Coinfection dynamics of two pathogens of barley with different transmission modes

Background/Question/Methods

Infections of a single host by multiple pathogens are common in nature but relatively poorly studied in comparison to single infections.  Pathogens within the same host can compete with or facilitate each other.  Competition can be direct, or mediated through the immune system of the host.  Instances of synergistic mortality, where coinfection increases the probability of host death, have also been observed.  Pathogens with different transmission modes may be expected to interact strongly within hosts, because the growth of one pathogen can affect the ability of the other to transmit to new hosts.  We examined the coinfection dynamics between barley yellow dwarf virus (BYDV), a pathogen that is transmitted horizontally between unrelated hosts by aphids, and barley stripe mosaic virus (BSMV), a pathogen that is transmitted vertically through seeds as well as horizontally through contact.  In this study, barley hosts were either uninoculated, inoculated with BYDV or BSMV only, or inoculated with one of 3 coinfection treatments (inoculated one week apart or concurrently). Estimates of viral concentration (titer) were taken weekly using enzyme-linked immunosorbent assay.  Vegetative biomass, reproductive biomass, and number of seeds were quantified for each host.

Results/Conclusions

Concurrent coinfection with both viruses resulted in significant reduction in host biomass and reproduction.  The effects of other coinfection treatments did not differ from the effect of BSMV alone.  Single inoculations with BYDV did not cause a significant change in host growth compared to controls.  Surprisingly, we found few ecologically significant interactions between BYDV and BSMV within barley.  There were no consistent differences between the viral concentration estimates for single infections or multiple infections over time.  Infection with one virus did not affect the ability of the other virus to invade a host plant either one week later or concurrently.  Multiple infections increased neither the chance of one virus being competitively excluded from a host plant nor the probability of host mortality.  Transmission experiments suggest no strong effects of coinfection on horizontal transmission of either virus, although vertical transmission assessments are ongoing.  These results suggest coinfection may have strong impacts on hosts without displaying strong competitive effects.