Plasmid acquisition is an important mechanism of rapid adaptation and niche expansion in prokaryotes. Positive selection for plasmid-coded functions is a major driver of plasmid evolution, while plasmids that do not confer a selective advantage are considered costly and expected to go extinct. Yet, plasmids are ubiquitous in nature, and their persistence remains an evolutionary paradox. Here, we demonstrate that non-mobile plasmids persist over evolutionary timescales without selection for the plasmid function. Evolving a minimal plasmid encoding for antibiotics resistance in
Escherichia coli
, we discover that plasmid stability emerges in the absence of antibiotics and that plasmid loss is determined by transcription-replication conflicts. We further find that environmental conditions modulate these conflicts and plasmid persistence. Silencing the transcription of the resistance gene results in stable plasmids that become fixed in the population. Evolution of plasmid stability under non-selective conditions provides an evolutionary explanation for the ubiquity of plasmids in nature.
From protists to humans, all animals and plants are inhabited by microbial organisms. There is an increasing appreciation that these resident microbes influence the fitness of their plant and animal hosts, ultimately forming a metaorganism consisting of a uni- or multicellular host and a community of associated microorganisms. Research on host-microbe interactions has become an emerging cross-disciplinary field. In both vertebrates and invertebrates a complex microbiome confers immunological, metabolic and behavioural benefits; conversely, its disturbance can contribute to the development of disease states. However, the molecular and cellular mechanisms controlling the interactions within a metaorganism are poorly understood and many key interactions between the associated organisms remain unknown. In this perspective article, we outline some of the issues in interspecies interactions and in particular address the question of how metaorganisms react and adapt to inputs from extreme environments such as deserts, the intertidal zone, oligothrophic seas, and hydrothermal vents.
SummaryAcquisition of foreign DNA by horizontal gene transfer is seen as a major source of genetic diversity in prokaryotes. However, strongly divergent DNA is not genomically integrated by homologous recombination and would depend on illegitimate recombination (IR) events which are rare. We show that, by two mechanisms, during natural transformation of Acinetobacter baylyi two IR events can integrate DNA segments. One mechanism is double illegitimate recombination (DIR) acting in the absence of any homology (frequency: 7 ¥ 10 -13 per cell). It occurs about 10 10 -fold less frequent than homologous transformation. The other mechanism is homologyfacilitated double illegitimate recombination (HFDIR) being about 440-fold more frequent (3 ¥ 10 -10 per cell) than DIR. HFDIR depends on a homologous sequence located between the IR sites and on recA + . In HFDIR two IR events act on the same donor DNA molecule as shown by the joint inheritance of molecular DNA tags. While the IR events in HFDIR occurred at microhomologies, in DIR microhomologies were not used. The HFDIR phenomenon indicates that a temporal recA-dependent association of donor DNA at a homology in recipient DNA may facilitate two IR events on the 5Ј and 3Ј heterologous parts of the transforming DNA molecule.
Plasmids are extra-chromosomal genetic elements whose ecology and evolution depend on their genetic repertoire and interaction with the host. We review the events that lead to transitions between plasmid lifestyle modes - invasion, host range, plasmid persistence and adaptation - from a plasmid perspective. Plasmid lifestyle is determined by various traits, including mobility, stability and indispensability that vary in their magnitude. Transitions between the plasmid lifestyles, invasion, host range, plasmid persistence and adpatation, are caused by the interplay between plasmid traits and host biology. Mobility and indispensability are important in plasmid ecology, whereas plasmid stability is more relevant for long-term plasmid evolution. In transitioning into additional chromosomes plasmids loose their independence and enter the host lineage. Though plasmids are confined to their hosts, their evolution may be independent of prokaryotic chromosomes.
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