No measurable fitness cost to experimentally evolved host defence in the Caenorhabditis elegans-Serratia marcescens host-parasite system.

Host susceptibility to parasites can vary over space and time. Costs associated with the maintenance of host defence are thought to account for a portion of this variation. Specifically, trade-offs wherein elevated defence is maintained at the cost of fitness in the absence of the parasite may cause levels of host defence to change over time and differ between populations. In previous studies, we found that populations of the host nematode, Caenorhabditis elegans, evolved greater levels of parasite avoidance and resistance against the bacterial parasite, Serratia marcescens. Here, we passaged these host populations either in the presence or absence of the parasite to test for a cost of elevated host defences. After 16 generations, we found that elevated levels of host defence were maintained during evolution in both the presence and absence of the parasite. Further, this maintenance of defence was not the result of limited standing genetic variation, but rather the absence of a measurable cost associated with defence. Therefore, costs associated with host defence may not broadly account for differences in host susceptibility across space and time.