Molecular Evolution of Plant RNA Viruses

Abstract

Viruses with single-stranded-RNA genomes and parasitic on eukaryotic cells form a very diverse group with a wide variation in genome structure and expression (56). This is particularly true for plant RNA viruses, which may have either segmented or nonsegmented genomes that are expressed using a variety of translation strategies (12, 14, 26). Based on these differences, and on other characteristics like host range, vector specificity, and particle morphology, the plant RNA viruses have been classified into a rather large number of groups. The most important of these are summarized in Table 1. During the last few years the complete nucleotide sequences of the genome of AIMVI (6, 10, 11), BMV (2, 3), CMV (50, 51), TMV (18), and CPMV (35, 61) have been elucidated. This has created the possibility of determining possible relationships between these and other eukaryotic RNA viruses in terms of RNA and protein sequence. Such studies have revealed striking sequence homologies among nonstructural proteins of plant and animal RNA viruses, which must reflect (until recently unsuspected) evolutionary rela­ tionships. In this review I summarize the main conclusions from these pro­ tein-sequence comparisons, which throw a totally new light on the evolution and systematics of RNA viruses. In addition, I discuss the possible evolution­ ary mechanisms that may underly the interviral protein homologies detected so far. .