Transposon removal Reveals Their Adaptive Fitness Contribution.

Transposable Elements (TE) are molecular parasites that persist in their host genome by generating new copies to outpace natural selection. TE exert a large influence on host genome evolution, in some cases providing adaptive changes. Here we measure the fitness effect of the TE insertions in the fission yeast Schizosaccharomyces pombe type strain by removing all insertions of its only native TE family, the LTR retrotransposon Tf2. We show that Tf2 elements provide a positive fitness contribution to its host. Tf2 ablation results in changes to the regulation of a mitochondrial gene and, consistently, the fitness effect is sensitive to growth conditions. We propose that Tf2 influence host fitness in a directed manner by dynamically rewiring the transcriptional response to metabolic stress.