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International audience; Sexual selection may contribute to the evolution of plant sexual dimorphism by favoring architectural traits in males that improve pollen dispersal to mates. In both sexes, larger individuals may be favored by allowing the allocation of more resources to gamete production (a ‘budget’ effect of size). In wind‐pollinated plants, large size may also benefit males by allowing the liberation pollen from a greater height, fostering its dispersal (a ‘direct’ effect of size). To assess these effects and their implications for trait selection, we measured selection on plant morphology in both males and females of the wind‐pollinated dioecious herb Mercurialis annua in two separate experimental common gardens at contrasting density. In both gardens, selection strongly favored males that disperse their pollen further. Selection for pollen production was observed in the high‐density garden only and was weak. In addition, male morphologies associated with increased mean pollen dispersal differed between the two gardens, as elongated branches were favored in the high‐density garden while shorter plants with longer inflorescence stalks were favored in the low‐density garden. Larger females were selected in both gardens. Our results point to the importance of both a direct effect of selection on male traits that affect pollen dispersal, and, to a lesser extent, a budget effect of selection on pollen production.

1. Hybridization is a key evolutionary process with major consequences for conservation and speciation. However, sexual barriers interact with the local context to determine hybridization rates in a way that is still poorly explored. For instance, in the context of an expanding or introduced plant population, where a few individuals are isolated in populations dominated by heterospecific individuals, what is the hybridization potential? 2. To obtain baseline predictions on hybridization rate between two species differing in abundance and to evaluate the effect of pollen limitation, we first used a mean-field model. We then explored a spatially explicit individual-based mating model relying on pollen dispersal kernels to predict hybridization rates in a mixed-species stand, under different scenarios for (i) the strength of sexual barriers; (ii) the spatial distribution of individuals within the site; (iii) the variation in individual fecundity; and (iv) the magnitude, shape and asymmetry of pollen dispersal kernels. 3. Pollen limitation was shown to have the potential to greatly increase hybridization rates. Similarly, fine-scale variation in species composition can result in elevated hybridization rates compared to mean-field predictions, especially with strong sexual barriers. However, species clustering in combination with reduced pollen dispersal has the opposite effect, protecting from hybridization. 4. Synthesis. Our simulation results show that variation of the pollen pool composition at the scale of individuals or stigmas due to spatial configurations or pollen limitation can substantially modify hybridization rates. We explain this by the disproportionate effect of some pollen environments on average hybridization rates. This suggests thoroughly evaluating individual behaviour in terms of hybridization, especially for rare or patchy species.

Interindividual variance of male reproductive success (MRS) contributes to genetic drift, which in turn interacts with selection and migration to determine the short-term response of populations to rapid changes in their environment. Individual relative MRS can be estimated through paternity analysis and can be further dissected into fecundity and spatial components. Existing methods to achieve this decomposition either rely on the strong assumption of a random distribution of pollen donors (TwoGener) or estimate only the part of the variance of male fecundity that is explained by few covariates. We developed here a method to estimate jointly the whole variance of male fecundity and the pollen dispersal curve from the genotypic information of sampled seeds and their putative fathers and geographical information of all individuals in the study area. We modelled the relative individual fecundities as a log-normally distributed random effect. We used a Bayesian approach, well suited to the hierarchical nature of the model, to estimate these fecundities. When applied to Sorbus torminalis, the estimated variance of male fecundity corresponded to an effective density of trees 13 times lower than the observed density (d(obs)/d(ep ) approximately 13). This value is between the value (approximately 2) estimated with a classical mating model including three covariates (neighbourhood density, diameter, flowering intensity) that affect fecundity and the value (approximately 30) estimated with TwoGener. The estimated dispersal kernel was close to previous results. This approach allows fine monitoring of ongoing genetic drift in natural populations, and quantitative dissection of the processes contributing to drift, including human actions.

International audience; 1. Characterizing fine scale mating patterns in plant populations makes it possible to investigate genetic drift, gene flow and selection gradients at a contemporary time scale.Molecular markers are valuable tools for this type of analysis, and numerous statistical methods have been developed to make the best of the information they can provide. In particular, we recently proposed a Bayesian approach based on a paternity analysis wherein we estimate jointly the variance in male fecundity and the pollen dispersal kernel. 2. Here, we use simulated data sets to investigate the accuracy of the Bayesian approach compared to (i) classical maximumlikelihood approaches (e.g. Neighbourhood model) that ignore variance in male fecundity or explain it through a few covariates and (ii) indirect methods (KinDist and Two-Gener) that integrate the variance in fecundity in an ‘effective population density’. 3. The Bayesian estimates correctly considered the over-dispersion resulting from the variance infecundity, resulting in wider but more accurate confidence intervals, in particular in high-densitypopulations. The maximum likelihood methods resulted in confidence intervals with low coverageprobabilities and in widespread false-positive tests when testing the effect of covariates on malefecundity. 4. Estimated individual fecundities and estimated empirical variance in fecundity were robust tothe distribution assumed for the individual random fecundities (log-normal or Gamma). In contrast,the theoretical variance estimate critically depended on the assumed distribution. 5. The indirect methods provided much more variable estimators, as expected because they use lessinformation about pollen sources and consider the molecular information only through genetic structure indices. 6. Disentangling the fecundity from the spatial effects in paternity analyses is necessary when studying selection in natura and or when addressing the effects of spatial distribution on effective gene flow. The Bayesian approach studied here successfully accounts for the variance in fecundity when a large fraction of it is not explained by the studied covariates. The Mixed Effect Mating Model computer program introduced here is devoted to its implementation.

La phase de renouvellement constitue l’étape clé de la transmission du patrimoine génétique d’un peuplement forestier et du maintien de sa diversité génétique. Son succès est conditionné par la production de pollen et de graines. Cette thèse s’intéresse à l’impact de la structure des peuplements reproducteurs à différentes échelles spatiales sur les flux de gènes lors de régénérations naturelles de pin sylvestre (Pinus sylvestris L.). Elle identifie les principaux déterminants de la production quantitative et qualitative de graines à l’échelle d’un peuplement. Nos résultats démontrent la nécessité de prendre en compte, à l’échelle d’un massif forestier, le caractère isolé ou non de l’unité de gestion à régénérer pour évaluer son potentiel fructifère. Dans des peuplements isolés, une production réduite de pollen peut ainsi conduire à une diminution de la production de graines viables et donc à une limitation de la population de semis naturels. Ce phénomène est accru quand les reproducteurs sont eux-mêmes isolés au sein d’un peuplement mélangé chêne-pin. Dans de larges massifs au contraire, la plupart des événements de fécondation sont issus d’un nuage pollinique suffisamment abondant pour ne pas limiter la production de graines. Dans ce cas les échanges polliniques dépassent l’échelle de l’unité de gestion. Il en résulte une faible différenciation des nuages polliniques captés par les différents arbres-mères. Enfin, nos travaux montrent que la dispersion des graines est concentrée dans le temps et se produit majoritairement à moins de 50m des semenciers. Nos résultats sont discutés dans le cadre de la gestion courante des massifs forestiers pour limiter les risques génétiques liés à certaines pratiques sylvicoles. Nous proposons enfin des descripteurs de terrain des principaux déterminants de la production et de la dispersion des graines en vue d’assurer le succès de la régénération naturelle., Natural regeneration phase is thus the key step of stand renewal in order to maintain its genetic diversity. Its success highly depends on seed production. This study focuses on the impact of structure of reproductive stand at different spatial scales on gene flow during natural regeneration of Scots pine (Pinus sylvestris L.). We identify main components of heterogeneity of seed production at stand scale. Moreover our results demonstrate the importance of isolation at landscape level to evaluate seed production potential of management units. In isolated stands, reduced pollen production can lead to limited seed production and consequently to limited seedling production. This phenomenon is even more intense when reproductive trees are isolated within a mixed oak-pine stand. On the reverse within large forest area, most of pollination events are due to the presence of a pollen cloud huge enough to ensure a non-limited seed production. In such a situation, pollen flow occurs at wider spatial scale than the management unit one. As a result, we observed a non significant differentiation of pollen clouds captured by mother trees. We found that duration of seed deposition process is short and seed dispersal mainly occurs at less than 50m from the seed trees. Our results are discussed within the context of current forest management in order to limit genetic risks linked to some sylvicultural practices. We also propose to forest managers field descriptors of important determinants of seed production and dispersal for successful natural regeneration.

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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.

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Crop-to-wild gene flow has received close attention over the past ten years in connection with the development and cultivation of transgenic crops. In this paper, we review key examples of crop/wild sympatry and overlapping flowering phenology, pollen and seed dispersal, the barriers to hybridisation and introgression, the evolution and fate of interspecific hybrids, their fitness, and the potential cost of transgenes. We pay particular attention to ways in which the evolution and divergence between crops and their wild relatives may interfere with these successive steps. Our review suggests that crop-to-weed gene flow is highly idiosyncratic and that crop gene dispersion will certainly be very difficult to preclude totally. Future directions for research should thus focus on the long-term establishment and effects of transgenes on natural communities.

Des expérimentations réalisées en 1998 par l'AGPM ont permis de mesurer la dispersion efficace du pollen de maïs en conditions climatiques réelles en utilisant une parcelle de plantes contenant un marqueur dominant (ici une coloration en bleu du grain) située au sein d'un champ de plantes ne contenant pas le marqueur. Ces expérimentations, accompagnées d'une modélisation mécaniste de la fonction de dispersion du pollen, avaient permis d'ajuster des modèles de dispersion sur les données observées (Klein, 2000 ; Klein et al., 2003). Il est alors possible de réaliser des prédictions de taux de pollinisations croisées entre deux parcelles de maïs sur la base de ces résultats (Angevin et al., 2001). Cependant, tout ce travail a été réalisé en milieu continu (c'est-à-dire pour un champ entièrement couvert de maïs) et il est très probable qu'une discontinuité du couvert végétal implique une modification de la fonction de dispersion pour cette espèce. La dispersion est en effet anémophile et la structure des écoulements d'air est fondamentalement différente à l'intérieur d'une parcelle de maïs et en dehors de celle-ci. Des expérimentations ont donc été réalisées en 1999 et 2000 pour mesurer la dispersion du pollen de maïs en présence d'une discontinuité du couvert végétal et pour vérifier dans quelle mesure celle-ci est effectivement différente de la dispersion en milieu continu. Nous présentons, dans une première partie, les résultats des expérimentations de 2000, conduites dans deux champs de maïs avec deux types de discontinuités du couvert végétal (tournesol ou trèfle). Dans une seconde partie, nous présentons quelques prédictions réalisées pour ces expérimentations en utilisant les modèles ajustés sur des données de dispersion en milieu continu ainsi que des données météorologiques acquises indépendamment., "Modelling of pollen dispersal appears necessary to assess the impact of transgenic crops in agro-ecosystems. Indeed pollen dispersal is a major component of geneflow and will therefore determine both cross-pollination levels between fields and from fields to potential related species. Numerous pollen dispersal experiments have therefore been published in the last ten years. Published pollen dispersal experiments on corn are seldom (Bateman (1947) reanalysed by Tufto et al. (1997), Klein et al. (2003)). For our part, the AGPM (a French technical institute of corn) performed a first experiment on corn in 1998 to measure its efficient pollen dispersal. The experimental design was a central plot of blue grain corn surrounded by classical yellow corn. The development of quasi-mechanistic dispersal models made it possible to derive individual dispersal functions from the results of these experiments (Klein et al. 2003). The knowledge of such a function makes it possible to predict cross-pollination rates between two adjacent corn fields (Angevin et al. 2001; Lavigne et al. 1996; Lavigne et al. 1998; Tufto et al. 1997). However, these models were developed for a continuous corn canopy (the all area modelled is covered with corn plants) and it is probable that discontinuities in the canopy will result in modifications of the individual dispersal function. Indeed, corn is wind pollinated and air movement differs within and outside fields (Mc Cartney 1997; Wilson et al. 1982). Two experiments were therefore performed in 2000 to measure pollen dispersal over a discontinuity in the canopy and results were compared to expected dispersal patterns over a continuous canopy to estimate the impact of such a discontinuity on efficient dispersal.

La majorité des chercheurs s'accorde à penser aujourd'hui qu'il est impossible de confiner un transgène strictement dans les parcelles cultivées, même si certaines hybridations ne se produisent qu'à des fréquences très faibles. Ainsi, si l'on veut créer une filière de production agro-alimentaire sans OGM, il faut quantifier la pollution génétique d'un champ vers un autre afin de mettre en place des consignes de culture garantissant une pureté de production satisfaisant les normes choisies. L'objectif de notre travail est d'estimer la fonction de dispersion individuelle du pollen de colza, en présence d'une discontinuité dans la culture., In order to predict the pollen dispersal of a plant species under several environmental conditions, it is necessary to determine its individual pollen dispersal function. Several parametric models have already been proposed for homogeneous crop experiments. However in an agricultural landscape, a discontinuity effect on pollen flows in a cultivated area (e.g. a road crosses a field) has to be taken into account. This effect is modelled and estimated: according to the size of the discontinuity, it may correspond to a significant acceleration of the pollen flow. Graphical diagnosis methods show that the modelling of the individual pollen dispersal distribution and of the discontinuity effect, best fits the data when using constant piecewise functions.

De nombreuses études expérimentales et théoriques se sont attachées à décrire la dispersion du pollen à courtes et moyennes distances (moins de 100 m en général), que ce soit en milieu continu (la surface considérée est couverte de manière homogène par l'espèce considérée) ou lors de l'étude de l'effet de discontinuités (surfaces non cultivées ou couvertes par une autre espèce) sur la dispersion du pollen. Ces études sont fondamentales lorsque l'on cherche à comprendre et prédire les taux de pollinisation croisée entre deux champs voisins ou les taux de pollution génétique engendrés par des plantes indésirables au sein d'une parcelle ou à proximité. Cependant, que ce soit dans une optique de ségrégation de filières par une spatialisation des variétés ou pour des questions de biologie des populations relatives aux flux de gènes entre champs et populations hors-champs ou sauvages, il devient nécessaire de comprendre les flux de gènes à l'échelle plus large d'un paysage agricole (de l'ordre de quelques kilomètres). Il est donc nécessaire de travailler sur les possibilités d'extrapolation à l'échelle du paysage des fonctions de dispersion obtenues en milieu continu pour les courtes et moyennes distances. Nous présentons donc ici quelques travaux qui explorent les propriétés des queues de fonctions de dispersion. Ces travaux sont basés sur la dispersion du pollen de colza pour laquelle des fonctions de dispersion ont été obtenues par ajustement sur des données expérimentales (Inra Rennes). Nous présentons ces données et les ajustements réalisés dans la première partie. Des prédictions de taux de pollinisation croisée ont alors été réalisées en extrapolant les fonctions obtenues à de plus grandes distances (300 m) et confrontées à des données obtenues par le Cetiom (partie 2). Enfin, des modèles à l'échelle du paysage permettent de comprendre l'importance du poids de la queue de dispersion sur le mélange des pollens provenant de différentes sources. En particulier, le fait que la fonction de dispersion ait une queue à décroissance plus lourde que la dispersion exponentielle à un effet bien plus fondamental que la distance moyenne de dispersion (partie 3)., Pollen dispersal over short and intermediate distances (generally less than 100m) has been the topic of numerous experimental and theoretical studies (Eastham & Sweet, 2002). Particular importance has been given to the intra-field dispersal (i.e. homogeneous canopy and distances around 100m) and to the effects of discontinuities in the canopy (non cultivated areas, or areas cultivated with non-studied species). A precise description of intermediate distance dispersal and a better understanding of gap effects on pollen dispersal are necessary to predict the cross-pollination rates between two close fields or the genetic pollution within a field that originates from undesired plants (cultivated, feral or wild) in the vicinity. However, it has become relevant to understand pollen dispersal on the larger scale of agricultural landscapes (distances of several kilometers). This is true in agronomy for issues relative to GM and non GM co-existence by defining spatial patterns of fields grown with different varieties, as well as in population biology for issues relative to gene flow between fields and feral or wild populations. Thus, it is necessary to work on the means to extrapolate the dispersal functions fitted to experimental data collected in an homogeneous canopy and only over intermediate distances in order to apply them to a broad range of distances and situations. In this paper, we present concepts to investigate the role and importance of long-distance dispersal regarding mainly the properties of dispersal function tails.

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