The first DNA plant virus was discovered in 1968. It was the cauliflower mosaic virus and was found to contain double-stranded (ds) DNA as the genome. Soon after other dsDNA containing viruses were reported. They were all placed in the caulimovirus group. The geminivirus group is the second plant virus group known to contain DNA as the genome but DNA here is single-stranded (ss). The cauliflower mosaic virus (CaMV) is a large icosahedron with a 50 nm diameter, contains 16% double-stranded DNA on the basis of its phosphorus content and quantitative diphenylamine tests, is possibly highly hydrated, and appears to have a hollow center of about 15 or 20 nm in diameter. The DNA of all caulimoviruses have a specific number of discontinuities at specific sites. Their number varies from 2 to 4 in different viruses or even virus isolates but the number of gaps is always fixed in each virus and occur at specific sites.
Southern bean mosaic virus (SBMV) is the archetype of group sobemovirus, which also includes turnip rosette virus (TRoSV), cocksfoot mottle virus, rice yellow mottle virus, and sowbane mosaic virus. SBMV, a pathogen of cultivated legumes, is widespread in the tropical and warm temperate parts of the world. SBMV virions are punned by precipitation with 4 to 8% polyethylene glycol, acid clarification, and differential centrifugation. SBMV virions are spheroidal. By electron microscopy they measure 30 nm in diameter, a value which agrees with the data obtained from X-ray diffraction and neutron scattering studies. SBMV interacts strongly with a lipid composite of phosphatidylcholine, cholesterol, and stearylamine, which forms a uniform 5-nm thick shell around the individual virions. Native SBMV is stable in the presence of protein denaturants and enzymes, indicating a highly integrated and coordinated macromolecular organization.
Banana bunchy top virus (BBTV), a type member of the genus Babuvirus of the family Nanoviridae, is a multicomponent single-stranded DNA virus which infects banana and plantain in most parts of the world where it is grown, leading to serious losses in production. It was reported as early as in 1880 in Fiji and later it spread to all the continents except the Americas. Banana bract mosaic virus (BBrMV), a species in the genus Potyvirus (family Potyviridae), is of recent origin in banana. It causes serious losses to the banana in the Philippines and India. It was authentically reported from the Philippines in 1989 and later from India and Sri Lanka. The genetic diversity and population structure of the viruses is very important to develop strategies to design better management options. In cases of multicomponent viruses like BBTV, reassortment, recombination, and mutations are the main mechanisms of genetic changes and evolution, whereas recombination is more common in BBrMV. RNA viruses are known to evolve very quickly owing to their high replication rate, short generation times, and error-prone RNA polymerases which lead to mutations. These genetic variabilities occurring in viruses will be fixed if they are advantageous for the virus to survive and adapt to various conditions. The selection pressure determines the fixation of mutation, recombination, and reassortment in a population. Many systematic studies have been performed on diversity and population structure of BBTV, whereas work on the BBrMV is limited. Based on the studies, the movement of BBTV appears to be neither consistent with the movement of banana across the globe nor with the human-mediated transcontinental spread, but it is relatively through infrequent movement events over the past 300 years from its centers of diversity in India and Southeast Asia (SEA). In this chapter, the genetic diversity of two banana viruses having DNA and RNA genomes—BBTV and BBrMV, respectively—are dealt with in detail. The causes of genetic diversity such as reassortment, recombination, and mutations are discussed for developing effective future management strategies for viruses.
