INTERACTIONS BETWEEN FREQUENCY-DEPENDENT AND VERTICAL TRANSMISSION INHOST-PARASITE SYSTEMS

We investigate host-pathogen dynamics and conditions for coexistence i n two models incorporating frequency-dependent horizontal transmission in conjunction with vertical transmission. The first model combines f requency-dependent and uniparental vertical transmission, while the se cond addresses parasites transmitted vertically via both parents. For the first model, we ask how the addition of vertical transmission chan ges the coexistence criteria for parasites transmitted by a frequency- dependent horizontal route, and show that vertical transmission signif icantly broadens the conditions for parasite invasion. Host-parasite c oexistence is further affected by the form of density-dependent host r egulation. Numerical analyses demonstrate that within a host populatio n, a parasite strain with horizontal frequency-dependent transmission can be driven to extinction by a parasite strain that is additionally transmitted vertically for a wide range of parameters. Although models of asexual host population? predict that vertical transmission alone cannot maintain a parasite over time, analysis of our second model sho ws that vertical transmission via both male and female parents can mai ntain a parasite at a stable equilibrium. These results correspond wit h the frequent co-occurrence of vertical with sexual transmission in n ature and suggest that these transmission modes call lead to host-path ogen coexistence for a wide range of systems involving hosts with high reproductive rates.