Gophna Uri, Kristensen David M, Wolf Yuri I, Popa Ovidiu, Drevet Christine, Koonin Eugene V
1] National Evolutionary Synthesis Center, Durham, NC, USA [2] Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel.
National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD, USA.
ISME J. 2015 Sep;9(9):2021-7. doi: 10.1038/ismej.2015.20. Epub 2015 Feb 24.
The CRISPR (clustered, regularly, interspaced, short, palindromic repeats)-Cas (CRISPR-associated genes) systems of archaea and bacteria provide adaptive immunity against viruses and other selfish elements and are believed to curtail horizontal gene transfer (HGT). Limiting acquisition of new genetic material could be one of the sources of the fitness cost of CRISPR-Cas maintenance and one of the causes of the patchy distribution of CRISPR-Cas among bacteria, and across environments. We sought to test the hypothesis that the activity of CRISPR-Cas in microbes is negatively correlated with the extent of recent HGT. Using three independent measures of HGT, we found no significant dependence between the length of CRISPR arrays, which reflects the activity of the immune system, and the estimated number of recent HGT events. In contrast, we observed a significant negative dependence between the estimated extent of HGT and growth temperature of microbes, which could be explained by the lower genetic diversity in hotter environments. We hypothesize that the relevant events in the evolution of resistance to mobile elements and proclivity for HGT, to which CRISPR-Cas systems seem to substantially contribute, occur on the population scale rather than on the timescale of species evolution.
古细菌和细菌的CRISPR（成簇规律间隔短回文重复序列）-Cas（CRISPR相关基因）系统提供了针对病毒和其他自私元件的适应性免疫，并且被认为会减少水平基因转移（HGT）。限制新遗传物质的获取可能是维持CRISPR-Cas适应性代价的来源之一，也是CRISPR-Cas在细菌间以及不同环境中分布不均的原因之一。我们试图验证这样一个假设，即微生物中CRISPR-Cas的活性与近期水平基因转移的程度呈负相关。通过三种独立的水平基因转移测量方法，我们发现反映免疫系统活性的CRISPR阵列长度与近期水平基因转移事件的估计数量之间没有显著相关性。相反，我们观察到水平基因转移的估计程度与微生物生长温度之间存在显著的负相关，这可以用较热环境中较低的遗传多样性来解释。我们推测，CRISPR-Cas系统似乎在其中发挥了重要作用的针对移动元件的抗性进化和水平基因转移倾向的相关事件，发生在种群尺度而非物种进化的时间尺度上。
