Role of trade-off between sexual and vertical routes for evolution of pathogen transmission

Many pathogens that are predominantly sexually transmitted can also be transmitted vertically. On the other hand, nonbeneficial pathogens that are predominantly vertically transmitted appear to be rare to absent. Many infections also exist that are only transmitted sexually. Using an empirically suggested trade-off between the horizontal and vertical transmission modes, we develop and analyze a model to study evolutionary dynamics of sterilizing, sexually transmitted infections which can also be transmitted vertically. We assume several flexible forms of the trade-off and ask under which conditions evolution in nonbeneficial pathogens favors vertical transmission, sexual transmission, or a mixture of the two. The evolutionary analysis of our model reveals a rich spectrum of evolutionary outcomes. In particular, evolution of pure sexual, pure vertical, and mixed transmission is possible, in addition to a frequent occurrence of evolutionary suicide. These outcomes can also arise via evolutionary branching and be combined in several evolutionary bistability regimes. We show that the shape of the trade-off between the two transmission modes significantly affects pathogen evolution. In particular, while vertical transmission dominates for concave and sigmoid trade-offs, sexual transmission is most commonly observed under convex trade-offs. Our analysis can shed more light on when an infection adopts a particular evolutionary behavior, and which region of the parameter space is realistic, so something about the evolutionary process itself.