Correspondence

The genetic basis for virulence in potyviruses is largely unknown. Earlier studies showed that there are two host types of Turnip mosaic virus (TuMV); the Brassica/Raphanus (BR)-host type infects both Brassica and Raphanus systemically, whereas the Brassica (B)-host type infects Brassica fully and systemically, but not Raphanus. The genetic basis of this difference has been explored by using the progeny of an infectious clone, p35Tunos; this clone is derived from the UK1 isolate, which is of the B-host type, but rarely infects Raphanus systemically and then only asymptomatically. Two inocula from one such infection were adapted to Raphanus by passaging, during which the infectivity and concentration of the virions of successive infections increased. The variant genomes in the samples, 16 in total, were sequenced fully. Four of the 39 nucleotide substitutions that were detected among the Raphanus sativus-adapted variant genomes were probably crucial for adaptation, as they were found in several variants with independent passage histories. These four were found in the protein 1 (P1), protein 3 (P3), cylindrical inclusion protein (CI) and genome-liked viral protein (VPg) genes. One of four ‘parallel evolution ’ substitutions, 3430GRA, resulted in a 1100MetRIle amino acid change in the C terminus of P3. It seems likely that this site is important in the initial stages of adaptation to R. sativus. Other independent substitutions were mostly found in the P3, CI and VPg genes