Host shifts result in parallel genetic changes when viruses evolve in closely related species.

Host shifts, where a pathogen invades and establishes in a new host species, are a major source of emerging infectious diseases. They frequently occur between related host species and often rely on the pathogen evolving adaptations that increase their fitness in the novel host species. To investigate genetic changes in novel hosts, we experimentally evolved replicate lineages of an RNA virus (Drosophila C Virus) in 19 different species of Drosophilidae and deep sequenced the viral genomes. We found a strong pattern of parallel evolution, where viral lineages from the same host were genetically more similar to each other than to lineages from other host species. When we compared viruses that had evolved in different host species, we found that parallel genetic changes were more likely to occur if the two host species were closely related. This suggests that when a virus adapts to one host it might also become better adapted to closely related host species. This may explain in part why host shifts tend to occur between related species, and may mean that when a new pathogen appears in a given species, closely related species may become vulnerable to the new disease.

Parallel and costly changes to cellular immunity underlie the evolution of parasitoid resistance in three Drosophila species.

A priority for biomedical research is to understand the causes of variation in susceptibility to infection. To investigate genetic variation in a model system, we used flies collected from single populations of three different species of Drosophila and artificially selected them for resistance to the parasitoid wasp Leptopilina boulardi, and found that survival rates increased 3 to 30 fold within 6 generations. Resistance in all three species involves a large increase in the number of the circulating hemocytes that kill parasitoids. However, the different species achieve this in different ways, with D. melanogaster moving sessile hemocytes into circulation while the other species simply produce more cells. Therefore, the convergent evolution of the immune phenotype has different developmental bases. These changes are costly, as resistant populations of all three species had greatly reduced larval survival. In all three species resistance is only costly when food is in short supply, and resistance was rapidly lost from D. melanogaster populations when food is restricted. Furthermore, evolving resistance to L. boulardi resulted in cross-resistance against other parasitoids. Therefore, whether a population evolves resistance will depend on ecological conditions including food availability and the presence of different parasite species.

Vertically transmitted rhabdoviruses are found across three insect families and have dynamic interactions with their hosts.

A small number of free-living viruses have been found to be obligately vertically transmitted, but it remains uncertain how widespread vertically transmitted viruses are and how quickly they can spread through host populations. Recent metagenomic studies have found several insects to be infected with sigma viruses (Rhabdoviridae). Here, we report that sigma viruses that infect Mediterranean fruit flies (Ceratitis capitata), Drosophila immigrans, and speckled wood butterflies (Pararge aegeria) are all vertically transmitted. We find patterns of vertical transmission that are consistent with those seen in Drosophila sigma viruses, with high rates of maternal transmission, and lower rates of paternal transmission. This mode of transmission allows them to spread rapidly in populations, and using viral sequence data we found the viruses in D. immigrans and C. capitata had both recently swept through host populations. The viruses were common in nature, with mean prevalences of 12% in C. capitata, 38% in D. immigrans and 74% in P. aegeria We conclude that vertically transmitted rhabdoviruses may be widespread in a broad range of insect taxa, and that these viruses can have dynamic interactions with their hosts.

The causes and consequences of changes in virulence following pathogen host shifts.

Emerging infectious diseases are often the result of a host shift, where the pathogen originates from a different host species. Virulence--the harm a pathogen does to its host-can be extremely high following a host shift (for example Ebola, HIV, and SARs), while other host shifts may go undetected as they cause few symptoms in the new host. Here we examine how virulence varies across host species by carrying out a large cross infection experiment using 48 species of Drosophilidae and an RNA virus. Host shifts resulted in dramatic variation in virulence, with benign infections in some species and rapid death in others. The change in virulence was highly predictable from the host phylogeny, with hosts clustering together in distinct clades displaying high or low virulence. High levels of virulence are associated with high viral loads, and this may determine the transmission rate of the virus.

A Flexible, Pooled CRISPR Library for Drug Development Screens.

Functional genomic screening with CRISPR has provided a powerful and precise new way to interrogate the phenotypic consequences of gene manipulation in high-throughput, unbiased analyses. However, some experimental paradigms prove especially challenging and require carefully and appropriately adapted screening approaches. In particular, negative selection (or sensitivity) screening, often the most experimentally desirable modality of screening, has remained a challenge in drug discovery. Here we assess whether our new, modular genome-wide pooled CRISPR library can improve negative selection CRISPR screening and add utility throughout the drug development pipeline. Our pooled library is split into three parts, allowing it to be scaled to accommodate the experimental challenges encountered during drug development, such as target identification using unlimited cell numbers compared with target identification studies for cell populations where cell numbers are limiting. To test our new library, we chose to look for drug-gene interactions using a well-described small molecule inhibitor targeting poly(ADP-ribose) polymerase 1 (PARP1), and in particular to identify genes which sensitise cells to this drug. We simulate hit identification and performance using each library partition and support these findings through orthogonal drug combination cell panel screening. We also compare our data with a recently published CRISPR sensitivity dataset obtained using the same PARP1 inhibitor. Overall, our data indicate that generating a comprehensive CRISPR knockout screening library where the number of guides can be scaled to suit the biological question being addressed allows a library to have multiple uses throughout the drug development pipeline, and that initial validation of hits can be achieved through high-throughput cell panels screens where clinical grade chemical or biological matter exist.

Intracellular survival of Staphylococcus aureus during persistent infection in the insect Tenebrio molitor.

Survival of bacteria within host cells and tissues presents a challenge to the immune systems of higher organisms. Escape from phagocytic immune cells compounds this issue, as immune cells become potential vehicles for pathogen dissemination. However, the duration of persistence within phagocytes and its contribution to pathogen load has yet to be determined. We investigate the immunological significance of intracellular persistence within the insect model Tenebrio molitor, assessing the extent, duration and location of bacterial recovery during a persistent infection. Relative abundance of Staphylococcus aureus in both intracellular and extracellular fractions was determined over 21 days, and live S. aureus were successfully recovered from both the hemolymph and within phagocytic immune cells across the entire time course. The proportion of bacteria recovered from within phagocytes also increased over time. Our results show that to accurately estimate pathogen load it is vital to account for bacteria persisting within immune cells.