Defective subviral particles modify ecological equilibria and enhance viral coexistence
Cooperation is a main driver of biological complexity at all levels. In the viral world, gene sharing among viral genomes, complementation between genomes or interactions within quasispecies are frequently observed. In this contribution, we explore the advantages that flexible associations between fully fledged viruses and subviral entities, such as virus satellites, might yield. We devise a mathematical model to compare different situations of competition between two viruses and to quantify how the association with a satellite qualitatively modifies dynamical equilibria. The relevant parameter is the invasion fitness of each virus or of the virus-satellite tandem, which in the model depends on the transmission rate of viruses and on their effect on host survival. While in a virus-virus competition one of the viruses becomes eventually extinct, an association with a satellite might change the outcome of the competition to favor the less competitive virus (regardless of whether it is the helper virus or not) or to allow for the stable coexistence of the two viruses and the satellite. We hypothesize that the latter scenario, in particular, constitutes a parsimonious evolutionary pathway towards more stable cooperative associations, such as bipartite viral forms.