Cytological alterations induced by BYV in infected cells of Chenopodium foliosum: BYV-type vesicles in a leaf parenchyma cell (bar represents 1 Ám). Inset presents details of clusters of c.100-200 nm diameter 'pseudo-spherical' vesicles containing irregular fibrous strands (bar represents 200 nm). The strands resemble structures proven to be dsRNA in similarly structured vesicles induced by other positive sense ssRNA-containing viruses (e.g. members of the genus Tombusvirus). The vesicles originate from budding into a membrane-surrounded compartment of unknown origin at the points indicated by arrows.