This fifth volume in the series The Plant Viruses, dealing with viruses
with bipartite genomes, completes the coverage of viruses with isometric
parti- cles and genomes consisting of single-stranded, positive-sense
RNA: viruses that have tripartite and monopartite genomes of this kind
were dealt with in Volumes 1 and 3, respectively. How close are the
affinities among the viruses within the groupings distinguished in this
way? All those with tripartite genomes are considered to be sufficiently
closely related to be included in the family Bromoviridae, whereas the
monopartite-genome viruses covered in Volume 3 clearly are a much more
diverse collection. Affinities among the viruses with bipartite genomes
are considered in Chapter 1 of this volume, along with the possible
origins, advantages, and disadvantages of these ge- nomes. The
conclusion reached from this assessment is that the bipartite- genome
viruses fall into four categories, those within each category having
closer affinities with viruses not included in this book than with
viruses in the other categories. No evidence was found that possession
of a bipartite genome gives a virus overwhelming advantages over viruses
of other sorts. More probably, any advantages are largely balanced by
disadvantages, and bipartite genomes may be best considered simply as an
alternative design for the hereditary material of a virus.