Meeting Summary: Viruses are ubiquitous parasites: they infect from other viruses and unicellular organisms up to large multicellular ones. In many cases, viruses peacefully coexist with their hosts but in most cases, they induce diseases. Hence, viruses are essential agents in populations and ecosystems’ dynamics. Researchers from different disciplines study viruses at specific levels of biological organization, but the ultimate host of any virus is an individual cell. It is inside the cell wherein the virus expresses its genes, kidnaps the cellular machinery to produce its own proteins and uses these proteins to interact with a number of host cell factors in order to ensure reproduction and spread to neighboring cells, from there to colonize tissues and organs, then be transmitted to other susceptible hosts, to spread in the host populations and, ultimately, to reach diverse ecosystems.
At each level (cellular, organismal, population, and ecosystems), different selective pressures operate upon viral populations. For example, whereas at the cellular and within-organism levels fast replication may provide short-term advantages, at the population level such strategy may jeopardize long-term between host transmission. Recent advances in sequencing techniques allow characterizing single-cell infection dynamics, making possible to get a concise picture of the evolutionary dynamics of virus spreading among cells within tissues and organs. For the first, time, we may be able of weighting the contribution of mutation/recombination, drift and selection in shaping viral populations at this very fundamental level. Furthermore, these techniques have been used to characterize the dynamics of viral populations at the entire individual host level and have allowed tracing down chains of between-individual transmissions, thus disentangling the roles of different evolutionary forces at the host-population level. Finally, they now allow exploring the virome of entire ecosystems, hunting for viruses in their reservoirs and understanding which factors may perturb virus’ stability.
Despite all these advances, we still miss a complete theory linking all these levels and that allows to predict the behavior of viral populations at higher levels by understanding basic dynamics at the lower levels and back. The aim of this Working Group (WG), that follows up from the SFI Workshop “Integrating Multi-Scale Virus Evolution” we organized last autumn, is to tackle this problem. The WG will be attended by a subgroup of the participants in the Workshop and will include experimental molecular virologists and theoreticians to further discuss some of the most challenging ideas that pop up last year. In particular, we will now focus in the nature and drivers of phase transitions linking the different levels at which viral populations can be studied.