Introduction

Brandes and Wetter (1959) first showed that potato virus Y together with 13 other viruses had filamentous flexuous virions about 750 nm in length, some of which were serologically related to one another. They proposed that these viruses formed a natural group, and this was subsequently named the potyvirus group by Harrison et al. (1971).

Large numbers of viruses have been added to the group, and it now contains at least 200 distinct species, or more than a fifth of all known plant viruses. It is the largest and most rapidly growing of the 50 or so families (or groups) of viruses that infect plants and is now named the Potyviridae comprising, at present, three genera, the potyviruses, rymoviruses and bymoviruses (Ward & Shukla, 1991; Barnett, 1992).

Poty viruses are found in all climatic zones, especially the tropics (Hollings & Brunt, 1981). They cause diseases in almost all crop plants and in many uncultivated species; in 1974 they were reported to infect 1112 plant species of 369 genera in 53 families (Edwardson, 1974), and that list too has grown. Their economic impact was also highlighted in a recent survey of important viruses with filamentous virions as 73% of those named were potyviruses (Milne, 1988). Thus it is important to try to understand how the potyviruses have evolved because it might give clues as to why they are such successful viruses.

Most potyviruses belong to the genus Potyvirus, and are transmitted in nature by aphids and through seeds of infected plants. These two properties, together with the diversity of crops they infect, assure the continuous presence of potyviruses in nature throughout the year (Hollings & Brunt, 1981).