Temperature modifies trait-mediated infection outcomes in a Daphnia-fungal parasite system

One major concern related to climate change is that elevated temperatures will drive increases in parasite outbreaks. Increasing temperature is known to alter host traits and host-parasite interactions, but we know relatively little about how these are connected mechanistically – that is, about how elevated temperatures impact the relationship between epidemiologically relevant host traits and infection outcomes. Here, we used a zooplankton-fungus ( Daphnia dentifera-Metschnikowia bicuspidata ) disease system to investigate whether temperature interacted with host susceptibility traits in determining infection outcomes. We did this by exposing D. dentifera to M. bicuspidata or leaving them unexposed at either control (20°C) or warming temperatures (24°C) in a fully factorial design. We found that elevated temperatures altered the physical barrier and immune responses to parasites during the initial infection process, and that infected hosts at elevated temperatures suffered a greater reduction of fecundity and lifespan. Furthermore, the relationship between a key trait – gut epithelium thickness, which is a physical barrier – and the likelihood of terminal infection reversed at warmer temperatures. Together, our results highlight the complex ways that temperatures can modulate host-parasite interactions, and the importance of considering key host susceptibility traits when predicting disease dynamics in a warmer world. This article is part of the theme issue ‘Infectious disease and evolution in a changing world’. ### Competing Interest Statement The authors have declared no competing interest.