Characterization of transposon-free strains of fission yeast

Transposable Elements are molecular parasites that persist in their host genome by generating new copies to outpace natural selection. Here we measure the parameters governing the copy number dynamics of the fission yeast Tf2 retrotransposons, using experimental and natural populations and a strain where all Tf2 copies are removed. In the absence of selection, Tf2 elements are deleted by mitotic recombination at rates that far exceed transposition. However natural population genomes display active and persistent Tf2 colonies. We show that Tf2 elements provide a fitness contribution to their host by dynamically rewiring the transcriptional response to metabolic stress. Therefore, Tf2 elements exhibit a mutualistic rather than parasitic behavior toward their host.