The abiotic environment may play an important role in disease dynamics, particularly for parasites that are environmentally transmitted. The bacterium Pasteuria ramosa, a common parasite of Daphnia species in temperate lakes and ponds around the world, is known to experience decreased fitness when exposed to light in the upper layers of lakes. However, lakes vary in water clarity; thus exposure to light varies among Pasteuria populations. If light exposure is harmful to Pasteuria and this selection pressure varies across ecosystems, are parasites locally adapted to the light conditions they experience? We collected Pasteuria-infected Daphnia from lakes spanning a gradient of light penetration during epidemics in the fall of 2015 to examine this question. For each Pasteuria strain, half of the spores were exposed to light for four days in the upper layer of the lightest study lake, while half of the spores were kept in the dark during the in-lake incubation. We then exposed a susceptible Daphnia clone to these spores in the lab to quantify the effect that light had on each lake-strain. We hypothesized that parasites originally collected from clearer lakes would infect more Daphnia after exposure to light, indicating lower sensitivity to light exposure.
Results/Conclusions
We found that, indeed, the parasites were very sensitive to light. In the treatment where parasites were exposed to ambient light, only one animal became infected, and that was from a parasite strain collected from the clearest lake. This is consistent with our hypothesis that parasites from clearer lakes will be less sensitive to UV. While only 1 out of 40 animals from the light treatments became infected, 16 of 49 from the dark treatment became infected. We are now carrying out additional experiments to increase sample size. Lakes are becoming darker as part of global change. Our data suggest that, as lakes get darker, this might increase disease outbreaks in lakes and reduce selection for light resistance in parasites.