This thesis constitutes a comprehensive analysis of the molecular and biological characteristics of three potyviruses (genus Potyvirus, family Potyviridae) naturally occurring in cultivated and wild species of family Solanaceae: Peru tomato virus (PTV), Potato virus V (PVV) and Wild potato mosaic virus (WPMV). In addition, the studies presented in this thesis focus on the genetic variability of isolates of PTV and PVV and on the role of the Potato virus A (PVA) 6K2 protein as a host-specific determinant of virus movement and symptom induction. Determination of the complete genomic sequences of PVV, PTV and WPMV demonstrated that these viruses are typical members of the genus Potyvirus. Furthermore, comparison of the polyprotein amino acid sequences and the biological and serological characteristics of these three viruses supported their current taxonomic position as independent species of the genus Potyvirus. The nucleotide sequences of the P1 protein, coat protein and non-translated regions of European and South American PVV isolates were determined and compared. Results showed limited genetic variability among the European isolates, in contrast to the higher variability found among the South American isolates of PVV. Phylogenetic analysis defined two distinct clusters, grouping the European isolates together but placing two South American isolates to a different group; these two isolates of PVV did not induce a hypersensitive response in an Nv gene-carrying potato cultivar in contrast to the European PVV isolates. Thus, European and South American PVV isolates belong to different strain groups. In addition, great genetic variability was detected among PTV isolates. Analysis of phylogenetic relationships among PTV, PVV, WPMV and other members of the genus Potyvirus commonly found infecting solanaceous crop plants showed that PTV, PVV and WPMV are the most closely related viruses which together with Potato virus Y, Pepper mottle virus, Pepper severe mosaic virus and Pepper yellow mosaic virus constitute a group distinguishable from other potyviruses. Thus, members of this group seem to share a common ancestor. The 6K2 protein of PVA was modified by deleting various portions or by introducing six histidine residues (6xHis) into various positions of this protein. These modifications disturbed functions required for viral infection in Nicotiana tabacum. Furthermore, inoculation of the insertion constructs to N. benthamiana plants did not result in systemic infection with the exception of one plant. This plant lacked typical PVA symptoms but had virus titers similar to the plants infected with the wild type virus: a single point mutation (Gly2 ® Cys2) in the 6xHis-containing 6K2 had restored the viral movement functions. However, partial deletion of the 6xHis-tag to gain the original size of the 6K2 protein was required to restore the induction of symptoms in N. benthamiana and to enable systemic infection of N. tabacum. Taken together, these results indicate the 6K2 is a host-specific determinant for long-distance movement and exemplify that mutations that arise during viral propagation represent a mechanisms by which viruses can evolve and adapt to different hosts
