Journal
ECOLOGY AND EVOLUTION
Volume 12, Issue 6, Pages -Publisher
WILEY
DOI: 10.1002/ece3.9011
Keywords
evolutionary rescue; host switching; mutualism; post-association barrier; Steinernema; Xenorhabdus
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Funding
- National Science Foundation [DEB-1906465, DEB-0919015]
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Following a host shift, repeated co-passaging is expected to increase fitness in a novel mutualistic relationship between Steinernema nematodes and Xenorhabdus bacteria. However, experimental evolution did not improve the fitness of either partner, suggesting that post-association costs may hinder the formation of novel partnerships among sympatric mutualists.
Following a host shift, repeated co-passaging of a mutualistic pair is expected to increase fitness over time in one or both species. Without adaptation, a novel association may be evolutionarily short-lived as it is likely to be outcompeted by native pairings. Here, we test whether experimental evolution can rescue a low-fitness novel pairing between two sympatric species of Steinernema nematodes and their symbiotic Xenorhabdus bacteria. Despite low mean fitness in the novel association, considerable variation in nematode reproduction was observed across replicate populations. We selected the most productive infections, co-passaging this novel mutualism nine times to determine whether selection could improve the fitness of either or both partners. We found that neither partner showed increased fitness over time. 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. Thus, post-association costs of host switching may represent a formidable barrier to novel partnerships among sympatric mutualists.
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