Competition and coexistence among three barley yellow dwarf viruses, and impacts on host plant communities
Most species host a diverse array of interacting pathogens and parasites, which together can impact the fitness of their hosts. Interference within hosts and vectors, competition for host resources, induced host immunity, and other mechanisms shape the composition of parasite communities within shared host populations. Here, we address two questions: (i) How is parasite diversity maintained? (ii) How does parasite diversity impact host populations? As a model system, we use the barley and cereal yellow dwarf viruses (B/CYDVs) in California grasslands, a group of aphid-vectored viruses that can infect hundreds of grass species. We use a mathematical model of transmission for three viruses and their two shared aphid vectors, parameterized with extensive empirical data from the literature and greenhouse experiments, to understand the factors that regulate virus diversity.
We find that the two closely related virus species, PAV and MAV, compete most strongly within their shared aphids and plants. Competition is symmetric within plants, but asymmetric within aphids, with MAV dominating over PAV. By contrast, a more distantly related virus, RPV, does not compete strongly with the other two. Incorporating the empirical data into the model shows that virus diversity and community composition depend strongly on aphid density and plant life history. Because of their strong competition, both PAV and MAV can competitively exclude the other across a broad range of aphid densities. By contrast, RPV can coexist with the other viruses across most aphid densities. Both the ability to simultaneously infect (“coinfect”) plants or aphids and fitness tradeoffs between viruses are necessary for virus coexistence—removing these processes from the model leads to competitive exclusion or priority effects. Because interactions with other viruses shape the abundance of the most virulent virus, PAV, and because coinfection can lead to more severe disease than single infection, the composition of the virus community impacts host population dynamics. We discuss implications of virus community composition for shaping host plant communities in invaded California grasslands, where viruses are transmitted between exotic annual grasses and native perennial grasses.