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Abstract

This dissertation investigates the genetic population structure of the pondweed species Potamogeton pectinatus in northern Germany. The overarching hypothesis was that continuity (marine habitat) or isolation (freshwater lake habitat) of the habitat will have a profound effect on genetic diversity and differentiation. The sampling design comprised 14 Baltic Sea populations and 12 lake populations that span across approximately the same range of longitude and latitude. The analysis of genetic population structure was done using DNA microsatellites that were developed using enriched DNA libraries.
The detection of population structure was influenced by somatic mutations. Findings of three alleles at one gene locus, and a bimodal distribution of pairwise genotype mismatch distribution suggested the occurrence of somatic chimeras, in particular in those populations that showed extensive clonal propagation. These somatic mutations changed the detectable genetic variation. Nevertheless, inferences on systematic differences of population structure between habitats will still be valid because there was no habitat specific bias.
The influence of the postglacial recolonisation was detectable in the genetic variation within populations (as heterozygosity and number of alleles). In lake populations, the genetic diversity decreased from the putative refugial sites in the west towards eastern populations. This was not true for marine populations where genetic diversity remained constant throughout the study area. Since the Baltic Sea is a continuous habitat, its recolonisation by P. pectinatus has probably not suffered as much from founder effects than the lake populations did.
The longitudinal gradient of clonal diversity was more complex. While in marine populations, the clonal diversity increased from west to east, the inverse was true for the lake populations. In the Baltic Sea, sexual reproduction was probably limited in more western sites that are more saline and hence more stressfull than the eastern sampling sites. In the lake populations, the overall population density decreased towards the east, suggesting some ecological limitation to the occurrence of pondweeds that may have been correlated with the concomittant decrease of sexual reproduction towars the east.
A comparison of genetic differentiation between popoulations showed that at similar geographic distances, lake populations were markedly stronger differentiated than marine populations. Both habitat types showed a significant isolation-by-distance relationship. The slope of the regression line in lake habitats was twice as steep as among the marine populations, indicating higher gene flow in the continuous habitat. In conclusion, I have demonstrated that the isolation barrier 'land' between isolated lake habitats markedly reduces gene flow