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Abstract Horizontal transfer of genetic material in eukaryotes has rarely been documented over short evolutionary timescales. Here, we show that two retrotransposons,ShellderandSpoink, invaded the genomes of multiple species of themelanogastersubgroup within the last 50 years. Through horizontal transfer,Spoinkspread inD. melanogasterduring the 1980s, while bothShellderandSpoinkinvadedD. simulansin the 1990s. Possibly following hybridization,D. simulansinfected the island endemic speciesD. mauritiana(Mauritius) andD. sechellia(Seychelles) with both TEs after 1995. In the same approximate time-frame,Shellderalso invadedD. teissieri, a species confined to sub-Saharan Africa. We find that the donors ofShellderandSpoinkare likely AmericanDrosophilaspecies from thewillistoni,cardini, andrepletagroups. Thus, the described cascade of TE invasions could only become feasible afterD. melanogasterandD. simulansextended their distributions into the Americas 200 years ago, likely aided by human activity. Our work reveals that cascades of TE invasions, likely initiated by human-mediated range expansions, could have an impact on the genomic and phenotypic evolution of geographically dispersed species. Within a few decades, TEs could invade many species, including island endemics, with distributions very distant from the donor of the TE. 

Climate warming is expected to shift the distributions of mosquitoes and mosquito-borne diseases, promoting expansions at cool range edges and contractions at warm range edges. However, whether mosquito populations could maintain their warm edges through evolutionary adaptation remains unknown. Here, we investigate the potential for thermal adaptation inAedes sierrensis, a congener of the major disease vector species that experiences large thermal gradients in its native range, by assaying tolerance to prolonged and acute heat exposure, and its genetic basis in a diverse, field-derived population. We found pervasive evidence of heritable genetic variation in mosquito heat tolerance, and phenotypic trade-offs in tolerance to prolonged versus acute heat exposure. Further, we found genomic variation associated with prolonged heat tolerance was clustered in several regions of the genome, suggesting the presence of larger structural variants such as chromosomal inversions. A simple evolutionary model based on our data estimates that the maximum rate of evolutionary adaptation in mosquito heat tolerance will exceed the projected rate of climate warming, implying the potential for mosquitoes to track warming via genetic adaptation. 

Suppression of transposable elements (TEs) is paramount to maintain genomic integrity and organismal fitness. InD.melanogaster, theflamencolocus is a master suppressor of TEs, preventing the mobilization of certain endogenous retrovirus-like TEs from somatic ovarian support cells to the germline. It is transcribed by Pol II as a long (100s of kb), single-stranded, primary transcript, and metabolized into ~24–32 nt Piwi-interacting RNAs (piRNAs) that target active TEs via antisense complementarity.flamencois thought to operate as a trap, owing to its high content of recent horizontally transferred TEs that are enriched in antisense orientation. Using newly-generated long read genome data, which is critical for accurate assembly of repetitive sequences, we find thatflamencohas undergone radical transformations in sequence content and even copy number acrosssimulansclade Drosophilid species.Drosophila simulans flamencohas duplicated and diverged, and neither copy exhibits synteny withD.melanogasterbeyond the core promoter. Moreover,flamencoorganization is highly variable acrossD.simulansindividuals. Next, we find thatD.simulansandD.mauritiana flamencodisplay signatures of a dual-stranded cluster, with ping-pong signals in the testis and/or embryo. This is accompanied by increased copy numbers of germline TEs, consistent with these regions operating as functional dual-stranded clusters. Overall, the physical and functional diversity offlamencoorthologs is testament to the extremely dynamic consequences of TE arms races on genome organization, not only amongst highly related species, but even amongst individuals.