Ökologische und genetische Differenzierung von Daphnia-galeata-Populationen in Europa

Here I analyse 23 populations of D. galeata, a large-lake cladoceran, distributed mainly across the Palaearctic. I detected high levels of clonal diversity and population differentiation using variation at six microsatellite loci across Europe. Most populations were characterised by deviations from H-W equilibrium and significant heterozygote deficiencies. Observed heterozygote deficiencies might be a consequence of simultaneous hatching of individuals produced during different times of the year or of the coexistence of ecologically and genetically differentiated subpopulations. A significant isolation by distance was only found over large geographic distances (> 700 km). This pattern is mainly due to the high genetic differentiation among neighbouring populations. My results suggest that historic populations of Daphnia were once interconnected by gene flow but current populations are now largely isolated. Thus local ecological conditions which determine the level of biparental sexual reproduction and local adaptation are the main factors mediating population structure of D. galeata. The population genetic structure and diversity in D. galeata was investigated at a European scale using six microsatellite loci and 12S rDNA sequence data to infer and compare historical and contemporary patterns of gene flow. D. galeata has the potential for long-distance dispersal via ephippial resting eggs by wind and other dispersing vectors (waterfowl), but shows in general strong population differentiation even among neighbouring populations. A total of 427 individuals were analysed for microsatellite and 85 individuals for mitochondrial (mtDNA) sequence data from 12 populations across Europe. I detected genetic differentiation among populations across Europe and locations within sampling regions for both genetic marker systems (average values: mtDNA FST = 0.574; microsatellite FST = 0.389), resulting in a lack of isolation by distance. Furthermore, several microsatellite alleles and one haplotype were shared across populations. Partitioning of molecular variance was inconsistant for both marker systems. Microsatellite variation was higher within than among populations, whereas mtDNA data yielded an inverse pattern. Relative high levels of nuclear DNA diversity were found across Europe. The amount of mitochondrial diversity was low in Spain, Hungary and Denmark. Gene flow analysis at a European scale did not reveal typical pattern of population recolonization in the light of postglacial colonization hypotheses. Populations, which recently experienced an expansion or population-bottleneck were observed both in middle and northern Europe. Since these populations revealed high genetic diversity in both marker systems, I suggest these areas to represent postglacial zones of secondary contact among divergent lineages of D. galeata. In order to reveal the relationship between population genetic structure of D. galeata and the relative contribution of environmental factors, I used a statistical framework based on canonical correspondence analysis. Although I detected no single ecological gradient mediating the genetic differentiation in either lake regions, it is noteworthy that the same ecological factors were significantly correlated with intra- and interspecific genetic variation of D. galeata. For example, I found a relationship between genetic variation of D. galeata and differentiation with higher and lower trophic levels (phytoplankton, submerged macrophytes and fish) and a relationship between clonal variation and species diversity within Cladocera. Variance partitioning had only a minor contribution of each environmental category (abiotic, biomass/density and diversity) to genetic diversity of D. galeata, while the largest proportion of variation was explained by shared components. My work illustrates the important role of ecological differentiation and adaptation in structuring genetic variation, and it highlights the need for approaches incorporating a landscape context for population divergence.In dieser Promotionsarbeit habe ich die genetische Populationsstruktur von Daphnia galeata Hyalodaphnia-Komplex (Crustacea: Anomopoda) anhand von sechs nukleären Markern (Microsatelliten Loci) von 23 Seen aus ganz Europa untersucht (Kapitel 2). Um einen möglichen Zusammenhang zwischen historischen Prozessen und der rezenten genetischen Struktur von D. galeata-Populationen herzustellen, wurden anhand einer weiteren vergleichenden Analyse basierend auf Sequenz- (mitochondriale 12S rDNA) und Microsatelliten-Daten 12 Populationen auf phylogeographische Muster untersucht (Kapitel 3). Anhand von Korrelationsanalysen wurde weiterhin getestet, ob und wie diese genetische Struktur von Umweltfaktoren beeinflusst wird (Kapitel 4). Ziel der Arbeit war es Faktoren und Prozesse herauszuarbeiten, die die genetische Struktur innerhalb und zwischen Populationen einer zyklisch parthenogenetischen Art beinflussen und formen. Meine Analysen ergaben signifikant niedrige Heterozygotie-Werte bezüglich der Erwartungen des Hardy-Weinberg Gleichgewicht, was auf eine hohe genetische Strukturierung hinweist. Eine Erklärung für die niedrigen Heterozygotie-Werte kann eine allochronische, d.h. zeitlich versetzte Isolation von genetischen Linien sein, die entweder durch Generationen oder eine ökologische Differenzierung getrennt sind. Meine Ergebnisse in Kapitel 2 ähneln eher der Populationsstruktur von Arten, die in instabilen Gewässern vorkommen, wo sexuelle Reproduktion häufig stattfindet. Somit zeigen diese Ergebnisse, dass für D. galeata eine sexuelle Reproduktion weitaus häufiger stattfindet als erwartet. Aufgrund dessen komme ich zu dem Schluss, dass die ökologische Stabilität des Habitats wahrscheinlich nicht für die Populationsstruktur von D. galeata verantwortlich ist, sondern z.B. der Grad der sexuellen Reproduktion und möglicherweise die lokale Anpassung von klonalen Linien. Weiterhin zeigen beide Markersysteme ähnliche populationsgenetische Strukturen von D. galeata in Europa auf. Das allgemeine Bild von Süd nach Nord gerichteter genetischer Verarmung aufgrund von postglazialer Expansion konnte nicht komplett bestätigt werden, da sowohl mitteleuropäische wie auch nordeuropäische Populationen relativ hohe nukleäre wie auch mitochondriale genetische Diversitäten aufwiesen. Aufgrund des Verteilungsmusters uniquer mitochondrialer Haplotypen und Mikrosatellitenallelen nehme ich an, dass nördliche D. galeata Populationen nicht aus südeuropäischen Refugien kolonisiert wurden. Die in Kapitel 3 angeführten Ergebnisse lassen weiterhin vermuten, dass die Monopolisierung durch die Nachkommen erster Immigranten im Zuge schneller lokaler Adaptation nach der Kolonisierung einen starken Einfluss auf die genetische Struktur von D. galeata hatte. Ich konnte aufzeigen, dass mehrere Faktoren Einfluss auf die genetische Struktur von D. galeata Populationen haben. Jedoch kam es zu einer regionalen Differenzierung in der Quantität und Qualität der Umweltfaktoren auf die intra- und inter-populationsgenetische Struktur von D. galeata. Somit konnte keine Kategorie der Umweltfaktoren (Abiotik, Biomasse oder Artenvielfalt verschiedener trophischer Ebenen) für sich als erklärende Variable der genetischen Populationsstruktur in D. galeata ermittelt werden. Dieser Promotion bildet die Grundlage für experimentell motivierte Studien, um die hier erarbeiteten Hypothesen zu testen. Es konnte anhand von D. galeata gezeigt werden, dass eine detaillierte Aufnahme sowohl abiotischer als auch biotischer Faktoren mehrerer trophischer Ebenen notwendig ist, um die microevolutiven Vorgänge innerhalb eines Habitates zu verstehen. Somit können nun basierend auf dieser empirischen Arbeit z.B. durch Life-History Experimente überprüft werden, ob bestimmte Umweltfaktoren artspezifisch wirken, oder generell für Cladoceren eine Rolle in Bezug auf ihre Populationsstruktur in Abhängigkeit ihres Habitattyps aufweisen

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