Clonal variations and virus regulation by aphids in transmission of a French PAV-type isolate of barley yellow dwarf virus

Twenty-one aphid clones of Rhopalosiphum padi and 21 clones of Sitobion avenae were evaluated for vector efficiency in transmitting a French PAV-type isolate (PAV-RG) of barley yellow dwarf virus (BYDV). All aphid clones transmitted the isolate, but vector efficiency was variable. The most efficient R. padi clone transmitted PAV-RG about twice as often as the least efficient one, Rp-CH (93 versus 38%). The most efficient S. avenae clone, however, transmitted PAV-RG eight times more often than the least efficient one, Sa-RS (76 versus 8%). All aphid clones acquired virus as determined by triple antibody sandwich-enzyme-linked immunosorbent assay (TAS-ELISA), but the amount of virus acquired differed among the clones. After a 5-day inoculation access period on healthy plants, virus titer in some aphid clones was not detectable by TAS-ELISA in samples of 10 aphids, but immunocapture-polymerase chain reaction (IC-PCR) could detect the virus in the extract of single aphids of all the clones. In most cases, a rapid reduction of PAV-RG titer in the aphids was associated with lower transmission efficiency. In a serial transmission test of 11 days, clonal variations in vector efficiency were consistently observed. After a 5-day transfer, vector efficiency of the six clones tested declined. Vector efficiency was significantly correlated with the level of virus titer in the aphids. Following the serial transfer, decline of virus titer in feeding aphids was triphasic, with an initial decrease occurring rapidly after the first transfer, then decreasing slowly. A second rapid reduction in virus titer often occurred after 7 days of transfer. In the serial transmission test, all three R. padi clones tested transmitted and retained virus until the last transfer at 11 days. The Sa-Chat1 and Sa-V clones of S. avenae successively transmitted and retained PAV-RG for 11 and 9 days, respectively. The Sa-RS clone transmitted PAV-RG until the 9-day transfer, but retained the virus for 11 days. Thus, the clonal variations in vector efficiency were not ascribed to poor ability to acquire the virus, but were associated with a possible transmission barrier of virions, as well as a more rapid reduction of virus titer in aphids.