@ARTICLE{TreeBASE2Ref22543, author = {Joel L Sachs and Ryan G Skophammer and Nidhanjali Bansal and Jason E Stajich}, title = {Evolutionary origins and diversification of proteobacterial mutualists}, year = {2014}, keywords = {evolutionary transition, mutualism, parasitism, symbiont}, doi = {10.1098/rspb.2013.2146}, url = {http://rspb.royalsocietypublishing.org/content/281/1775/20132146}, pmid = {24285193}, journal = {Proceedings of the Royal Society B: Biological Sciences}, volume = {281}, number = {1775}, pages = {}, abstract = {Mutualistic bacteria infect most eukaryotic species in nearly every biome. Nonetheless, two dilemmas remain unresolved about bacterial-eukaryote mutualisms: how do mutualist phenotypes originate in bacterial lineages and to what degree do mutualists traits drive or hinder bacterial diversification? Here, we reconstructed the phylogeny of the hyperdiverse phylum proteobacteria and investigated the origins and evolutionary diversification of mutualistic bacterial phenotypes compared to other lifestyles. We categorized host-association phenotypes using an information gathering heuristic that incorporates ambiguous information from a manually curated database. Our ancestral state reconstructions inferred a range of 34-39 independent origins of mutualist phenotypes in proteobacteria, revealing the surprising frequency with which host-beneficial traits have evolved in this phylum. We found proteobacterial mutualists to be more often derived from parasitic than from free-living ancestors, consistent with the untested paradigm that bacterial mutualists most often evolve from pathogens. We inferred that mutualists exhibit a negative net diversification rate (speciation minus extinction), which suggests that mutualism evolves primarily via transitions from other states rather than diversification within mutualist taxa. Moreover, our ancestral state reconstructions infer that proteobacterial mutualist lineages exhibit a paucity of reversals to parasitism or to free-living status. This evolutionary conservatism of mutualism is contrary to longstanding theory, which predicts that selection should often favor mutants in microbial mutualist populations that exploit or abandon more slowly evolving eukaryotic hosts. } }

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