Contrasting parasite-mediated reductions in fitness within vs. between patches of a nematode host

<p>Host and parasites interact across spatial scales, but parasite-mediated fitness effects are often measured at local scales only. Recent work suggests that parasites can reduce host fitness during dispersal between patches, highlighting the potential for both within- and between-patch effects to contribute to the net fitness consequences of parasitism. Building on this work, we measured the contribution of the dispersal phase to parasite-mediated reductions in host fitness. We used the nematode <em>Caenorhabditis elegans</em> and its natural microsporidian parasite <em>Nematocida parisii</em> to quantify the fitness consequences of parasitism at the individual, population, and metapopulation level. <em>N. parisii</em> reduced individual fecundity and population growth but had its greatest fitness impact at the dispersal stage: parasitism reduced the fitness of dispersing larvae by 63 – 100%. These results indicate that the cost of parasitism in this system is greatly underestimated if the metapopulation level is not taken into account. We also found that the effects of <em>N. parisii</em> vary with host genotype, and the relative advantage of the most resistant genotype increases with inclusion of the dispersal stage. Taken together, our findings demonstrate that host-parasite interactions at the dispersal stage magnify selection for parasite resistance.</p>