Abstract
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.