“Quantitative genetic variation in, and environmental effects on, pathogen resistance and temperature-dependent disease severity in a wild trout”

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Paul Vincent Debes, Riho Gross, and Anti Vasemägi

Genetic and environmental variation of a temperature-governed disease

Disease severity may differ among individuals because of the pathogen amount that an individual carries and the health damage that a specific pathogen amount can cause. Disease-severity differences may underlie genetic and environmental variation. Importantly, only genetic variation can enable evolution by natural selection towards more pathogen-resistant or disease-tolerant populations. However, the magnitude and evolutionary importance of the genetic variation may change with the environment. A study appearing in The American Naturalist shows how strongly genetic and environmental variation for pathogen infection and the severity of pathogen-induced disease can differ between two natural populations and informs us about possible evolution in the presence of a disease.

In salmonid fishes, such as trout, a microscopic parasite induces the potentially fatal proliferative kidney disease (PKD), whose severity increases with water temperatures. By electrofishing trout siblings in two neighboring Estonian rivers with different water temperatures and using a combined morphological and molecular approach, Paul Debes, Riho Gross, and Anti Vasemägi are able to disentangle and quantify the genetic and environmental variation for parasite amount carried (resistance-1) and PKD severity. Nonetheless, the researchers find that genetic variance for parasite resistance is much higher in the colder river, whereas the disease manifests more severely with a somewhat larger genetic variance in the warmer river. This suggests that selection for parasite resistance proceeds more rapidly under warmer water temperatures, when a stronger selection by a more severe PKD is expected, and which may deplete the genetic resistance variance. However, because water temperature varies spatially and temporally, natural selection and thus the intensity of evolutionary change may also vary spatially and temporally. Thus, the study highlights how environmental variation governs disease severity, affects genetic variance for resistance and disease traits, and may hamper evolution towards more resistant or tolerant populations. Read the Article

The colder river Altja (left) and the warmer river Mustoja (right) from where the studied Estonian brown trout (Salmo trutta) individuals were sampled. (Credit: Anti Vasemägi, Paul Debes, 2014)