Within-host competition between barley yellow dwarf-PAV and -PAS

• Quantitative nucleotide sequencing was used to monitor relative growth of BYDV-PAV and -PAS in doubly infected plants.
• In most double inoculation treatments the ratio of PAV:PAS was 70:30 by 45 days post primary virus inoculation.
• In most double inoculation treatments the transmission rate by single aphids was greater for PAV than PAS.
• Transmission frequency was greater for PAS if PAS was given a 15-day head start.
• This in a BYDV epidemic, PAV transmission will be favored from most, but not all, doubly infected plants.

The PAV and PAS species of barley yellow dwarf virus (Luteoviridae) share hosts, vectors, and have sympatric distributions. To better understand how competition between species influences virus growth within the host, transmission rate between hosts, and ultimately virus population structure two experiments were conducted. The first experiment varied the order of PAV and PAS inoculation and the time interval between the first and second inoculation. Relative virus concentration was measured at 8, 20, 33, and 45 days after primary virus inoculation (dpi). Regardless of the order of inoculation or the length of time between inoculations, PAV dominated the virus population by 33 dpi (PAV concentration ranged from 55% to 89%). The second experiment measured the rate of vector transmission from single and multiple infections. From single infections, the transmission rate was 67% for PAV and 60% for PAS. PAV had significantly greater odds of transmission for all competition treatments—except if PAS was given a 15-day head start before inoculation with PAV. In the latter treatment, PAS was transmitted with a greater frequency than PAV, but the difference was not statistically significant. Our data show persistent co-infection between PAV and PAS, but PAV is more likely to be transmitted from mixed infections. Thus, within-host interactions between PAV and PAS create conditions that promote both the competitive exclusion of PAS, as well as co-existence between species and the maintenance of genetic diversity in the host community.