Abstract
Gene flow is a key factor in the evolution of species, influencing effective population size, hybridisation and local adaptation. We analysed local gene flow in eight stands of white oak (mostly Quercus petraea and Q. robur, but also Q. pubescens and Q. faginea) distributed across Europe.Adult trees within a given area in each stand were exhaustively sampled (range [239, 754], mean 423), mapped, and acorns were collected ([17,147], 51) from several mother trees ([3], [47], 23). Seedlings ([65,387], 178) were harvested and geo-referenced in six of the eight stands. Genetic information was obtained from screening distinct molecular markers spread across the genome, genotyping each tree, acorn or seedling. All samples were thus genotyped at 5–8 nuclear microsatellite loci. Fathers/parents were assigned to acorns and seedlings using likelihood methods. Mating success of male and female parents, pollen and seed dispersal curves, and also hybridisation rates were estimated in each stand and compared on a continental scale.On average, the percentage of the wind-borne pollen from outside the stand was 60%, with large variation among stands (21–88%). Mean seed immigration into the stand was 40%, a high value for oaks that are generally considered to have limited seed dispersal. However, this estimate varied greatly among stands (20–66%). Gene flow was mostly intraspecific, with large variation, as some trees and stands showed particularly high rates of hybridisation.Our results show that mating success was unevenly distributed among trees. The high levels of gene flow suggest that geographically remote oak stands are unlikely to be genetically isolated, questioning the static definition of gene reserves and seed stands.
Highlights
Plants are static organisms whereas their genes are often highly mobile
For each stand and paternity assignment, we used the most successful test in terms of percentages of correct father choice (Cfc) and of father correctly assigned among the assigned paternities (Pca)
The quality of the tests varied among stands, but in most cases correct choices were . 80%. a and b errors were less than 5% except in two cases for each parameter (Italy and Great Britain for a, Sweden and Switzerland for b)
Summary
Plants are static organisms whereas their genes are often highly mobile. Gene flow predominantly occurs through dispersal of both seed and pollen, and the contemporary distribution of neutral genetic diversity across the landscape is largely, though not entirely, due to the extent and relative importance of these two dispersal processes [1]. Detailed studies of local gene flow performed in both pure and mixed stands of European white oaks indicate that pollen immigration into stands is generally high but very variable between studies [4,5,6,7,8,9]. Our aim was to measure the extent of gene flow in white oaks (Quercus robur (pedunculate oak, 73.5% of the total tree sample), Q. petraea (sessile oak, 23%), Q. pubescens (pubescent oak, 2%), Q. faginea (Portuguese oak, 1.5%)) in eight stands distributed throughout the species’ natural ranges across Europe, using the same molecular markers to perform paternity and parentage analyses. Stands with more than one species of oak are not common across Europe, which constrained the choice of woods available for this study. We expected to control the heterogeneity across different ecological, demographic and sampling settings to enable general conclusions to be drawn
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