Evolution and structure of Tomato spotted wilt virus populations: evidence of extensive reassortment and insights into emergence processes

Tomato spotted wilt virus (TSWV; genus Tospovirus, family Bunyaviridae) genetic diversity was evaluated by sequencing parts of the three RNA genome segments of 224 isolates, mostly from pepper and tomato crops in southern Europe. Eighty-three per cent of the isolates showed consistent clustering into three clades, corresponding to their geographical origin, Spain, France or the USA, for the three RNA segments. In contrast, the remaining 17% of isolates did not belong to the same clade for the three RNA segments and were shown to be reassortants. Among them, eight different reassortment patterns were observed. Further phylogenetic analyses provided insights into the dynamic processes of the worldwide resurgence of TSWV that, since the 1980s, has followed the worldwide dispersal of the western flower thrips (Frankliniella occidentalis) tospovirus vector. For two clades composed essentially of Old World (OW) isolates, tree topology suggested a local re-emergence of indigenous TSWV populations following F. occidentalis introductions, while it could not be excluded that the ancestors of two other OW clades were introduced from North America contemporarily with F. occidentalis. Finally, estimation of the selection intensity that has affected the evolution of the NSs and nucleocapsid proteins encoded by RNA S of TSWV suggests that the former could be involved in the breakdown of resistance conferred by the Tsw gene in pepper.