Experimental selection for genome streamlining and the conservation of phenotypic plasticity in a bacterial isolate exposed to predation

27 Maig 2016
Sala d'Actes
Impartida per: 
Dr. Jakob Pernthaler
University of Zurich, Switzerland


Aquatic bacteria often face an adaptive trade-off between nutrient limitation and predation mortality. One possible strategy to mitigate this dilemma is to maintain a plastic phenotype; this leads to more ecological stability but exposes a susceptible subpopulation to the predominant selection factor. We investigated the evolutionary trajectories of a freshwater bacterial isolate with a plastic defence phenotype (protist-inedible cell aggregates) that was maintained under nutrient restriction and high grazing mortality. Strains evolving only under bottom-up limitation drastically reduced aggregation whereas concomitant top-down mortality led to the opposite, i.e., high constitutive aggregate formation. Three isolates from independent replicates of the high predation treatments had undergone virtually identical genome streamlining events. Competition assay between strains with and without reduced genomes showed that the excision had a positive influence on growth efficiency and competitiveness, albeit at the cost of reduced metabolic flexibility. Interestingly, the evolved strains from both treatments had reduced phenotypic plasticity and had lost their predator sensing ability. A follow-up experiment unveiled that the perception of predators by bacteria was only maintained if they were exposed to alternating regimes of predator presence and absence. By contrast, phenotypic plasticity slowly eroded in strains evolving under constant conditions. Our experiments show bacteria may simultaneously adapt to two selection factors by genome reduction, thereby overcoming an apparently inescapable trade-off. We also provide first experimental evidence for the importance of a fluctuating environment to maintain phenotypic plasticity.


Brief biography

Jakob Pernthaler is professor of Limnology and Aquatic Microbiology at the University of Zurich, Switzerland, where he has spent the last 10 years of his career. He did his PhD in limnology at the University of Innsbruck, and he subsequently worked as postdoctoral fellow and research scientist at the Max-Planck Institute for Marine Microbiology. Jakob Pernthaler has developed methods for the microscopic identification of bacteria and of their activities; and using these tools he has studied the ecology and diversity of bacterioplankton populations in marine and freshwater systems. Another focus of his research has been on the interactions of pelagic bacteria with their protistan predators both in model systems and natural habitats.