COMPARISON OF THE COAT PROTEIN, MOVEMENT PROTEIN AND RNA-POLYMERASE GENE-SEQUENCES OF AUSTRALIAN, CHINESE, AND AMERICAN ISOLATES OF BARLEY YELLOW DWARF VIRUS TRANSMITTED BY RHOPALOISPHUM-PADI

Barley yellow dwarf luteovirus-GPV (BYDV-GPV) is a common problem in C hinese wheat crops but is unrecorded elsewhere. A defining characteris tic of GPV is its capacity to be transmitted efficiently by both Schiz aphis graminum and Rhopaloshiphum padi. This dual aphid species transm ission contrasts with those of BYDV-RPV and BYDV-SGV, globally distrib uted viruses, which are efficiently transmitted only by Rhopaloshiphum padi and Schizaphis graminum respectively. The viral RNA sequences en coding the coat protein (22 K) gene, the movement protein (17 K) gene, the region surrounding the conserved GDD motif of the polymerase gene and the intergenic sequences between these genes were determined for GPV and an Australian isolate of BYDV-RPV (RPVa). In all three genes, the sequences of GPV and RPVa were more similar to those of an America n isolate of BYDV-RPV (RPVu) than to any other luteovirus for which th ere is data available. RPVa and RPVu were very similar, especially the ir coat proteins which had 97% identity at the amino acid level. The c oat protein of GPV had 76% and 78% amino acid identity with RPVa and R PVu respectively. The data suggest that RPVu and RPVa are correctly na med as strains of the same serotype and that GPV is sufficiently diffe rent from either RPV strain to be considered a distinct BYDV type. The coat protein and movement protein genes of GPV are very dissimilar to SGV. The polymerase sequences of RPVu, RPVa and GPV show close affini ties with those of the sobemo-like luteoviruses and little similarity with those of the carmo-like luteoviruses. The sequences of the coat p roteins, movement proteins and the polymerase segments of BYDV serotyp es, other than RPV and GPV, form a cluster that is separate from their counterpart sequences from dicot-infecting luteoviruses. The RPV and GPV isolates consistently fall within a dicot-infecting cluster. This suggests that RPV and GPV evolved from within this group of viruses. S ince these other viruses all infect dicots it seems likely that their common ancestor infected a dicot and that RPV and GPV evolved from a v irus that switched hosts from a dicot to a monocot.