Hedging bets for survival – how bacteria adapt to changing environments
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Figure 1. Bet-hedging as an evolutionary stable strategy
Population A switches stochastically between two bistable states (purple and green), each of which confers a fitness benefit in the environment of corresponding colour but is maladaptive in the alternative environment. Upon environmental change, population A is primed for rapid growth and recovery. Population A trades off optimal fitness in each environment (by continually generating a proportion of maladaptive types) with the ability to adapt rapidly to environmental change. Population B generates random variation by a mutation. By chance, a small number of individuals may carry mutations enabling growth in the alternative environment. Upon environmental change, population B recovers effective population size more slowly; if environmental fluctuations were to occur rapidly, population B may lack adaptive variants and could face extinction
he natural environments that bacteria encounter, from soil microcosms to the human gut, are diverse and stressful. In order to survive widely varying conditions, bacterial populations must possess mechanisms for rapid adaptation through the generation of new phenotypes. The basis of phenotypic diversification, and the mechanism upon which traditional adaptation studies have focused, is mutation. Mutational mechanisms of adaptation to changing environments, including the evolution of ‘contingency loci’ for locus-specific hypermutation in pathogens, have been wellcharacterized and reviewed extensively (Arber, 2011). But what is new and exciting, is a growing understanding of the role of non-mutational mechanisms, to produce phenotypes that are adaptive, but at the same time readily reversible. ‘Bistability’ or ‘stochastic switching’ is an epigenetic phenomenon whereby cells can switch between two distinct physiological states without mutation or DNA rearrangement (Dubnau & Losick, 2006). The adaptive benefit, is that it enables a population to ‘hedge its bets’ in the environmental change stakes, without a genetically-encoded commitment, maintaining a proportion of individuals maladapted for the current regime but primed and ready for growth should the environment change (Figure 1). ‘Bet-hedging’ is predicted to be the diversification strategy of choice when environments fluctuate unpredictably, whereas regular, predictably changing environments, and those with strong cues, have been shown to promote more deterministic (sensor-based) diversification strategies (Jablonka et al., 1995; and other reviewed in Libby & Rainey, 2011). Bet-hedging as a strategy Until recently, evolutionary thinking about bet-hedging as an adaptive strategy has focused on plants and animals with complex life-cycles (Simons, 2011). The relevance to microorganisms was revealed only with the revolution in the way we think about populations. Where once we thought of populations of bacteria as identical clones, each individual exhibiting the same genotype, patterns of gene expression and, subsequently, phenotype as its neighbours, we now know that heterogeneity is everywhere and that, even fleeting 31