title: Nematode-bacteria mutualism: Selection within the mutualism supersedes selection outside of the mutualism.
abstract: The coevolution of interacting species can lead to codependent mutualists. Little is known about the effect of selection on partners within verses apart from the association. Here, we determined the effect of selection on bacteria (Xenorhabdus nematophila) both within and apart from its mutualistic partner (a nematode, Steinernema carpocapsae). In nature, the two species cooperatively infect and kill arthropods. We passaged the bacteria either together with (M+), or isolated from (M-), nematodes under two different selection regimes: random selection (S-) and selection for increased virulence against arthropod hosts (S+). We found that the isolated bacteria evolved greater virulence under selection for greater virulence (M-S+) than under random selection (M-S-). In addition, the response to selection in the isolated bacteria (M-S+) caused a breakdown of the mutualism following reintroduction to the nematode. Finally, selection for greater virulence did not alter the evolutionary trajectories of bacteria passaged within the mutualism (M+S+ = M+S-), indicating that selection for the maintenance of the mutualism was stronger than selection for increased virulence. The results show that selection on isolated mutualists can rapidly breakdown beneficial interactions between species, but that selection within a mutualism can supersede external selection, potentially generating codependence over time.
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405. levi.morran@emory.edu.|Department of Biology, Emory University, 1510 Clifton Rd. NE, Atlanta, Georgia, 30322. levi.morran@emory.edu.
authors: Levi T
authors: LT
authors: Morran
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.|Department of Biology, Emory University, 1510 Clifton Rd. NE, Atlanta, Georgia, 30322.
authors: McKenna J
authors: MJ
authors: Penley
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.
authors: Victoria S
authors: VS
authors: Byrd
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.
authors: Andrew J
authors: AJ
authors: Meyer
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.|Department of Biology, Emory University, 1510 Clifton Rd. NE, Atlanta, Georgia, 30322.
authors: Timothy S
authors: TS
authors: O'Sullivan
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.
authors: Farrah
authors: F
authors: Bashey
authors: Department of Bacteriology, University of Wisconsin-Madison, 1550 Linden Dr. Madison, Madison, Wisconsin, 53706.
authors: Heidi
authors: H
authors: Goodrich-Blair
authors: Department of Biology, Indiana University, 1001 E. Third St., Bloomington, Indiana, 47405.
authors: Curtis M
authors: CM
authors: Lively
mesh_terms: D000818:Animals; D005075:Biological Evolution; D017168:Rhabditida / Q000382:microbiology*; D013559:Symbiosis; D020593:Xenorhabdus / Q000502:physiology*
keywords: Adaptation; coevolution; experimental selection; symbiosis