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Gene flow, gene diversity and hybridisation

Concerning the genetic diversity in France, for the Fraxinus excelsior populations located in northeastern France levels of genetic variability within and among stands were estimated for the seedling and adult stages. As expected for a forest tree, our results reveal high levels of intra-population diversity and a low genetic differentiation between stands. However, a general and significant heterozygote deficiency was found, with a mean FIS of 0.163 for the seedlings and of 0.292 for the adult trees.

We determined the genetic diversity of F. angustifolia using samples collected in the summer of 2003 in southern France in five populations situated west of the Pyrenees. If we compared the two species, the most important difference is the number of alleles (total and effective) that is lower for F. angustifolia. We also examined graphically the differences in genetic diversity between the two species, by simulating re-sampling curves. The simulation analyses indicate that clearly, the genetic diversity in terms of alleles is higher in F. excelsior. However, SD deviations of one species overlap that of the other, suggesting that differences are on the limit of being statistically significant. Differences are less significant for the number of common alleles (alleles effective), suggesting that is in the number of rare alleles where lies the difference between both species.

Concerning the gene flow in a F. excelsior population, pollen flow combines short distance events and long -distance dispersal events but in the case of small local stand, the ash used only long-distance dispersal. Concerning the hybridisation processes in France, along a large transect in the two sympatric areas, (Loire valley and Saone valley), Multivariate Canonical Discriminant Analyses indicate that intermediate morphologies do exist between the two species for the Loire valley, but not for the Saone populations. The chloroplast marker, however, was not correlated to the morphology. Genetic population assignment using the Bayesian procedures implemented in the software Structure V2 indicated that effectively, the populations from the Loire valley cluster close to those of the Mediterranean populations of F. angustifolia as indicated by the morphological analysis. Molecular grouping indicated strong population affinities to either species rather than intermediate genetic composition.

This grouping was mainly produced by allele clines for the microsatellite markers FEM-19 and M2-30 that consistently were smaller for F. angustifolia-like populations. Our results suggest that geographical separation may have favoured different adaptations in both groups that may have undergone secondary contact in areas of intermediate ecological conditions. The fact that genetic population grouping appeared to be strong towards either species suggests that divergent selection may play an important in maintaining both groups, resulting in introgressed populations of either species rather than hybrid groups.

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