LOSS OF POTYVIRUS TRANSMISSIBILITY AND HELPER-COMPONENT ACTIVITY CORRELATE WITH NON-RETENTION OF VIRIONS IN APHID STYLETS

The hypothesis that loss of aphid transmissibility of potyvirus mutant s is due to non-retention of virions in the mouthparts was tested by f eeding aphids through membranes on purified virions of aphid transmiss ible (AT or HAT) and non-aphid-transmissible (NAT) tobacco vein mottli ng virus (TVMV) or tobacco etch virus (TEV), in the presence of functi onal [potato virus Y (PW) HC or TVMV HC] or non-functional (PVC HC) he lper component (HC). TVMV virions were detected, by electron microscop ic examination of immunogold-labelled thin sections, in the food canal or cibarium of 57 % of 28 aphids fed on the transmissible combination of TVMV-AT and functional HC, while no virions were found in these st ructures in 25 aphids fed on the nontransmissible combinations: TVMV-N AT and PW HC, or TVMV-AT and PVC HC. Autoradiography of intact stylets allowed the examination of much larger numbers of aphids, fed on I-12 5-labelled TEV; 48 % of 523 aphids fed on the TEV-HAT and PW HC combin ation retained label in the stylets; this correlated well with the per centage transmission in bioassays. In contrast, in non-transmissible c ombinations, label was found in the stylets of 0.77% of 389 aphids fed on TEV-NAT and PVY HC, and 1.35 % of 223 aphids fed on TEV-HAT and PV C HC. No differences were found in the overall amount of label in the bodies of aphids fed on the transmissible and non-transmissible combin ations. There was a strong tendency for virions to be retained in the distal third of the stylets; 56 % of aphids positive for TVMV, and 82 % of those positive for TEV, had label in this area. These data suppor t the concept that virions retained within the stylets are those that are primarily involved in potyvirus transmission.