The aphid transmits anthriscus yellows virus (AYV) and parsnip yellow fleck virus (PYFV) in a semi-persistent manner and loses them on moulting. It transmits PYFV only when it is also carrying AYV. The particles of AYV have not been seen in preparations or sections from infected plants but those of PYFV are isometric, about 30 nm in diam. in negative stain. Groups of a few to several hundred virus-like particles about 20 to 28 nm in diam. were seen by electron microscopy in the foregut of transmitting AYV or the AYV/PYFV complex but not in aphids fed on healthy plants or on a source of PYFV alone. The particles were surrounded by densely staining material, which was in turn embedded in a matrix of lightly staining material (M-material), overlying a 15 to 20 µm-long portion of the intima lining the ventral wall of the pharynx (anterior portion of the foregut). In this region the intima is thickened and formed into several spike-like protrusions. The M-material also occurred in aphids fed on healthy plants, but less commonly than in those fed on infected plants. In some aphids that were fixed immediately after feeding on AYV-infected plants, additional groups of virus-like particles were found free in the foregut lumen. The particles in the matrix were not found until the aphids had fed on AYV-infected plants long enough to acquire transmissible virus, but they were still present in aphids that had fed for 2 h on 10% sucrose after feeding for 24 h on a source of AYV. In moulting aphids, the matrix, with associated virus-like particles, remained attached to the old pharynx lining. It is thought that the virus-like particles are those of AYV and that the matrix is the specific site of retention within the aphid. No differences were observed between aphids carrying AYV and those carrying both AYV and PYFV, and it is not known whether some of the particles found in insects carrying both viruses are those of PYFV.


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