The effects of temperature and host-parasite interactions on parasite persistence in a planktonic crustacean

Although the outcome of parasitic infections can be explained by a combination of environmental and host/parasite genetic factors, these factors are often confounded by geography. Thus, linking temperature, a locally variable environmental factor, with host and parasite genetics can reveal complex spatial host-parasite interactions. We used Daphnia magna genotypes from Central Europe, where the Hamiltosporidium tvaerminnensis parasite has not yet been reported, and from two regions where it is frequently found, Northern Europe and Mediterranean basin. In Central Europe habitats are usually permanent and hosts are typically in their planktonic phase during summer - the hottest time of the year. In Northern Europe and the Mediterranean basin, on the other hand, hosts inhabit ponds that frequently dry-up in summer. We predicted that high temperatures during host and parasite active phases would prevent long-term parasite persistence. By exposing all hosts to two parasite isolates at ambient and stressfully high temperatures we tested this prediction. At ambient temperatures, we confirmed that long-term parasite persistence is only possible in Northern and Mediterranean host genotypes, while we observed reduced persistence at high temperature, but only for the Mediterranean hosts. Virulence was higher in Northern host genotypes only at ambient temperature. These results were consistent among the two parasites isolates. Our findings, thus, do not corroborate our hypotheses and suggest that predictions about responses to future climate change are highly complex in this host-parasite system.