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An isolation by distance model is presented, which is better fitted to forest tree populations than those available to date. This model was developed in order to understand which factors might determine genetic diversity and spatial structure within populations of these species, for in situ management of their genetic resources. It allows a nonuniform spatial distribution of individuals, as well as some selfing. This model was studied by means of simulations, using a complete factorial design with 32 highly contrasted cases, which resulted from the combination of different values of the following input variables : global density, seed and pollen dispersal distances, generation overlap and overdominant selection. Our results show that much of the variation in spatial genetic structure and inbreeding at the individual level can be explained by variations in clumping and outcrossing rate. Inbreeding at the population level is mainly determined by the degree of generation overlap. As a general conclusion, all the input variables studied here do affect the genetic characteristics of simulated populations, and most interaction effects between these variables are highly significant. These results suggest that the explanation of differences in the level and/or structure of within-population genetic diversity between tree species could presumably be improved by using additional classification factors such as density, the level of clumping, generation overlapping, and/or selection, as well as by taking into account the interaction effects between classification factors. (Resume d'auteur)

The mating system is of major concern for the dynamic management of genetic resources. We investigated the outcrossing rate of six experimental wheat populations, derived from two genetic pools (PA and PB) which have been cultivated at four French sites. The replicates of the two segregating populations have undergone 10 years of multiplication in isolation, with no artificial selection. For each population, 78 individuals were randomly sampled and genotyped for 25 codominant loci revealed by 15 restriction fragment length polymorphism (RFLP) probes. Outcrossing rates were estimated on the basis of the multilocus heterozygote deficiency. Estimates ranged from 2.4 to 10.1 %, depending on the population. The outcrossing rate appeared to be affected by the cultivation site, and also by the interaction between the genetic pool and the cultivation site. No significant pool effect was found over sites. The use of natural or modified outcrossing rates is suggested for the management of gene flows between the subpopulations of a metapopulation. Consequences of recombination events within and between populations are discussed in the context of short-term adaptive response of populations and long-term elimination of weakly deleterious mutations., Le régime de reproduction est un paramètre essentiel pour le pilotage d’un programme de gestion dynamique des ressources génétiques. Cet article s’attache à l’estimation du taux d’allogamie dans six populations expérimentales de blé tendre, issues de deux pools génétiques différents (PA et PB) cultivés dans quatre sites distincts. Chaque population a subi dix années de multiplication en isolement, sans sélection consciente. Dans chacune d’elles, 78 individus ont été aléatoirement échantillonnés et génotypés pour vingt-cinq locus codominants, grâce à quinze sondes RFLP. Les taux d’allogamie ont été estimés à partir de l’hétérozygotie résiduelle multilocus. Un niveau d’allogamie élevé pour cette espèce cléistogame a été observé. Les estimations varient de 2,4 à 10,1 % suivant les populations. Ces taux d’allofécondation dépendent d’une part du site de culture, et d’autre part d’une interaction entre composition génétique initiale et site de culture. L’intérêt de l’allogamie tant pour la génération de nouvelles combinaisons génétiques que pour la purge des mutations faiblement délétères est discuté. L’utilisation des taux naturels d’allogamie pour instaurer des flux de gènes entre les populations est proposée dans l’optique d’une gestion en métapopulation.