Molecular evolution of the γ–Herpesvirinae

Abstract

Genomic sequences available for members of the γ– Herpesvirinae allow analysis of many aspects of the group's evolution. This paper examines four topics: (i) the phylogeny of the group; (ii) the histories of γ–herpesvirus–specific genes; (iii) genomic variation of human herpesvirus 8 (HHV–8); and (iv) the relationship between Epstein–Barr virus types 1 and 2 (EBV–1 and EBV–2). A phylogenetic tree based on eight conserved genes has been constructed for eight γ–herpesviruses and extended to 14 species with smaller gene sets. This gave a generally robust assignment of evolutionary relationships, with the exception of murine herpesvirus 4 (MHV–4), which could not be placed unambiguously on the tree and which has evidently experienced an unusually high rate of genomic change. The γ–herpesviruses possess a variable complement of genes with cellular homologues. In the clearest cases these virus genes were shown to have originated from host genome lineages in the distant past. HHV–8 possesses at its left genomic terminus a highly diverse gene ( K1 ) and at its right terminus a gene ( K15 ) having two diverged alleles. It was proposed that the high diversity of K1 results from a positive selection on K1 and a hitchhiking effect that reduces diversity elsewhere in the genome. EBV–1 and EBV–2 differ in their alleles of the EBNA–2 , EBNA–3A , EBNA–3B and EBNA–3C genes. It was suggested that EBV–1 and EBV–2 may recombine in mixed infections so that their sequences outside these genes remain homogeneous. Models for genesis of the types, by recombination between diverged parents or by local divergence from a single lineage, both present difficulties.