Fitness costs of Doc expression are insufficient to stabilize its copy number in Drosophila melanogaster

The stable coexistence of transposable elements (TEs) with their host genome over long periods of time suggests TEs have to impose some deleterious effect upon their host fitness. Three mechanisms have been proposed to account for the deleterious effect caused by TEs: host gene interruptions by TE insertions, chromosomal rearrangements by TE-induced ectopic recombination, and costly TE expression. However, the relative importance of these mechanisms remains controversial. Here, we test specifically if TE expression accounts for the host fitness cost imposed by TE insertions. In the retrotransposon Doc, expression requires binding of the host RNA polymerase to the internal promoter. If expression of Doc elements is deleterious to their host, Doc copies with promoters would be more strongly selected against and would persist in the population for shorter periods of time compared with Docs lacking promoters. We tested this prediction using sequence-specific amplified polymorphism (SSAP) analyses. We compared the populations of these two types of Doc elements in two sets of lines of Drosophila melanogaster: selection-free isogenic lines accumulating new Doc insertions and isogenized isofemale lines sampled from a natural population. We found that (1) there is no difference in the proportion of promoter-bearing and promoter-lacking copies between sets of lines, and (2) the site occupancy distribution of promoter-bearing copies does not skew toward lower frequency compared with that of promoter-lacking copies. Thus, selection against promoter-bearing copies does not appear to be stronger than that of promoter-lacking copies. Our results show that expression is not playing a major role in stabilizing Doc copy numbers.