Molecular evolution of the gamma-Herpesvirinae

Genomic sequences available for members of the gamma -Herpesvirinae allow a nalysis of man) aspects of the group's evolution. This paper examines four topics: (i) the phylogen of the group; (ii) tile histories of gamma -herpes virus-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 c onstructed for eight gamma -herpesviruses and extended to 14 species with s maller gene sets. This gave a generally robust assignment of evolutionary r elationships, with tile exception of murine herpesvirus 4 (MHV-4), which co uld not be placed unambiguously on the tree and which has evidently experie nced an unusually high rate of genomic change. The gamma -herpesviruses pos sess a variable complement of genes with cellular homologues. In the cleare st cases these virus genes were shown to have originated from host genome l ineages in the distant past. HHV-8 possesses at its left genomic terminus a highly diverse gene (KI) and at its right terminus;l gene (K15) having two diverged alleles. It was proposed that the high diversity of XI results fr om a positive selection on K1 and a hitchhiking effect that reduces diversi ty 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 sequence:; outside t hese genes remain homogeneous. Models for genesis of the types, by recombin ation between diverged parents or by local divergence from a single lineage , both present difficulties.