genetic diversity within populations, referred to as fine‐scale spatial genetic structure (FSGS), also carries information about recent demographic changes, yet it has rarely been connected to range scale processes. We studied eight silver fir (Abies alba Mill.) population pairs (sites), growing at high and low elevations, representative of the main genetic lineages of the species. A total of 1,368 adult trees and 540 seedlings were genotyped using 137 and 116 single nucleotide polymorphisms (SNPs), respectively. Sites revealed a clear east‐west isolation‐by‐distance pattern consistent with the post‐glacial colonization history of the species. Genetic differentiation among sites (F CT = 0.148) was an order of magnitude greater than between elevations within sites (F SC = 0.031), nevertheless high elevation populations consistently exhibited a stronger FSGS. Structural equation modelling revealed that elevation and, to a lesser extent, post‐glacial colonization history, but not climatic and habitat variables, were the best predictors of FSGS across populations. These results suggest that high elevation habitats have been colonized more recently across the species range. Additionally, paternity analysis revealed a high reproductive skew among adults and a stronger FSGS in seedlings than in adults, suggesting that FSGS may conserve the signature of demographic changes for several generations. Our results emphasize that spatial patterns of genetic diversity within populations provide information about demographic history complementary to non‐spatial statistics, and could be used for genetic diversity monitoring, especially in forest trees.

Highlights

Dispersal capacity of organisms plays a fundamental role in the establishment, persistence and range dynamics of species, especially during environmental changes (Travis et al, 2013, Saastamoinen et al, 2018)
We studied natural silver fir populations with no or close-to-nature management, so we assumed that differences in fine-scale spatial genetic structure (FSGS) were due to demographic history and/or environmental constraints/adaptation
The DAPC analysis revealed that the first linear discriminant function (LD1) ordered the populations from east to west with the exception of PYR, which situated close to the French populations (VEN, Lure 190 (LUR), ISS, VES) (Figure 1a)

Summary

Introduction

Dispersal capacity of organisms plays a fundamental role in the establishment, persistence and range dynamics of species, especially during environmental changes (Travis et al, 2013, Saastamoinen et al, 2018). The long-term consequence of limited dispersal is that populations become subdivided and distant populations become genetically more differentiated than nearby populations; a phenomenon referred to as isolation-by-distance (IBD) in subdivided populations (Wright, 1943; Malécot, 1948; Kimura & Weiss, 1964). FSGS captures principally the effect of limited dispersal with respect to the past few generations, while, in contrast, IBD reflects deeper demographic time scales. This distinction is important because, ignoring mutations, a drift-dispersal equilibrium can be reached within a few generations on a spatial scale up to one order of magnitude larger than the average parent-offspring distance, while many more generations can be necessary on larger spatial scales (Slatkin, 1993; Vekemans & Hardy, 2004; Bradburd & Ralph, 2019)

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