Experimental evolution of Dictyostelium discoideum cheating under relaxed selection

<p>Many microbes interact with one another, but the difficulty of directly observing these interactions in nature makes interpreting their adaptive value complicated.  The social amoeba <em>Dictyostelium discoideum </em>forms aggregates<em> </em>wherein some cells are sacrificed for the benefit of others. Within chimeric aggregates containing multiple unrelated lineages, cheaters can gain an advantage by undercontributing, but the extent to which wild <em>D. discoideum </em>has adapted to cheat is not fully clear. In this study, we experimentally evolved <em>D. discoideum </em>in an environment where there were no selective pressures to cheat or resist cheating in chimeras. <em>D. discoideum </em>lines grown in this environment evolved reduced competitiveness within chimeric aggregates and reduced ability to migrate during the slug stage. By contrast, we did not observe a reduction in cell number, a trait for which selection was not relaxed.  The observed loss of traits that our laboratory conditions had made irrelevant suggests that these traits were adaptations driven and maintained by selective pressures <em>D. discoideum </em>faces in its natural environment. Our results suggest that <em>D. discoideum</em> faces social conflict in nature, and illustrate a general approach that could be applied to searching for social or non-social adaptations in other microbes.</p>