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International audience; Common ash (Fraxinus excelsior L.) and narrow-leaved ash (F. angustifolia Vahl) are the most common ash species in Western Europe. The former is considered to be a highly valuable timber tree and contamination of its seed lots by the latter is strongly undesirable. We studied molecular, physiological and morphological characteristics that can help to detect the presence of F. angustifolia at the population level, either in seed and/or seedling samples, or in adult ash leave samples from natural or managed populations. First we developed two molecular markers, which correspond to a RAPD-SCAR marker and a nuclear microsatellite-derived marker. Results indicate that these markers are almost specific to one or the other species for a set of populations sampled across Europe. Second, first year dormancy was studied using germination tests without stratification. F. excelsior seeds did not germinate at all, while germinations were observed in samples of F. angustifolia and introgressed populations after 16 weeks. In addition, F. angustifolia embryo/seed length ratios at the dispersal stage were significantly higher than those from F. excelsior populations. This study provides straightforward and robust tools for avoiding commercial problems of impurity of seed lots and can help forest managers to certify common ash stands.; Le frêne commun (Fraxinus excelsior L.) et le frêne oxyphylle (F. angustifolia Vahl) sont les deux espèces de frêne les plus répandues en Europe occidentale. Le frêne commun étant le seul à posséder une importante valeur commerciale, la contamination de lots de graines par du frêne oxyphylle est fortement indésirable. Nous avons étudié des caractéristiques moléculaires, morphologiques et physiologiques qui pourront se révéler très utiles à la détection de la présence de F. angustifolia à l'échelle populationnelle, dans des lots de graines ou de semis, ainsi que dans des échantillons de feuilles d'arbres adultes issus de populations naturelles ou exploitées. Nous avons développé deux marqueurs moléculaires, un SCAR-RAPD et un marqueur issu d'un locus microsatellite nucléaire. Testés sur un ensemble de populations européennes, ces marqueurs se sont révélés quasi-spécifiques de l'une ou l'autre des deux espèces. D'autre part, nous avons étudié la dormance des graines en première année par des tests de germination sans stratification. Aucune germination n'a été observée sur un ensemble de descendances de frêne commun, alors que des germinations ont été observées sur toutes les descendances de frêne oxyphylle après seize semaines, ainsi que sur certaines descendances issues de populations introgressées. Enfin, nous avons montré que les embryons de F. excelsior à maturité dispersive occupent significativement moins de place dans la graine que ceux de F. angustifolia. Cette étude permet de fournir des outils rapides et efficaces qui pourront permettre d'éviter d'éventuels problèmes commerciaux liés à la pureté de lots de graines, et d'aider à la certification des peuplements de frêne.

Background The structure and evolution of hybrid zones depend mainly on the relative importance of dispersal and local adaptation, and on the strength of assortative mating. Here, we study the influence of dispersal, temporal isolation, variability in phenotypic traits and parasite attacks on the male mating success of two parental species and hybrids by real-time pollen flow analysis. We focus on a hybrid zone population between the two closely related ash species Fraxinus excelsior L. (common ash) and F. angustifolia Vahl (narrow-leaved ash), which is composed of individuals of the two species and several hybrid types. This population is structured by flowering time: the F. excelsior individuals flower later than the F. angustifolia individuals, and the hybrid types flower in-between. Hybrids are scattered throughout the population, suggesting favorable conditions for their local adaptation. We estimate jointly the best-fitting dispersal kernel, the differences in male fecundity due to variation in phenotypic traits and level of parasite attack, and the strength of assortative mating due to differences in flowering phenology. In addition, we assess the effect of accounting for genotyping error on these estimations. Results We detected a very high pollen immigration rate and a fat-tailed dispersal kernel, counter-balanced by slight phenological assortative mating and short-distance pollen dispersal. Early intermediate flowering hybrids, which had the highest male mating success, showed optimal sex allocation and increased selfing rates. We detected asymmetry of gene flow, with early flowering trees participating more as pollen donors than late flowering trees. Conclusion This study provides striking evidence that long-distance gene flow alone is not sufficient to counter-act the effects of assortative mating and selfing. Phenological assortative mating and short-distance dispersal can create temporal and spatial structuring that appears to maintain this hybrid population. The asymmetry of gene flow, with higher fertility and increased selfing, can potentially confer a selective advantage to early flowering hybrids in the zone. In the event of climate change, hybridization may provide a means for F. angustifolia to further extend its range at the expense of F. excelsior.

We examined large-scale patterns of morphology, genetic structure and ecological correlates of Fraxinus excelsior and the closely related species Fraxinus angustifolia in France, in order to determine the degree of hybridization between them. We sampled 24 populations in two putative hybrid zones (Loire and Saône), and five control populations of each species. We measured foliar characteristics of adult trees and used five nuclear microsatellites as molecular markers. Canonical discriminant analysis indicated that the two species differ in morphology, but that intermediate types are common in the Loire region but less frequent in the Saône region. Bayesian population assignment identified one F. angustifolia and two F. excelsior gene pools. Most Loire individuals clustered genetically with the F. angustifolia gene pool. In contrast, the Saône region presented individuals belonging mostly to F. excelsior pools, although the F. angustifolia type was frequent in certain populations. The lowest FST values were found between the Loire and F. angustifolia controls that also exhibited no significant isolation by distance. The proportion of the F. angustifolia gene pool in each locality was negatively correlated with winter temperatures, suggesting that a cold climate may be limiting. Hybridization is probably favoured by the intermediate climatic conditions in the Loire region that allow both species to occur, but is somewhat hampered by the harsher winters in the Saône area where morphological introgression has apparently not yet occurred.