Borgs are giant linear extrachromosomal elements (ECEs) of methane-oxidizing Methanoperedens archaea whose evolutionary origin and ecosystem distribution remain unknown. Here we detected 240 Borgs in diverse saline, soil, and freshwater ecosystems. 27 encode methyl-coenzyme M reductases central to methane metabolism, eight have full rRNA operons related to those of Methanoperedens, and some contain up to four ribosomal proteins. We also identified 323 mini-Borgs whose proteome content and gene phylogenies classify as a distinct ECE type. Based on 105 complete and near-complete genomes, Borgs and mini-Borgs share core genes in conserved order, indicating common ancestry. Phylogenetic and diversity analyses suggest that Borgs evolved via gene acquisition into a backbone inherited from mini-Borgs. This evolutionary trajectory mirrors those proposed for giant viruses of both eukaryotes and bacteria.
Shi, L.-D., Penev, P., Kolody, B. C., Miloslavich, L., Lei, S., Sachdeva, R., Rasmussen, A. N., Tolar, B. B., Francis, C. A., Probst, A. J., Vazquez-Campos, X., Payne, T. E., Jiang, Z., Li, J., Wang, C., He, Z., Ni, J., Hug, L. A., Banfield, J.
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