Molecular evolution under increasing transposable element burden in Drosophila: A speed limit on the evolutionary arms race

Surprisingly, we find neither of the first two hypotheses to be true. Specifically, we found that genome-wide levels of purifying selection, measured by the ratio of non-synonymous to synonymous substitution rates (ω), were greater in species with greater TE abundance. In addition, species with greater TE abundance have greater levels of purifying selection in the piRNA machinery. In contrast, it appears that increasing TE abundance has primarily driven adaptation in the piRNA machinery by increasing codon bias.These results indicate that within the Drosophila genus, a historically reduced strength of selection relative to drift is unlikely to explain patterns of increased TE success across species. Other factors, such as ecological exposure, are likely to contribute to variation in TE abundances within species. Furthermore, constraints on the piRNA machinery may temper the evolutionary arms race that would drive increasing rates of evolution at the amino acid level. In the face of these constraints, selection may act primarily by improving the translational efficiency of the machinery of genome defense through efficient codon usage.In sexual species, genetic parasites such as transposable elements (TEs) can proliferate to the detriment of the host [1]. Natural selection is widely considered the dominant force limiting TE proliferation [2-4]. The selective forces limiting TE abundance are thought to act against three primary consequences of TE proliferation- gene mutation by TE insertion, chromosomal rearrangement caused by ectopic recombination among dispersed copies, and the energetic burden imposed on the host arising from the costs of replication, transcription and translation of TE copies. TE insertion alleles often segregate at low frequencies in populations, consistent with natural selection limiting their increase. However, studies in recent years have shown that different modes of RNA silencing also play an important role in constraining TE proliferation [5