from Part II - Basic virology and viral gene effects on host cell functions: betaherpesviruses
Published online by Cambridge University Press: 24 December 2009
Introduction
Despite biological divergence, human cytomegalovirus (HCMV, HHV-5), on the one hand, and the three human roseolaviruses (HHV-6A, HHV-6B, HHV-7), on the other hand, share approximately 70 evolutionarily conserved and collinear genes (italics, Table 15.1). Mammalian betaherpesviruses probably have a common ancestor dating back over 50 million years. The genomes of all betaherpesviruses vary in the regions flanking a large conserved block of genes spanning UL23 to UL124 in HCMV. There appear to be two distinct evolutionary lineages represented by cytomegaloviruses and roseolaviruses infecting primates (Chapter 14). These lineages are not preserved in lower mammals such as rodents where divergence within the cytomegaloviruses is striking. Similarity between rodent cytomegaloviruses and either primate cytomegaloviruses or roseolaviruses is about the same, and includes the same set of 70 conserved genes. Thus, despite the fact that this subgroup of herpesviruses is the most highly distributed amongst mammals, evolutionary divergence is dramatic. The betaherpesvirus-common genes are composed of 41 herpesvirus core functions discussed in Chapters 3, 4and 14 plus approximately 30 betaherpesvirus-specific gene products that are involved in replication and cell tropism. Biological similarities include tropism for hematopoietic cells in the myeloid lineage (Kondo et al., 2002a; Sissons et al., 2002). In addition to the 70 conserved replication genes human betaherpesviruses HCMV and HHV-6B also have latent genes with a common structure and genomic location (Kondo et al., 1996, 2002b 2003b) suggesting evolutionary conservation in this important process as well.
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