| Summary: | Biological invasions can be considered as an experiment performed by nature. They offer a unique way to study adaptation to new environments with all facets of new biotic and abiotic challenges like climate change and disease. On top of that, rising temperatures and risk of disease can interact and intensify selection pressure on invasive species to immunologically adapt to local parasites. The invasion of the Pacific oyster, Crassostrea gigas into the North Sea, is an ideal system to study the interactive effects of climate change and disease. Two independent invasions lead to the establishment of two genetically distinct populations, that differ in their selective history of disease outbreaks. While the Southern population is frequently subjected to natural selection induced by oyster summer mortality, with mortality rates exceeding 60%, Northern populations and hybrids in a secondary contact zone have been spared so far. Here, the interaction of high temperatures and bacteria of the genus Vibrio are believed to be the main causative agents of such mortalities. To test for differential selection gradients within the two invasion waves, we infected wild and artificially bred oysters with allopatric and sympatric Vibrio splendidus strains at prevailing and proposed future water temperatures. Based on mortality rates, immune response, and bacterial infection loads we observed that at high water temperatures oysters were severely impacted by Vibrio infection. However, while we consistently observed specific host immunological adaptation to sympatric parasites also at lower temperatures, direct advantages in terms of host fitness could only be detected at high temperatures indicating that the selective environment can unveil otherwise cryptic patterns of local adaptation.
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