Evolution of North American PVY(NTN) strain Tu 660 from local PVY(N) by mutation rather than recombination

A North American (NA) isolate of tobacco veinal necrotic strain of Potato virus Y (PVY(N)) (N-Jg) and a NA isolate of potato tuber necrotic strain of Potato virus Y (PVY(NTN) (Tu 660) were tested for their phenotypes by inoculation to potato plants of three potato cultivars. Upon inoculation with Tu 660, tubers of the cultivars 'Norchip' and 'Ranger Russet' developed potato tuber necrotic ringspot disease (PTNRD) but not the tubers of 'Russet Burbank'. N-Jg failed to induce PTNRD in the tested cultivars. The genomic RNAs of both strains were completely sequenced and analysed. High homology (98% and 99% identity on nucleotide and polyprotein, respectively) was found between Tu 660 and N-Jg. When polyproteins were compared with other isolates, high identity was observed between Tu 660 and an European (Eu) PVY(N)-605 (98%) and with an Eu-PVY(NTN)-H (96%). However, when individual mature proteins were compared, much lower identities (86.5-94%) were found between Tu 660 and PVY9(NTN))-H compared to 98-99.5% between Tu 660 and PVY(N)-605 in the P3, 6K1 and CI regions. Further sequence analysis indicated that the PVY(NTN)-H is a hybrid molecule of the genomic RNA recombination of PVY(O) and Eu-PVY(N) as shown by Glais et al. (Arch Virol 147, 363-378), whereas NA-PVY(NTN) Tu 660 is free of recombination points. Phylogenetic analysis confirmed this observation, and suggested that, in light of high homology, the Tu 660 might have evolved from NA-PVY(N) by mutations rather than the genome recombinations. The non-recombinant nature of NA-PVY(NTN) Tu 660 strongly suggests that the recombinant structure of genome is not a necessary prerequisite for the PTNRD phenotype.