Potato Virus Y Biological Strain Group Y^D: Hypersensitive Resistance Genes Elicited and Phylogenetic Placement

Potato virus Y (PVY) disrupts healthy seed potato production and causes tuber yield and quality losses globally. Its subdivisions consist of strain groups defined by potato hypersensitive resistance (HR) genes and whether necrosis occurs in tobacco, and phylogroups defined by sequencing. When PVY isolate PP was inoculated to potato cultivar differentials with HR genes, the HR phenotype pattern obtained resembled that caused by strain group PVY^D isolate KIP1. A complete genome of isolate PP was obtained by high-throughput sequencing. After removal of its short terminal recombinant segment, it was subjected to phylogenetic analysis together with 30 complete nonrecombinant PVY genomes. It fitted within the same minor phylogroup PVY^O3 subclade as KIP1. Putative HR gene Nd was proposed previously to explain the unique HR phenotype pattern that developed when differential cultivars were inoculated with PVY^D. However, an alternative explanation was that PVY^D elicits HR with HR genes Nc and Ny instead. To establish which gene(s) it elicits, isolates KIP1 and PP were inoculated to F₁ potato seedlings from (i) crossing 'Kipfler' and 'White Rose' with 'Ruby Lou' and (ii) self-pollinated 'Desiree' and 'Ruby Lou', where 'Kipfler' is susceptible (S) but 'White Rose', 'Desiree', and 'Ruby Lou' develop HR. With both isolates, the HR:S segregation ratios obtained fitted 5:1 for 'Kipfler' × 'Ruby Lou', 11:1 for 'White Rose' × 'Ruby Lou', and 3:1 for 'Desiree'. Those for 'Ruby Lou' were 68:1 (isolate PP) and 52:0 (isolate KIP1). Because potato is tetraploid, these ratios suggest PVY^D elicits HR with Ny from 'Ruby Lou' (duplex condition) and 'Desiree' (simplex condition) and Nc from 'White Rose' (simplex condition) but provide no evidence that Nd exists. Therefore, our differential cultivar inoculations and inheritance studies highlight that PVY^D isolates elicit an HR phenotype in potato cultivars with either of two HR genes Nc or Ny, so putative gene Nd can be discounted. Moreover, phylogenetic analysis placed isolate PP within the same minor phylogroup PVY^O3 subclade as KIP1, which constitutes the most basal divergence within overall major phylogroup PVY^O.