Pre- and post-association barriers to host switching in sympatric mutualists.

Coevolution between mutualists can lead to reciprocal specialization, potentially causing barriers to host switching. Here, we conducted assays to identify pre- and post-association barriers to host switching by endosymbiotic bacteria, both within and between two sympatric nematode clades. In nature, Steinernema nematodes and Xenorhabdus bacteria form an obligate mutualism. Free-living juvenile nematodes carry Xenorhabdus in a specialized intestinal receptacle. When nematodes enter an insect, they release the bacteria into the insect hemocoel. The bacteria aid in killing the insect and facilitate nematode reproduction. Prior to dispersing from the insect, juvenile nematodes must form an association with their symbionts; the bacteria must adhere to the intestinal receptacle. We tested for pre-association barriers by comparing the effects of bacterial strains on native versus non-native nematodes via their virulence towards, nutritional support of, and ability to associate with different nematode species. We then assessed post-association barriers by measuring the relative fitness of nematodes carrying each strain of bacteria. We found evidence for both pre- and post-association barriers between nematode clades. Specifically, some bacteria were highly virulent to non-native hosts, and some nematode hosts carried fewer cells of non-native bacteria, creating pre-association barriers. In addition, reduced infection success and lower nematode reproduction were identified as post-association barriers. No barriers to symbiont switching were detected between nematode species within the same clade. Overall, our study suggests a framework that could be used to generate predictions for the evolution of barriers to host switching in this and other systems.