Mutations in Turnip mosaic virus genomes that have adapted to Raphanus sativus

Abstract

The genetic basis for virulence in potyviruses is largely unknown. Earlier studies showed that there are two host types ofTurnip mosaic virus(TuMV); theBrassica/Raphanus(BR)-host type infects bothBrassicaandRaphanussystemically, whereas theBrassica(B)-host type infectsBrassicafully and systemically, but notRaphanus. 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 infectsRaphanussystemically and then only asymptomatically. Two inocula from one such infection were adapted toRaphanusby 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 theRaphanus 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,3430G→A, resulted in a1100Met→Ile amino acid change in the C terminus of P3. It seems likely that this site is important in the initial stages of adaptation toR. sativus. Other independent substitutions were mostly found in the P3, CI and VPg genes.