Institute for Genome Sciences, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Department of Biology, Institute of Ecology and Evolution, University of Oregon, Eugene, OR 97403-5289, USA.
Genes (Basel). 2021 Jan 20;12(2):127. doi: 10.3390/genes12020127.
Symbioses between animals and bacteria are ubiquitous. To better understand these relationships, it is essential to unravel how bacteria evolve to colonize hosts. Previously, we serially passaged the free-living bacterium, , through the digestive tracts of germ-free larval zebrafish () to uncover the evolutionary changes involved in the initiation of a novel symbiosis with a vertebrate host. After 20 passages, we discovered an adaptive missense mutation in the gene of the pilus operon, which improved host colonization, increased swimming motility, and reduced surface adhesion. In the present study, we determined that this mutation was a loss-of-function mutation and found that it improved zebrafish colonization by augmenting representation in the water column outside larvae through a reduced association with environmental surfaces. Additionally, we found that strains containing the mutation were able to immigrate into host digestive tracts at higher rates per capita. However, mutant and evolved strains exhibited no evidence of a competitive advantage after colonizing hosts. Our results demonstrate that bacterial behaviors outside the host can play a dominant role in facilitating the onset of novel host associations.
动物与细菌之间的共生关系无处不在。为了更好地理解这些关系，揭示细菌如何进化以定殖宿主至关重要。此前，我们通过无菌幼虫斑马鱼的消化道连续传代自由生活细菌，以揭示与脊椎动物宿主建立新共生关系所涉及的进化变化。经过 20 次传代，我们在菌毛操纵子的基因中发现了一个适应性错义突变，该突变提高了宿主的定殖能力，增加了游动能力，减少了表面黏附。在本研究中，我们确定该突变是一个功能丧失突变，并发现它通过减少与环境表面的关联，增加了水柱中菌的表达，从而提高了斑马鱼的定殖能力。此外，我们发现含有突变的菌株能够以更高的人均速率移民到宿主的消化道中。然而，突变体和进化菌株在定殖宿主后没有表现出竞争优势的证据。我们的结果表明，宿主外的细菌行为可以在促进新的宿主关联方面发挥主导作用。
