![]() ![]() 1990 Daniels and Chovnick 1993 Gloor et al. Interruption of this gap-repair mechanism leads to ID insertions ( Engels et al. Instead, the copy number increase is achieved by repair of double-stranded breaks-resulting from excision of the TE-using the sister chromatid as template ( Engels et al. DNA TEs multiply by a “cut-and-paste” mechanism which does not inherently lead to an increase in copy numbers. IDs arise at a high frequency during propagation of DNA transposons, such as the P-element. Here, we test the hypothesis whether it may be feasible to reconstruct the invasion route of the P-element based on extant population samples using internal deletions (IDs) of the P-element as markers. Reconstructing the invasion route of the P-element was only possible because of the availability of many fly strains sampled during its invasion from different geographic locations ( Anxolabéhère et al. The P-element first spread in American populations and later invaded European and African populations ( Anxolabéhère et al. It subsequently spread in worldwide D. melanogaster populations between 19 ( Anxolabéhère et al. willistoni in South America ( Daniels and Chovnick 1993). melanogaster likely acquired the P-element by horizontal transfer (HT) from the distantly related D. simulans within the last 100 years ( Kidwell 1983 Kofler et al. For example, the P-element, one of the best understood eukaryotic TEs, invaded Drosophila melanogaster and D. In spite of these elaborate defense mechanisms, TEs are successful invaders that have been found in most prokaryotic and eukaryotic genomes studied so far ( Biémont and Vieira 2006 Wicker et al. Initially, a TE invasion will be stopped by multiple segregating piRNA cluster insertions, but fixed insertions may emerge later on ( Kofler 2019 Kelleher et al. It is assumed that a newly invading TE multiplies within populations until the spread is stopped by TE copies that randomly jumped into piRNA clusters, which triggers the production of piRNAs that silence the TE ( Bergman et al. piRNAs are largely derived from discrete genomic loci that have been termed piRNA clusters ( Brennecke et al. ![]() piRNAs associate with PIWI clade proteins, which act to silence TEs with complementary sequences at the transcriptional as well as the posttranscriptional level ( Sienski et al. In mammals and many invertebrates, the defense system against TEs relies on piRNAs, small RNAs ranging in size from 23 to 29 nt ( Brennecke et al. To control the spread of these deleterious elements, host organisms evolved elaborate defense mechanisms. Most TE insertions likely have negative effects on host-fitness ( Mackay 1989 Houle and Nuzhdin 2004 Mackay et al. Transposable elements (TEs) are short stretches of DNA that selfishly proliferate within host genomes. Our approach might be applicable to other DNA transposons in different host species. Simulations of TE invasions in spatially distributed populations confirm that IDs may allow us to infer invasion routes. Our approach also sheds light on the unknown timing of the invasion in African populations: We suggest that African populations were invaded after American but before European populations. This approach allowed us to reconstruct the invasion route of the P-element with reasonable accuracy. We suggest that inferring invasion routes is possible as: 1) the fraction of IDs increases in successively invaded populations, which also explains the striking differences in the ID content between American and European populations, and 2) successively invaded populations end up with similar sets of IDs. These IDs arise at a high rate when DNA transposons, such as the P-element, are active. Here, we test the hypothesis that the invasion route of the P-element may be reconstructed from extant population samples using internal deletions (IDs) as markers. Inferring this invasion route was made possible by a unique resource available in D. melanogaster: Many strains sampled from different locations over the course of the last century. It invaded American populations first and later spread to the Old World. The P-element, one of the best understood eukaryotic transposable elements, spread in natural Drosophila melanogaster populations in the last century.
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