Importance des recombinaisons ARN dans l'évolution des lutéovirus

Résumé - Les lutéovirus ont été reconnus comme un groupe de phytovirus par l'ITCV (International Committee on Taxonomy of Viruses) en 1975. Ils ont en commun un certain nombre de propriétés comme la structure isométrique de leurs particules, leur restriction au tissu phloémique de la plante hôte et leur transmission obligatoire par puceron. Ils ont été initialement divisés en 2 sous-groupes, sur la base de leurs relations sérologiques et leurs effets cytopathologiques. Les séquences complètes ou partielles du génome de plusieurs lutéovirus ont été déterminées. En fonction de la région du génome considérée, des relations phylogénétiques différentes sont observées entre les lutéovirus et d'autres phytovirus. Ces observations suggèrent un rôle important des recombinaisons ARN dans l'évolution des lutéovirus. Des modèles d'évolution des lutéovirus par recombinaison ARN ainsi que des propositions sur la taxonomie de cet ensemble de virus sont discutés.

Abstract - Importance of RNA recombinations in the evolution of luteoviruses. Luteoviruses were recognized as a plant virus group by the International Committee on Taxonomy of Viruses (ITCV) in 1975. They have a number of biological properties in common such as their isometric particles, their restriction to phloem tissue and their dependence on aphid vectors for transmission. They were initially separated into 2 distinct subgroups on the basis of their serological relatedness and cytopathological effects. During the last few years, complete genome sequences and organisations have been determined for many luteoviruses and, depending on the region of the genome, various relationships are observed between luteoviruses and other plant viruses. More sequence homologies are observed between luteoviruses, for the 3' half of their genome coding for the structural proteins (capsid, readthrough) and the putative movement protein (figs 1 and 2a). Luteoviruses differ mainly in the evolutionary origin of their RNA-dependent RNA polymerases (RdRp) encoded by the 5' half of the genome (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.