Post-association barrier to host switching maintained despite strong selection in a novel mutualism

<p class="MsoNormal"><span>Following a host shift, repeated co-passaging of a mutualistic pair is expected to increase fitness over time in one or both species.<span>  </span>Without adaptation, a novel association may be evolutionarily short-lived as it is likely to be outcompeted by native pairings.<span>  </span>Here we test whether experimental evolution can rescue a low-fitness novel pairing between two sympatric species of <em>Steinernema</em> nematodes and their symbiotic <em>Xenorhabdus</em> bacteria.<span>  </span>Despite low mean fitness in the novel association, considerable variation in nematode reproduction was observed across replicate populations.<span>  </span>We selected the most productive infections, co-passaging this novel mutualism nine times to determine whether selection could improve fitness of either or both partners.<span>  </span>We found that neither partner showed increased fitness over time.<span>  </span>Our results suggest that the variation in association success was not heritable and that mutational input was insufficient to allow evolution to facilitate this host shift.<span>  </span>Thus, post-association costs of host switching may represent a formidable barrier to novel partnerships among sympatric mutualists. </span></p>