Are host-parasite interactions influenced by adaptation to predators? A test with guppies and Gyrodactylus in experimental stream channels

Natural populations often face multiple mortality sources. Adaptive responses to one mortality source might also be beneficial with respect to other sources of mortality, resulting in reinforcing adaptations; or they might be detrimental with respect to other sources of mortality, resulting in conflicting adaptations. We explored these possibilities by testing experimentally if the responses of guppies (Poecilia reticulata) to the monogenean ectoparasitic worm Gyrodactylus differed between populations adapted to different predation regimes. In experimental stream channels designed to replicate the natural environment, we exposed eight guppy populations (high-predation and low-predation populations from each of four separate rivers) either to their local Gyrodactylus parasites (infection treatment) or to the absence of those parasites (control). We found that infection dynamics varied dramatically among populations in a repeatable fashion, but that this variation was not related to the predation regime of origin. Consistent with previous work, high-predation guppy females gained more mass, had lower reproductive investment, and had more but smaller embryos than did low-predation females. Relative to control (no parasite) channels, guppies from treatment (infected) channels gained less mass but produced similar numbers and sizes of embryos-and thus had a higher reproductive effort. However, no interaction was evident between infection treatment and predation regime. We conclude that parasitism by Gyrodactylus and predation are both likely selective forces for guppies, but that adaptation to predation does not have an obvious deterministic effect on host-parasite dynamics or on life-history traits of female guppies.