Abstract
Mobile genetic elements (MGEs), such as transposons and insertion sequences, propagate within bacterial genomes, but persistence times in individual lineages are short. For long-term survival, MGEs must continuously invade new hosts by horizontal transfer. Theoretically, MGEs that persist for millions of years in single lineages, and are thus subject to vertical inheritance, should not exist. Here we make the case for an exception — a class of MGE termed REPIN. REPINs are non-autonomous MGEs whose duplication depends on non-jumping RAYT transposases. Comparisons of REPINs and typical MGEs show that replication rates of REPINs are orders of magnitude lower, REPIN population size fluctuations correlate with changes in available genome space, REPIN conservation depends on RAYT function, and REPIN diversity accumulates within host lineages. From these data it follows that REPINs persist for millions of years within single host lineages. Such long-term persistence is expected to generate conflicts arising from the diverging effects of selection acting simultaneously on REPINs and host genomes. Evidence of conflict comes from analyses of REPIN abundance and diversity in two distantly related bacterial species. Taken together, our data lead to the conclusion that REPINs are ancient Darwinian replicators that have evolved enduring, beneficial relationships, with eubacterial genomes.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We significantly changed the title, abstract and introduction. We made minor changes to the rest of the manuscript.