The descent of human herpesvirus 8

Analysis of conserved herpesvirus genes shows that human herpesvirus 8 (HHV 8) belongs to a clade of primate herpesviruses within the gamma 2 sublineag e. Overall, this clade has developed by cospeciation with host species. Sev enteen genes of HHV8 and its nearest relatives lack simple homologues in ot her herpesviruses, and eight of these have known mammalian homologues, pres umed to be the so sources of the HHV8 versions. Comparative analyses yield further insights into the source or time of acquisition for several of thes e genes. All were acquired by the HHV8 lineage in the very distant past. Tw o unusual processes are manifest in the contemporary evolution of HHV8. Fir st, the K1 gene is uniquely under going a process of extensive and positive ly selected substitution, of unknown significance. Secondly, the K15 gene h as been substituted in some lineages by a distant homologue, presumably by recombination with an unknown herpesvirus. While available data are otherwi se compatible with a cospeciational relationship between HHV8 and the human species, they do not exclude relatively recent transfer to humans from ano ther primate host.