Effects of Wolbachia on transposable element activity largely depend on Drosophila melanogaster host genotype

Transposable elements (TEs) are repetitive DNA sequences capable of changing position in host genomes, thereby causing mutations. TE insertions typically have deleterious effects but they can also be beneficial. Increasing evidence of the contribution of TEs to adaptive evolution further raises interest in understanding what factors impact TE activity. Based on previous studies associating the bacterial endosymbiont Wolbachia to changes in the abundance of piRNAs, a mechanism for TE repression, and to transposition of specific TEs, we hypothesized that Wolbachia infection would interfere with TE activity. We tested this hypothesis by studying expression of 14 TEs in a panel of 25 Drosophila melanogaster host genotypes, naturally infected with Wolbachia and annotated for TE insertions. The host genotypes differed significantly in Wolbachia titers inside individual flies, with broad-sense heritability around 20%, and in the number of TE insertions, which depended greatly on TE identity. By removing Wolbachia from the target host genotypes, we generated a panel of 25 pairs of Wolbachia -positive and Wolbachia -negative lines in which we quantified transcription levels our target TEs. We found variation in TE expression that was dependent on Wolbachia status, TE identity, and host genotype. Comparing between pairs of Wolbachia -positive and Wolbachia -negative flies, we found that Wolbachia removal affected TE expression in 23.7% of the TE-genotype combinations tested, with up to 4.6 times differences in median level of transcript. Our data shows that Wolbachia can impact TE activity in host genomes, underscoring the importance this endosymbiont can have in the generation of genetic novelty in hosts. ### Competing Interest Statement The authors have declared no competing interest.