Diversity and composition of viral communities: coinfection of barley and cereal yellow dwarf viruses in California grasslands.

Most species host multiple pathogens, yet field studies rarely examine the processes determining pathogen diversity within a single host or the effects of coinfection on pathogen dynamics in natural systems. Coinfection can affect pathogen transmission and virulence. In turn, coinfection can be regulated within hosts by interactions such as cross-protective immunity or at broader spatial scales via vector distributions. Using a general model, we demonstrate that coinfection by a group of vectored pathogens is highest with abundant generalist vectors and weak cross-protection and coinfection-induced mortality. Using these predictions, we investigate the distribution of five coexisting aphid-vectored, viral pathogens (barley and cereal yellow dwarf luteoviruses and poleroviruses) in a native perennial grass (Elymus glaucus) in both space (700 km) and time (4 years). Observed coinfection rates were much higher than expected at random, suggesting that within-host processes exerted weak effects on within-host pathogen diversity. Covariance among viruses in space and time was highest for viral species sharing a vector. Temporal correlation arose from the synchronous invasion of two viruses transmitted by a shared aphid species. On the basis of our modeling and empirical results, we expect that factors external to individual hosts may affect the coinfection dynamics in other communities hosting vectored pathogens.