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Summary of the thesis
Rapid, poleward range expansions are observed for an increasing number of species in the past decades. These distributional changes are commonly attributed to global environmental change. Recent research, however, indicates that genetic adaptation might also play an important role in explaining the success of range expansions. Considering the fast pace of many range expansions, such contemporary evolutionary processes are unlikely to rely on the emergence of new mutations. Instead, standing genetic variation acts as important resource to fuel adaptation. This variation can be present in a population´s gene pool or it is introduced by secondary contact and admixture of formerly isolated genetic lineages. In the past years, ample evidence has been compiled for an association of admixture, adaptation and range expansions for numerous plant and animal species.
Here, I present an analysis of the recent range expansion of the European wasp spider Argiope bruennichi. Originally, this species inhabited the Mediterranean region and warm oceanic climates in France and South-Western Germany. From around 1930 onwards, the spider started expanding its range into increasingly continental climates and can now be found as far north as Finland. This thesis aims to disentangle environmental and genetic factors, involved in the species´ range expansion. In particular, I analyze the interconnection of genetic admixture and invasion success. I approach these questions using population genetic and phylogeographic methods, morphological analyses, ecological experiments and finally whole genome- and transcriptome sequencing.
In chapter one, I conduct a detailed genetic and ecological analysis of the spider´s range expansion. I base this study on a dense sampling of more than 2.000 contemporary specimens. In addition, I include about 500 historical spiders from natural history collections. I present genetic and morphological data, as well as several ecological experiments on thermal tolerance and preference and a reciprocal transplant study. My results indicate that the spider´s range expansion is associated with admixture of formerly isolated genetic lineages from around 1930 onwards. The ecological experiments indicate that invasive spider populations have simultaneously adapted to colder temperatures by shifting their thermal preference and tolerance.
Like many other spider species, Argiope bruennichi has a wide ranging Palearctic distribution. In chapter two, I conduct a phylogeographic survey over the species´ whole range, from the Macaronesian islands over Europe to East Asia. Next to Argiope bruennichi, I include a second widely distributed spider species, the nursery web spider Pisaura mirabilis. The study is based on mitochondrial and nuclear genetic markers. I highlight the importance of outer-European glacial refugia for the wasp spider. I then show the effects of secondary contact in shaping the postglacial genetic structure of the two species. The analysis identifies several instances of incongruent phylogenetic patterns for mitochondrial and nuclear DNA markers, possibly due to recurrent selection on mitochondria.
DNA from natural history collections provides a valuable resource to trace historical genetic changes during range expansions. For that reason, I present an analysis of DNA sequencing and microsatellite genotyping success in historical spider specimens in chapter three. In addition, I exemplarily illustrate the utility of historical specimens to trace historical genetic changes in populations.
In the above chapters, I have presented evidence for admixture leading to differential adaptation in spider populations. However, the functional basis of this adaptation remains unknown. For that reason I embark towards unraveling its genomic architecture in chapter four. Initially, I generate the first available draft genome sequence of a spider species. Based on this data, I analyze genome-wide differences of native and invasive wasp spider populations across an environmental gradient.
Gene regulatory evolution is a possible mechanism to provide the means for rapid contemporary adaptation to environmental stress. For this reason, I conduct a genome-wide gene expression analysis of native and invasive wasp spiders, which have been exposed to temperature stress in chapter five. I discuss the gene expression divergence between Northern and Southern European spiders in relation to the possibility of recent contemporary adaptation.
