Host avoidance and resistance vary independently and are specific to parasite genotype

Abstract

Hosts can reduce the negative fitness effects of parasite infection by avoiding contact with parasites or by resisting infection after contact. Because of their shared outcome, avoidance and resistance have been hypothesized to trade off with one another. Assuming these defenses carry fitness costs, hosts are expected to have high levels of one defense or the other, but not both. Alternatively, avoidance and resistance may covary positively, if, for example, they complement one another or are genetically or mechanistically linked. Testing these hypotheses requires measuring avoidance and resistance independently, which is challenging because they are functionally linked. In this study, we separated avoidance and resistance of the host C. elegans against the bacterial parasite S. marcescens and tested for a correlation between them. We phenotyped a panel of 12 genetically divergent hosts using two distinct bacterial strains and multiple experimental contexts. We found no evidence of a correlation between avoidance and resistance. This result suggests that avoidance and resistance can covary independently. Moreover, we found strong genetic specificity not only for resistance, but also for a measure of avoidance, motivating further research to examine the coevolutionary dynamics of avoidance.