Abstract
Extreme climate events such as heat waves are expected to increase in frequency under global change. As one indirect effect, they can alter magnitude and direction of species interactions, for example those between hosts and parasites. We simulated a summer heat wave to investigate how a changing environment affects the interaction between the broad-nosed pipefish (Syngnathus typhle) as a host and its digenean trematode parasite (Cryptocotyle lingua). In a fully reciprocal laboratory infection experiment, pipefish from three different coastal locations were exposed to sympatric and allopatric trematode cercariae. In order to examine whether an extreme climatic event disrupts patterns of locally adapted host-parasite combinations we measured the parasite's transmission success as well as the host's adaptive and innate immune defence under control and heat wave conditions. Independent of temperature, sympatric cercariae were always more successful than allopatric ones, indicating that parasites are locally adapted to their hosts. Hosts suffered from heat stress as suggested by fewer cells of the adaptive immune system (lymphocytes) compared to the same groups that were kept at 18°C. However, the proportion of the innate immune cells (monocytes) was higher in the 18°C water. Contrary to our expectations, no interaction between host immune defence, parasite infectivity and temperature stress were found, nor did the pattern of local adaptation change due to increased water temperature. Thus, in this host-parasite interaction, the sympatric parasite keeps ahead of the coevolutionary dynamics across sites, even under increasing temperatures as expected under marine global warming.
Highlights
The interaction between hosts and parasites is often conceptualized as an arms race where hosts and parasites need to adapt and counter adapt permanently to keep up with the evolution of the respective antagonist [1,2]
We examined if environmental change can disrupt locally adapted host-parasite combinations and whether the pattern of local adaptation is stable enough to endure the impact of a simulated heat wave
Not all specificity indices were significantly different from zero (Table 1), but for each host-parasite combination, at least one specificity index differentiated significantly from zero. This indicates a clear pattern of local adaptation of the parasite to its local host. This pattern persists even if we allocate the host-parasite interaction into a new, warmer environment by imposing a heat wave
Summary
The interaction between hosts and parasites is often conceptualized as an arms race where hosts and parasites need to adapt and counter adapt permanently to keep up with the evolution of the respective antagonist [1,2]. Parasite populations are expected to adapt to the locally most common host genotypes [3,4] while at an individual level matching host6parasite genotypic interactions provide the genetic basis for this adaptation [5,6,7,8,9]. Hosts are expected to lag behind in this arms race, resulting in parasite populations being more often adapted to their sympatric hosts than vice versa [10]. For example by imposing warmer temperature conditions, any such local adaptation process can be disrupted, whereas this disruption can again be spatially restricted
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