Pianezza, R.; Kofler, R.

The abundance and composition of transposable elements (TEs) varies widely across species, yet the evolutionary forces shaping this diversity remain poorly understood. Using 285 recently published genomes from drosophilid species, we investigated the evolutionary origins of the ~130 TE families present in D. melanogaster and found that 79 were exchanged via horizontal transposon transfer (HTT) with other drosophilids. Most HTT events involved closely related species such as D. sechellia, D. mauritiana, and D. teissieri, although transfers from more distantly related taxa were also observed. Notably, D. melanogaster appears to be a net recipient of HTTs, acquiring about three times as many TEs as it donated. Geographic patterns reveal that most HTTs involved Afrotropical species, reflecting D. melanogaster\'s ancestral range, with fewer involving species from the Neotropics, a region which D. melanogaster invaded only ~200 years ago. Despite colonizing the Nearctic, Australasian, and Palearctic regions between 200-2000 years ago, we found no evidence of HTT with species from those areas. Nonetheless, an analysis of drosophilids from each biogeographic realm shows that HTT is widespread in each realm, with 3-55% of the genome in each species derived from HTT. Strikingly, a considerable portion of the genome is shared among all species inhabiting the same realm --regardless of phylogenetic distance-- indicating that geographic overlap, rather than shared ancestry, is a primary driver of TE composition. These findings highlight biogeography as a major force shaping the TE landscape and underscore the importance of ecological interactions in genome evolution.