Abstract
In contrast to obligate bacteria, facultative symbiotic bacteria are mainly characterized by genome enlargement. However, the underlying relationship of this feature with adaptations to various habitats remains unclear. In this study, we used the global genome data of Nostoc strains, including 10 novel genomes sequenced in this study and 26 genomes available from public databases, and analyzed their evolutionary history. The evolutionary boundary of the real clade of Nostoc species was identified and was found to be consistent with the results of polyphasic taxonomy. The initial ancestral species of Nostoc was demonstrated to be consistent with a facultative symbiotic population. Further analyses revealed that Nostoc strains tended to shift from facultative symbiosis to a free-living one, along with an increase in genome sizes during the dispersal of each exterior branch. Intracellular symbiosis was proved to be essentially related to Nostoc evolution, and the adaptation of its members to free-living environments was coupled with a large preference for gene acquisition involved in gene repair and recombination. These findings provided unique evidence of genomic mechanisms by which homologous microbes adapt to distinct life manners and external environments.
Highlights
Microbes play a major role in driving various ecological processes, and they contribute crucial functionalities to ecosystems [1,2]
Modern cyanobacterial holds the Previous argued that the genomic evolution oftaxonomy
Modern cyanobacterial taxonomy holds t lineage, even for prokaryotic species. Such an understanding of the taxonomy of any viewpoint that the verified from a single common cyanobacteriaall species is difficult species to achieveshould because come of misidentification
Summary
Microbes play a major role in driving various ecological processes, and they contribute crucial functionalities to ecosystems [1,2]. Recent studies have suggested that these microbes prefer a large genome size to meet metabolic requirements and adapt to a more complicated life cycle [12] This inconsistent adaptation of microbial genomes—shifting from obligate symbiosis to free-living—warrants further investigation to gain insights into their underlying evolutionary mechanisms. A particular trait of symbiotic Nostoc species is their facultative life cycle: they can undergo alternative life cycles involving both free-living cycles and forming an efficient symbiont with their hosts [20]. These genetically and morphologically similar Nostoc species that have symbiotic and free-living populations substantially differ in terms of genome size [21]. Genome amplification in the free-living population was found to be accompanied by the enrichment of functional genes related to gene repair and recombination during the dispersal of exterior branches
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