Deletion Mutants of Double-Stranded RNA Genetic Elements Found in Plants and Fungi

Genetic elements composed of double-stranded (ds) RNA are found in plants and fungi in a variety of forms. Plant-infecting reoviruses contain 10 or 12 discrete segments of dsRNA packaged as a set within one virus particle. ^{1 , 3} The genome segments of a second group of plant dsRNA viruses, the cryptic or temperate viruses, ^{4 , 5} vary with respect to number (two to five) and size (~1.6 to 0.8 × 10⁶ daltons). ⁴ It is unclear whether the dsRNAs of these viruses are encapsidated in the same or in different particles. ⁵ The presence of dsRNAs in plants is often associated with infection by RNA viruses that possess single-stranded RNA genomes. These molecules, which represent replicative intermediate forms of viral genomes, are convenient diagnostic markers for such infections. ^{6 , 7} High-molecular-weight (greater than 5 kilobase-pairs) dsRNAs of ill-defined origin and function have been detected in a number of apparently healthy plants, e.g., the ~14-kilobase-pair dsRNAs associated with Phaseolus vulgaris cv. Black Turtle soup. ^{8 , 9} As early as 1974, viruses or virus-like particles (VLPs) had been detected in isolates of over 60 species of fungi. ¹⁰ A common feature of these mycoviruses is a dsRNA genome ^{10 , 11} (whose replication is reviewed by Breunn in Volume I). In certain fungal species, the associated dsRNA genetic elements resemble plasmids more than they resemble viral genomes and can influence the virulence of their fungal hosts. ^{11 12}