Dokument

Summary:
English summary Limitations of gene flow in populations lead to spatial aggregation of related individuals, which implicates inbreeding and a loss of genetic diversity. The theoretical consequences of these processes are generally understood. However, consequences for real populations, distributed across man made landscapes are not immediately apparent. The anthropogenic introduction of exotic species or domesticated cultivars into landscapes has increased the possibility of introgressive gene flow to their wild relatives. The two taxa Populus nigra and P. x canadensis were used as a model system to obtain knowledge on the quantitative patterns of dispersal within and between the mosaic of remnant native individuals of P. nigra and plantations of P. x canadensis. For this purpose, key features of nSSR markers (nuclear simple sequence repeats), namely high polymorphism, codominant inheritance and availability of species-specific alleles were utilised to create an easy-to-handle tool for obtaining multilocus genetic fingerprints. As publicly available allelic ladders of each locus have been created, the presented marker system enables the generation of transferable genetic data on poplar in different laboratory settings. By this means, the marker assay can help to enlarge present clonal molecular data bases. It is also essential for certification purposes in commercial forestry. Using the marker assay, a natural population of P. nigra was analysed for genetic diversity and spatial genetic structure. Parentage analyses of seedlings as well as juveniles from a restricted area of natural regeneration enabled the quantification of pollen and seed-mediated gene flow, respectively. Consequences for genetic diversity could be concluded and consequences for the management of natural recruitment could be deduced. Spatial genetic patterns of the P. nigra adult tree population suggest prevailing short-distance gene flow, the major part of which (i. e.70 %) takes place within distances of less than 1 km. This helps to explain the reduced diversity in investigated juveniles. In the context of surrounding plantations, introgressive gene flow between P. nigra and the bred taxon P. x canadensis was studied. For this purpose, progeny of both taxa was analysed. Introgressive gene flow was found in both directions. Particular rates varied greatly and were probably due to stochastic environmental conditions and the spatial distribution of trees. However, preferential hybridisation was found between female P. x canadensis and male P. nigra. Moreover, introgressed individuals could be found in natural recruitment. Practical consequences for both upcoming studies and the conservation of natural P. nigra populations are implicated. Results of this research are discussed with respect to hybridisation processes and concerning the risk assessment of transgene flow from genetically modified taxa. Findings may also be transferred to management plans of plant species exhibiting similar dispersal mechanisms as poplar.

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