Climate change-driven disease in sympatric hosts: Temporal dynamics of parasite burden and proliferative kidney disease in wild brown trout and Atlantic salmon.

Global climate change is altering the abundance and spread of various parasites, which has important consequences not only for host-parasite interactions but also for the relationships between different host species. Here, we focus on the myxozoan endoparasite Tetracapsuloides bryosalmonae that causes temperature-dependent proliferative kidney disease (PKD) in salmonids. We characterized the temporal changes in the parasite load and the severity of PKD signs (renal hyperplasia, haematocrit) in two sympatric populations of wild brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). We found that both the parasite load and disease signs vary considerably between individuals, species, rivers and sampling periods. We showed that Atlantic salmon was able to slow down the initial parasite proliferation rate and subsequently tolerate high parasite burden without obvious disease signs. In contrast, the initial parasite proliferation rate was much higher in brown trout, which was followed by the development of severe PKD signs. Thus, the speed of parasite proliferation, rather than the absolute number of the parasites in the host kidney, may play an important role in interspecific variation in PKD susceptibility. To conclude, this study illustrates the usefulness of temporal perspective for understanding host defence mechanisms and climate change-mediated impacts in the wild.

The strength and form of natural selection on transcript abundance in the wild.

Gene transcription variation is known to contribute to disease susceptibility and adaptation, but we currently know very little about how contemporary natural selection shapes transcript abundance. Here, we propose a novel analytical framework to quantify the strength and form of ongoing natural selection at the transcriptome level in a wild vertebrate. We estimated selection on transcript abundance in a cohort of a wild salmonid fish (Salmo trutta) affected by an extracellular myxozoan parasite (Tetracapsuloides bryosalmonae) through mark-recapture field sampling and the integration of RNA-sequencing with classical regression-based selection analysis. We show, based on fin transcriptomes of the host, that infection by the parasite and subsequent host survival is linked to upregulation of mitotic cell cycle process. We also detect a widespread signal of disruptive selection on transcripts linked to host immune defence, host-pathogen interactions, cellular repair and maintenance. Our results provide insights into how selection can be measured at the transcriptome level to dissect the molecular mechanisms of contemporary evolution driven by climate change and emerging anthropogenic threats. We anticipate that the approach described here will enable critical information on the molecular processes and targets of natural selection to be obtained in real time.

First record of proliferative kidney disease agent Tetracapsuloides bryosalmonae in wild brown trout and European grayling in Finland.

The myxozoan endoparasite Tetracapsuloides bryosalmonae causes temperature-driven proliferative kidney disease (PKD) in salmonid fishes. Despite the economic and ecological importance of PKD, information about the distribution of the parasite is still scarce. Here, we report for the first time the occurrence of T. bryosalmonae in wild brown trout Salmo trutta and European grayling Thymallus thymallus populations in Finland. We detected T. bryosalmonae at high prevalence in both brown trout and European grayling from the transboundary Finnish-Russian River Koutajoki system (Rivers Oulankajoki, Kuusinkijoki, Kitkajoki, Maaninkajoki, and Juumajoki) in north-eastern Finland. In southern Finland, T. bryosalmonae was detected in River Siuntionjoki young-of-the-year brown trout collected both in 2015 and 2016 (100% prevalence), while the parasite was not observed in fish from 3 other rivers (Ingarskila, Mustajoki, and Vantaanjoki) flowing to the Gulf of Finland. Our results, together with those from recent studies of Atlantic salmon, indicate that T. bryosalmonae is distributed over much higher latitudes in northern Europe than previously appreciated. We expect that increasing water temperatures will likely cause new PKD outbreaks in these more northerly regions in the future.