Forty-year-old beet ringspot virus isolate: tests for potato true seed transmission, genome sequencing, recombination analysis and phylogenetic placement

Beet ringspot virus (BRSV) isolate IF came from a 1980 bait test on a Scottish soil sample. Between 1982 and 1984, it was used to study possible BRSV seed transmission in potato (Solanum tuberosum). To encourage flowering, potato scions were graft-inoculated onto tomato plants and these grafted plants were inoculated with IF. Flowers on BRSV-infected potato shoots were self-pollinated and their seeds collected. BRSV was found readily in pollen from these potato flowers, but never detected in any seedlings that grew from their seeds. Thus, although BRSV was present in pollen, no evidence of potato true seed transmission was obtained. In 1985, IF was preserved by freeze-drying and kept thereafter in a historical virus isolate collection. In 2020, it was subjected to high throughput sequencing and a complete genomic sequence was obtained. No recombination was detected in the IF sequence but its RNA1 and RNA2 were major parents of three, and a minor parent of one, recombinant BRSV isolates, respectively. Phylogenetic analysis of the P1 and P2 components of the coat protein gene encoded on its RNA2 placed the isolate IF sequence within the BRSV lineage. However, too few other BRSV sequences from different countries were present to provide information on its geographical associations.