Abstract
Behaviours that facilitate persistence in the face of fluctuating environmental conditions are canonical examples of evolutionary adaptations. One such trait is bet-hedging: the stochastic switching between phenotypic states. While bet-hedging is found in organisms ranging from pathogenic bacteria to humans, the evolutionary emergence of this behaviour has never been observed. In this talk I will described a selection experiment in which bacterial populations were propagated in a fluctuating environment under conditions that mimic essential features of the host immune system. Initially, all populations adapted to rounds of fluctuating selection by mutation; however, in two (of twelve) populations this trend was broken by the de novo evolution of bet-hedging genotypes, which persisted on account of stochastic phenotype switching. Genome re-sequencing of one of these genotypes revealed the underlying mutations, and identified a single nucleotide change as the proximate cause of switching - the mechanism of the switch is epigenetic. Nevertheless, the evolution of bet-hedging was contingent upon mutations that fixed earlier during the experiment – the primary role of history being to alter the relative fitness effect wrought by the proximate mutation. These findings reveal a remarkable capacity for evolutionary innovation in a simple bacterium, and point to the fact that risk-spreading behaviours may have been the first evolutionary ‘solutions’ to fluctuating environments in the history of life.
Paul B. Rainey (together with Hubertus J. E. Beaumont, Jenna Gallie, Christian Kost)
New Zealand Institute for Advanced Study and Allan Wilson Centre for Molecular Ecology & Evolution, Massey University, Auckland, New Zealand.