Plague Gordon R, Dougherty Kevin M, Boodram Krystal S, Boustani Samantha E, Cao Huansheng, Manning Sarah R, McNally Camille C
Louis Calder Center-Biological Field Station, Department of Biological Sciences, Fordham University, Armonk, NY 10504, USA.
Genetica. 2011 Jul;139(7):895-902. doi: 10.1007/s10709-011-9593-x. Epub 2011 Jul 13.
Insertion sequences (ISs) are transposable genetic elements in bacterial genomes. IS elements are common among bacteria but are generally rare within free-living species, probably because of the negative fitness effects they have on their hosts. Conversely, ISs frequently proliferate in intracellular symbionts and pathogens that recently transitioned from a free-living lifestyle. IS elements can profoundly influence the genomic evolution of their bacterial hosts, although it is unknown why they often expand in intracellular bacteria. We designed a laboratory evolution experiment with Escherichia coli K-12 to test the hypotheses that IS elements often expand in intracellular bacteria because of relaxed natural selection due to (1) their generally small effective population sizes (N (e)) and thus enhanced genetic drift, and (2) their nutrient rich environment, which makes many biosynthetic genes unnecessary and thus selectively neutral territory for IS insertion. We propagated 12 populations under four experimental conditions: large N (e) versus small N (e), and nutrient rich medium versus minimal medium. We found that relaxed selection over 4,000 generations was not sufficient to permit IS element expansion in any experimental population, thus leading us to hypothesize that IS expansion in intracellular symbionts may often be spurred by enhanced transposition rates, possibly due to environmental stress, coupled with relaxed natural selection.
插入序列（ISs）是细菌基因组中的可移动遗传元件。IS元件在细菌中很常见，但在自由生活的物种中通常很少见，这可能是因为它们对宿主具有负面的适应性影响。相反，ISs在最近从自由生活方式转变而来的细胞内共生体和病原体中经常增殖。IS元件可以深刻影响其细菌宿主的基因组进化，尽管尚不清楚它们为何经常在细胞内细菌中扩增。我们设计了一项针对大肠杆菌K-12的实验室进化实验，以检验以下假设：IS元件经常在细胞内细菌中扩增是因为（1）它们通常较小的有效种群大小（Ne）导致自然选择放松，从而增强了遗传漂变；（2）它们所处的营养丰富的环境，使得许多生物合成基因变得不必要，从而成为IS插入的选择性中性区域。我们在四种实验条件下培养了12个种群：大Ne与小Ne，以及营养丰富的培养基与基本培养基。我们发现，在4000代中自然选择放松并不足以使IS元件在任何实验种群中扩增，因此我们推测细胞内共生体中的IS扩增可能常常是由转座率提高所推动的，这可能是由于环境压力，再加上自然选择放松。
