| Summary: | The stickleback Gasterosteus aculeatus is a resident of coastal marine and freshwaters throughout the temperate Holarctic and is a model species for evolutionary biology, particularly due to its recolonization events of freshwaters in previously glaciated catchments. This study concentrates on the species in Ireland, particularly in the Burrishoole system in the west coast of Ireland. The latter is an extensively studied catchment with freshwater streams and lakes running into a brackish lake over waterfalls largely impassable upstream to sticklebacks; then connecting to coastal waters. Previous genetic (using microsatellites), morphological and meristic studies in the Burrishoole catchment identified four statistically distinct populations; one in the major freshwater lake and three (completely-, partially- and low-plated forms) in the brackish lake. Here, a more extensive study was undertaken extending the geographic and temporal analyses of genetic aspects (using neutral and adaptive SNPs) and phenotypic traits including morphological and metabolic characters, and gut microbiome composition. Neutral SNPs showed largely the same population composition as previously suggested using microsatellites (i.e. isolation-by-adaptation reflective of time of divergence), though the partially-plated group in the brackish lake was not supported. This composition being temporarily stable and supported by morphological analysis. The confirmed adaptive SNPs suggest a different scenario with isolation-by-environment in relation to the two populations differentiated by ecomorph (plates, body shape, gill rakers and migratory behaviour) co-existing in Lough Furnace. Analysis of traits associated with metabolism and gut microbiome did not extend population differentiation, but yielded novel results individually. The individual results are discussed in four separate chapters, and the thesis then concludes with an overview chapter. Similarity or differences with other stickleback studies around the North Atlantic and Pacific are ...
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