Wolbachia are the most widespread maternally-transmitted bacteria in the animal kingdom. Their global spread in arthropods and varied impacts on animal physiology, evolution, and vector control are in part due to parasitic drive systems that enhance the fitness of infected females, the transmitting sex of Wolbachia. Male killing is one common drive mechanism wherein the sons of infected females are selectively killed. Despite decades of research, the gene(s) underlying Wolbachia-induced male killing remain unknown. Here using comparative genomic, transgenic, and cytological approaches in fruit flies, we identify a candidate gene in the eukaryotic association module of Wolbachia prophage WO, termed WO-mediated killing (wmk), which transgenically causes male-specific lethality during early embryogenesis and cytological defects typical of the pathology of male killing. The discovery of wmk establishes new hypotheses for the potential role of phage genes in sex-specific lethality, including the control of arthropod pests and vectors.

The P-element is one of the best understood eukaryotic transposable elements. It invaded Drosophila melanogaster populations within a few decades but was thought to be absent from close relatives, including Drosophila simulans. Five decades after the spread in D. melanogaster, we provide evidence that the P-element has also invaded D. simulans. P-elements in D. simulans appear to have been acquired recently from D. melanogaster probably via a single horizontal transfer event. Expression data indicate that the P-element is processed in the germ line of D. simulans, and genomic data show an enrichment of P-element insertions in putative origins of replication, similar to that seen in D. melanogaster. This ongoing spread of the P-element in natural populations provides a unique opportunity to understand the dynamics of transposable element spread and the associated piwi-interacting RNAs defense mechanisms.T he P-element, one of the best understood eukaryotic transposable elements (TEs), was originally discovered as the causal factor for a syndrome of abnormal phenotypes in Drosophila melanogaster. Crosses in which males derived from newly collected strains were mated with females from long established laboratory stocks produced offspring with spontaneous male recombination, high rates of sterility, and malformed gonadsthat is, "hybrid dysgenesis" (1-4). Eventually it was discovered that hybrid dysgenesis was due to the presence of a TE, the P-element (5, 6), which rapidly became the workhorse of Drosophila transgenesis (5, 7-9). Surveys of strains collected over 70 y show that the P-element spread rapidly in natural D. melanogaster populations, between 1950 and 1990 (10-12), and surveys of other Drosophila species revealed that the P-element had been horizontally transferred (HT) from a distantly related species, Drosophila willistoni (13). As there could be a considerable lag time between the initial transmission of a TE and its invasion of worldwide populations, it is unclear exactly when the P-element first entered D. melanogaster. However, the initial HT event likely occurred somewhere between the spread of D. melanogaster populations into the habitat of D. willistoni, around 1800 (14), and the onset of the worldwide invasion of D. melanogaster populations, around 1950 (10). In any case, the P-element had not been found in close relatives of D. melanogaster, including Drosophila simulans (13-18). The failure of the P-element to invade D. simulans is surprising, as both species are cosmopolitan, are mostly sympatric, and share insertions from many TE families via horizontal transfer (19,20). Furthermore, when artificially injected, the P-element can transpose in D. simulans, albeit at a reduced rate (21, 22). Results and DiscussionThe Recent Invasion of D. simulans Populations. Here, we show that the P-element has recently invaded natural D. simulans populations. We sequenced D. simulans collected from South Africa (in 2012) and from Florida (in 2010) as pools (Pool-seq) (23) and analyzed TE insertions in these ...

In a classic example of the invasion of a species by a selfish genetic element, the P-element was horizontally transferred from a distantly related species into Drosophila melanogaster. Despite causing ‘hybrid dysgenesis’, a syndrome of abnormal phenotypes that include sterility, the P-element spread globally in the course of a few decades in D. melanogaster. Until recently, its sister species, including D. simulans, remained P-element free. Here, we find a hybrid dysgenesis-like phenotype in the offspring of crosses between D. simulans strains collected in different years; a survey of 181 strains shows that around 20% of strains induce hybrid dysgenesis. Using genomic and transcriptomic data, we show that this dysgenesis-inducing phenotype is associated with the invasion of the P-element. To characterize this invasion temporally and geographically, we survey 631 D. simulans strains collected on three continents and over 27 years for the presence of the P-element. We find that the D. simulans P-element invasion occurred rapidly and nearly simultaneously in the regions surveyed, with strains containing P-elements being rare in 2006 and common by 2014. Importantly, as evidenced by their resistance to the hybrid dysgenesis phenotype, strains collected from the latter phase of this invasion have adapted to suppress the worst effects of the P-element.

Genes involved in immune defense against pathogens provide some of the most well-known examples of both directional and balancing selection. Antimicrobial peptides (AMPs) are innate immune effector genes, playing a key role in pathogen clearance in many species, including Drosophila. Conflicting lines of evidence have suggested that AMPs may be under directional, balancing, or purifying selection. Here, we use both a linear model and control-gene-based approach to show that balancing selection is an important force shaping AMP diversity in Drosophila. In Drosophila melanogaster, this is most clearly observed in ancestral African populations. Furthermore, the signature of balancing selection is even more striking once background selection has been accounted for. Balancing selection also acts on AMPs in Drosophila mauritiana, an isolated island endemic separated from D. melanogaster by about 4 Myr of evolution. This suggests that balancing selection may be broadly acting to maintain adaptive diversity in Drosophila AMPs, as has been found in other taxa.

1Viruses make up a considerable proportion of the pathogens infecting animals. They can spread 2 rapidly between hosts, and sicken or even kill their hosts to promote their own proliferation. Due Drosophila innubila, a species frequently exposed to a highly pathogenic DNA virus. We 9 investigate the evolution of the immune system and find little evidence for rapid evolution of the 1 0 antiviral RNAi genes, though we do find rapid evolution of several other pathways, suggesting evolution of resistance to DNA viruses differs greatly from that of RNA viruses.