Convergent Evolution in Intracellular Elements: Plasmids as Model Endosymbionts

• Published in: Trends in microbiology (Regular ed.) 2018-09, Vol.26 (9), p.755-768
• Main Authors: Dietel, Anne-Kathrin, Kaltenpoth, Martin, Kost, Christian
• Format: Article
• Language: English
• Subjects: Bacteria; Bacteria - genetics; Bacteria - metabolism; bacteria–insect interactions; Chromosome Segregation; convergent evolution; Cytoplasm; DNA Transposable Elements; Endosymbionts; endosymbiosis; Eukaryota; Eukaryotes; Evolution; Evolution, Molecular; Gene Transfer, Horizontal; genome evolution; Host Microbial Interactions - genetics; Host Microbial Interactions - physiology; model system; Mutation; Plasmids; Plasmids - genetics; Plasmids - metabolism; Prokaryotes; Symbiosis; Symbiosis - genetics; Symbiosis - physiology
• ISSN: 0966-842X; 1878-4380
• DOI: 10.1016/j.tim.2018.03.004

Endosymbionts are organisms that live inside the cells of other species. This lifestyle is ubiquitous across the tree of life and is featured by unicellular eukaryotes, prokaryotes, and by extrachromosomal genetic elements such as plasmids. Given that all of these elements dwell in the cytoplasm of their host cell, they should be subject to similar selection pressures. Here we show that strikingly similar features have evolved in both bacterial endosymbionts and plasmids. Since host and endosymbiont are often metabolically tightly intertwined, they are difficult to disentangle experimentally. We propose that using plasmids as tractable model systems can help to solve this problem, thus allowing fundamental questions to be experimentally addressed about the ecology and evolution of endosymbiotic interactions. All host-dependent elements (i.e., endosymbiotic bacteria and plasmids) unavoidably impose a metabolic burden on their hosts and face the risk of being lost during host reproduction. Adaptation to the intracellular environment has resulted in the convergent evolution of strikingly similar features in both intracellular bacteria and plasmids. Both endosymbiotic bacteria and plasmids evolve towards a reduction of metabolic costs and/or and increase their persistence by active segregation systems, manipulation of host reproduction, or horizontal transfer to other host lineages. The genetic amenability and experimental tractability of bacteria–plasmid interactions makes them powerful tools to experimentally address fundamental questions regarding the ecology and evolution of endosymbiotic interactions.