Evolution of trophic transmission in parasites: the need to reach a matingplace?

Although numerous parasite species have a simple life cycle (SLC) and compl ete their life cycle in one host, there are other parasite species that exp loit several host species successively. From an evolutionary perspective, u nderstanding the mix of adaptive and contingent forces shaping the transiti on from an ancestral single-host state to such a complex life cycle (CLC) h as proved an intriguing challenge. In this paper, we propose a new hypothes is, which states that CLCs involving trophic transmission (i.e. transmissio n to a predator) evolved because they are an efficient way for parasites to meet a sexual partner, assuming that selective benefits are associated wit h cross-fertilization. Predators that eat a lot of prey in a relatively sho rt time interval act to concentrate isolated parasites. We use an optimalit y model to develop our hypothesis and discuss further directions of potenti al research.