That which does not kill you, makes you stranger: E. coli populations selected for 560 generations in randomly fluctuating stressful environments could become superior invaders

Abstract

Populations with a history of living in fluctuating environments are predicted to be successful invaders. Enhanced invasive abilities can stem from capacity to inhabit novel environments. A recent study has shown that bacterial populations selected under fluctuating temperatures perform better when confronted with novel environments. But experimental evidence for evolution of better competitors/invaders, however, is still sparse. Moreover, the question as to how the nature of selection regime, i.e. complexity and predictability, affects the potential to respond to novel environments is largely unexplored. Here I compare the invasive ability of replicate E. coli populations – selected in a randomly fluctuating complex environment – with control populations that have not experienced such environmental fluctuations. For this purpose, relative fitness was assessed in three different biotic scenarios: (a) separate competition with two different bacteria, Serratia, and Staphylococcus; (b) the ability to infect Drosophila melanogaster (fruit fly). Results show that selected populations are better than the controls when competed against Serratia, without any disadvantage in any of the other novel biotic challenges. In the second part of the study, I checked whether the observed advantage in the competition with Serratia exhibits a correlation with an improvement in fitness in any of the component environments, i.e. environmental variables which were part of the selection. To answer this, I compared the fitness of control and selected populations over the duration of selection in all the three component environments. The results of these fitness assays in the component environments reveal absence of adaptation over time for selected populations. Therefore, my results suggest that complex randomly fluctuating environments can select for populations which can potentially be better invaders/competitors in some novel environments, and evolution of this ability need not be correlated with the adaptation to the component environments.