L'introgression est un processus fréquent et qui a d'importantes conséquences évolutives. L'objectif de ce travail était de tester un modèle neutre d'introgression chez des épicéas du Plateau tibétain et des régions voisines. Le travail a permis de mettre en évidence que la direction de l'introgression pouvait être prédite par la dynamique passée des populations d'arbres, et que l'importance de cette introgression était inversement proportionnelle à l'intensité des échanges génétiques au sein de l'espèce invasive, grâce à la comparaison de la structure génétique basée sur des marqueurs chloroplastiques (à hérédité paternelle) et mitochondriaux (à hérédité maternelle). / Introgression is a widespread phenomenon with potentially profound evolutionaryconsequences. Recently, significant progress in our understanding of introgression hasbeen made with the development of a neutral model. This model predicts that, whenone species invades an area already occupied by a related species, introgression ofneutral genes takes place mainly from the local species towards the invading ones. Inaddition, following a contact between two hybridizing species, the model predicts thatintrogression should be particularly frequent for genome components experiencinglittle gene flow. However, to date, there was no empirical example available, in whichone species expanded into the range of a closely related one and two markers withcontrasted rates of gene flow had been studied for both species. Only in such a casecould the two predictions outlined above be tested simultaneously. In addition, basedon these two predictions, species delimitation should be more efficient when usingmolecular markers experiencing high rates of gene flow. The present thesis was designed to test the hypotheses of this model. The biologicalmodel used was conifers, a group in which introgression and hybridization arecommon because of incomplete reproductive isolation. The species investigatedbelong to the genus Picea (spruce). We focused on two species complexes,represented by monographic clades in a phylogenetic study using the chloroplast genematK. All species studied occur in the Qinghai-Tibetan Plateau (QTP) and adjacenthighlands. The phylogeography of these species complexes was reconstructed usingorganelle markers (mitochondrial DNA, mtDNA and chloroplast DNA, cpDNA). Inconifers, mtDNA and cpDNA have contrasted modes of inheritance. The former ismaternally inherited, transmitted by seeds experiencing little gene flow while thelatter is paternally inherited, transmitted by both pollen and seeds experiencing highlevels of gene flow. Therefore, uniparentally inherited mtDNA and cpDNA markersexperience different rates of gene flow in such a group, providing an ideal model to test the relationship between rates of gene flow, introgression and species delimitation.Two mtDNA fragments (nad1intron b/c; nad5 intron1) and three cpDNA fragments(ndhK-C；trnL-trnF；trnS-trnG) were sequenced for nine species belonging to thePicea asperata and P. likiangensis species complex.(1) Nine mtDNA and nine cpDNA haplotypes were detected in 459 individualsfrom 46 natural populations in five species of P. asperata complex. As found in mostconifer species studied so far, low variation is present in the two mtDNA intronsalong with a high level of differentiation among populations (GST = 0.90). In contrast,higher variation and lower differentiation among populations was found at cpDNAmarkers (GST = 0.56). The cpDNA, although far from being fully diagnostic, is morespecies-specific than mtDNA: four groups of populations were identified usingcpDNA markers, all of them related to species or groups of species, whereas formtDNA, geographical variation prevails over species differentiation. A literaturereview shows that mtDNA variants are often shared among related conifer species,whereas cpDNA variants are more species-specific. Hence, increased intraspecificgene flow appears to decrease differentiation within species but not among species.[...]