RESUMEN
Transposable elements (TEs) are genomic parasites that proliferate within host genomes, and which can also invade new species. The P-element, a DNA-based TE, recently invaded two Drosophila species: Drosophila melanogaster in the 20th century, and D. simulans in the 21st. In both species, lines collected before the invasion are susceptible to "hybrid dysgenesis", a syndrome of abnormal phenotypes apparently due to P-element-inflicted DNA damage. In D. melanogaster, lines collected after the invasion have evolved a maternally acting mechanism that suppresses hybrid dysgenesis, with extensive work showing that PIWI-interacting small RNAs (piRNAs) are a key factor in this suppression. Most of these studies use lines collected many generations after the initial P-element invasion. Here, we study D. simulans collected early, as well as late in the P-element invasion of this species. Like D. melanogaster, D. simulans from late in the invasion show strong resistance to hybrid dysgenesis and abundant P-element-derived piRNAs. Lines collected early in the invasion, however, show substantial variation in how much they suffer from hybrid dysgenesis, with some lines highly resistant. Surprisingly, although, these resistant lines do not show high levels of cognate maternal P-element piRNAs; in these lines, it may be that other mechanisms suppress hybrid dysgenesis.