Unbiased, Cell-free Profiling of Single Influenza Genomes at High-throughput.

The segmented structure of the Influenza A virus (IAV) genome facilitates reassortment, segment exchange during co-infection. When divergent strains mix across human, agricultural, and wildlife reservoirs novel strains are generated, which has been the source of pandemics. Due to the limited throughput and infection-based assays, IAV reassortment studies has been limited to permissive reassortment. We have developed DE-flowSVP to achieve extremely high throughput, direct profiling of as many as 10 5 IAV particles in a single-day experiment and enabled quantitative profiling of reassortment propensity between divergent strains for the first time. By profiling reassortants between two naturally circulating low-pathogenicity avian IAVs, we confirmed that molecular incompatibility yields strong preference toward within-strain mixing. Surprisingly, we revealed that two-to-three particle aggregation contributed primarily to genome mixing (75-99%), suggesting that aggregation mediated by sialic acid binding by viral surface proteins provides a secondary pathway to genome mixing while avoiding the co-packaging fitness cost. We showed that genome mixing is sensitively dependent on co-infection timing, relative segment abundances, and viral surface-protein background. DE-flowSVP enables large-scale survey of reassortment potential among the broad diversity of IAV strains informing pandemic strain emergence.