Nucleotide sequence of the genome terminal region 3' to the capsid-readthrough cistron were compared for 10 PAV-like isolates of barley yellow dwarf virus (BYDV) from three continents. The sequenced region varied in length from 853 to 864 nucleotides and the extent of sequence homology among the isolates ranged from 84 to 99%. Sequence variations occur mainly in two locations, one in the ORF6 coding region and the other near the genome 3' terminus. Sequence homology grouping reveals three genetically distinct clusters of PAV isolates (A, B and C). Cluster A consists of the Australian isolates, cluster B of one Canadian and three French isolates, and cluster C of the French isolate, RG. Dissimilarities with the corresponding genome-3'- terminal region of the BYDV-MAV serotype were greater than those observed between the PAV isolates alone. Comparison with the sequence of the 3' untranslated region of soybean dwarf virus revealed two stretches of nucleotide similarity, suggesting a common ancestor. Study of the coding ability revealed that the ORF6 is present in all the sequenced PAV isolates but differs in size and deduced amino acids composition. However, the fact that the majority of nucleotide changes are restricted to the third base position of the ORF6-codons suggests that ORF6 codes for a functional protein.

Four PA V -like isolates of barley yellow dwarf virus (BYDV) were identified as causing very severe (RG), severe (2t), moderately severe (3b) and mild symptoms (13t) in barley (Hordeum vulgare) culti var Plaisan t in a growth chamber at 25 days after inoculation. These isola res had different effects on a range of barl ey ge no ty pes. Cu ltivar Vixen, whi ch contains the Y d2 resistance gene, and 80-8 1 BQCB I 0 were

(figs 1 and 2b). Thus, the RdRp of the subgroup 1 luteoviruses (PA V and MA V serotypes of the barley yellow dwarf virus (BYDV) and the soybean dwarf virus) show many similarities with those of the dianthoviruses, carmoviruses and tombusviruses (fig 2b). In contrast, subgroup 2 luteoviruses (potato leafroll virus, beet western yellow virus (BINYV), cucurbit aphid borne virus and the BYDV-RPV), have a sobemo-like polymerase (fig 2b). This dichotomy in sequence homology between the 5' and the 3' halves of their genome suggests an important role of RNA recombination in the evolution of luteoviruses. Mixed infections occur between luteoviruses and other plant viruses and are favourable situations for the generation of new virus entities. The newly generated viruses may have genomes composed of 2 taxonomically different RNA molecules, such as the ST9 strain of the BWYV or the pea enation mosaic enamovirus. They may also have genomes generated by RNA recombination. Subgroup 1 luteoviruses may have been generated by a RNA recombination event between a subgroup 2 luteovirus and a dianthovirus (fig 1). Additional RNA recombination events that occurred during luteovirus evolution could be deduced from sequence comparison of strains of the same luteovirus or different luteoviruses.luteovirus / RNA recombination / evolution / taxonomy / laphid borne * Correspondance et tirés à part

The Japanese barley cultivar, 'Chikurin Ibaraki 1', is partially resistant to the PAV serotype of barley yellow-dwarf virus (BYDV), but its induced mutant line, Ea52, is susceptible. The inheritance of resistance in cv. 'Chikurin Ibaraki 1' to BYDV-PAV was investigated. The F, and F2 plants of crosses of cvs 'Chikurin Ibaraki 1', Ea52, 'Vixen', carrying the Yd2 gene of resistance, and 'Plaisant', a susceptible French cultivar, were tested in growth chamber and field conditions. Isolate RG, against which 'Chikurin Ibaraki 1' is partially resistant in growth chamber and field conditions, and isolate 2t, which overcomes the partial resistance of 'Chikurin Ibaraki 1' in field conditions (Chalhoub et al. 1994) were used. The segregation of F2 plants of crosses between 'Chikurin Ibaraki r and the susceptible cultivars to isolate RG (one resistant to three susceptible) suggests that the resistance of'Chikurin Ibaraki 1' is controlled by a single recessive gene. All 537 Fj plants of'Chikurin Ibaraki 1' X 'Vixen' tested with isolate RG in growth chamber and field conditions were resistant. The F2 plants of this cross were all resistant to isolate 2t in growth chamber conditions but segregated with a ratio of one resistant to three susceptible in field conditions owing to the susceptibility of'Chikurin Ibaraki 1' to this isolate. Results suggest that the resistance gene in 'Chikurin Ibaraki 1' is tightly hnked or allelic with the Yd2 gene in 'Vixen'. However, it differs from this gene in 'Vixen' in that it can be overcome by isolate 2t in field conditions.