title: Molecular evolution under increasing transposable element burden in Drosophila: a speed limit on the evolutionary arms race.
abstract: BACKGROUND
  Genome architecture is profoundly influenced by transposable elements (TEs), and natural selection against their harmful effects is a critical factor limiting their spread. Genome defense by the piRNA silencing pathway also plays a crucial role in limiting TE proliferation. How these two forces jointly determine TE abundance is not well understood. To shed light on the nature of factors that predict TE success, we test three distinct hypotheses in the Drosophila genus. First, we determine whether TE abundance and relaxed genome-wide purifying selection on protein sequences are positively correlated. This serves to test the hypothesis that variation in TE abundance in the Drosophila genus can be explained by the strength of natural selection, relative to drift, acting in parallel against mildly deleterious non-synonymous mutations. Second, we test whether increasing TE abundance is correlated with an increased rate of amino-acid evolution in genes encoding the piRNA machinery, as might be predicted by an evolutionary arms race model. Third, we test whether increasing TE abundance is correlated with greater codon bias in genes of the piRNA machinery. This is predicted if increasing TE abundance selects for increased efficiency in the machinery of genome defense.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas 66045, USA.
authors.forename: Dean M
authors.initials: DM
authors.lastname: Castillo
authors.forename: Joshua Chang
authors.initials: JC
authors.lastname: Mell
authors.forename: Kimberly S
authors.initials: KS
authors.lastname: Box
authors.forename: Justin P
authors.initials: JP
authors.lastname: Blumenstiel
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D003062:Codon / Q000235:genetics; D019295:Computational Biology; D004251:DNA Transposable Elements / Q000235:genetics*; D004330:Drosophila / Q000235:genetics*; D019143:Evolution, Molecular; D040961:Genetic Drift; D014644:Genetic Variation; D016013:Likelihood Functions; D008957:Models, Genetic; D009154:Mutation / Q000235:genetics; D010802:Phylogeny; D034741:RNA, Small Interfering / Q000378:metabolism; D012044:Regression Analysis; D012641:Selection, Genetic; D013045:Species Specificity
chemical_list: D003062:Codon; D004251:DNA Transposable Elements; D034741:RNA, Small Interfering