Division of Biological Sciences, University of Montana, Missoula, MT, USA.
Nat Ecol Evol. 2018 Mar;2(3):510-519. doi: 10.1038/s41559-017-0435-9. Epub 2018 Jan 15.
The conventional view of bacterial adaptation emphasizes the importance of rapidly evolved changes that are highly repeatable in response to similar environments and subject to loss in the absence of selection. Consequently, genetic variation is not expected to persist over long time scales for these organisms. Here, we show that a geographically widespread gene content polymorphism has surprisingly been maintained for tens of millions of years of diversification of the multicellular cyanobacterium Fischerella thermalis. The polymorphism affects gas permeability of the heterocyst-the oxygen-sensitive, nitrogen-fixing cell produced by these bacteria-and spatial variation in temperature favours alternative alleles due to thermodynamic effects on both heterocyst function and organism fitness at physiological temperature extremes. Whether or not ancient balancing selection plays a generally important role in the maintenance of microbial diversity remains to be investigated.
传统的细菌适应观点强调快速进化变化的重要性，这些变化在类似环境中高度可重复，并且在没有选择的情况下容易丢失。因此，预计这些生物体的遗传变异不会在长时间尺度上持续存在。在这里，我们表明，一种具有广泛地理分布的基因内容多态性，在多细胞蓝藻 Fischerella thermalis 的数千万年的多样化过程中，一直令人惊讶地得以维持。这种多态性影响异形胞的气体通透性——这些细菌产生的对氧气敏感、固氮的细胞——并且温度的空间变化由于对异形胞功能和生物体在生理极端温度下的适应度的热力学效应，有利于替代等位基因。古老的平衡选择是否在维持微生物多样性方面发挥着普遍重要的作用，还有待研究。
