Experimental evolution of bet hedging

Bet hedging-stochastic switching between phenotypic states-is a canonical example of an evolutionary adaptation that facilitates persistence in the face of fluctuating environmental conditions. Although bet hedging is found in organisms ranging from bacteria to humans, direct evidence for an adaptive origin of this behaviour is lacking. Here we report the de novo evolution of bet hedging in experimental bacterial populations. Bacteria were subjected to an environment that continually favoured new phenotypic states. Initially, our regime drove the successive evolution of novel phenotypes by mutation and selection; however, in two (of 12) replicates this trend was broken by the evolution of bet-hedging genotypes that persisted because of rapid stochastic phenotype switching. Genome re-sequencing of one of these switching types revealed nine mutations that distinguished it from the ancestor. The final mutation was both necessary and sufficient for rapid phenotype switching; nonetheless, the evolution of bet hedging was contingent upon earlier mutations that altered the relative fitness effect of the final mutation. These findings capture the adaptive evolution of bet hedging in the simplest of organisms, and suggest that risk-spreading strategies may have been among the earliest evolutionary solutions to life in fluctuating environments.

风险分摊策略（bet hedging）——即表型状态间的随机切换——是一类经典的进化适应机制，可帮助生物在波动环境中维持种群存续。尽管从细菌到人类的各类生物中均已发现该策略的存在，但该行为的适应性起源仍缺乏直接实验证据。本研究首次在实验细菌种群中观测到风险分摊策略的从头进化（de novo evolution）。研究中将细菌置于持续筛选新型表型的环境中培养。实验初期，我们的培养体系通过突变与选择逐步推动新型表型的连续进化；然而在12个重复组中的2个里，这一进程被风险分摊型基因型的进化所打断——这类基因型凭借快速的随机表型切换得以稳定存续。对其中一类随机切换型菌株的全基因组重测序结果显示，其与祖先菌株之间存在9处差异突变。最终的突变既是实现快速表型切换的必要且充分条件，但风险分摊策略的进化仍依赖于此前已发生的、改变了该最终突变相对适应度效应的前置突变。本研究的发现揭示了最简单生物类群中风险分摊策略的适应性进化历程，同时表明风险分摊策略或许是波动环境中生命最早进化出的生存策略之一。