Switches in the mode of transmission select for or against a poorly aphid transmissible strain of potato virus Y with reduced helper component and virus accumulation

A poorly aphid-transmissible potato virus Y (PVY-PAT) variant emerged after several cycles of mechanical transmission of an initially aphid-transmissible (AT) isolate. Sequence analysis of the N-terminal region of the helper component-proteinase (HC-Pro) gene revealed a Lys to Glu change at a position previously found to abolish the HC-Pro aphid transmission activity in several potyviruses. Two cycles of aphid transmission allowed the virus population to evolve towards an AT form (PVY-ATnew) where a Glu to Lys change was observed. PW-PAT produced lower amounts of coat protein and the accumulation of its HC-Pro in infected plants decreased from 7 to 28 days post-inoculation, as compared to PVY-ATnew. RT-PCR and restriction analysis showed that the two virus populations co-existed in the PVY-AT isolate and that the AT form was counter-selected during mechanical transmission. These observations suggest that the Lys to Glu substitution leads to decreased stability of HC-Pro resulting in poor transmissions by aphids, and further strengthen the idea that HC-Pro is involved in the accumulation of potyvirus in infected plants.