MOLECULAR PHYLOGENY AND DISSEMINATION OF HUMAN T-CELL LYMPHOTROPIC VIRUS TYPE-I VIEWED WITHIN THE CONTEXT OF PRIMATE EVOLUTION AND HUMAN MIGRATION

.A renewed interest in the emergence and evolution of the primate T-ce ll lymphotropic viruses has followed the discovery of genetically dist inct variants of human T-cell lymphotropic virus type I (HTLV-I) in Me lanesia and Australia. Phylogenetic trees based on selected regions of the gag, pol, env and pX genes of HTLV-I from widely separated geogra phic regions and of simian T-cell lymphotropic virus type I (STLV-I) f rom African and Asian catarrhines, constructed using the neighbor-join ing and maximum parsimony methods, indicated that the Australo-Melanes ian and cosmopolitan strains of HTLV-I have evolved along separate geo graphically dependent lineages, with African STLV-I strains clustering with cosmopolitan HTLV-I strains and Asian STLV-I strains diverging f rom the common ancestral virus before the Australo-Melanesian HTLVI st rains. When viewed within the context of non-human primate evolution a nd human occupation of Australia and Melanesia, the rate of molecular change of HTLV-I and STLV-I is approximately 2.5-6.8 x 10(-7) substitu tions per site per year. Overall, the sequence and phylogenetic analys es are in accord with interspecies virus transmission among non-human primates, as well as between non-human primates and humans, with indep endent evolution of HTLV-I in Southeast Asia and in Africa, and with d issemination of HTLV-I by forced or voluntary movements of human popul ations. The immunosuppressive and T-cell activation properties of HTLV -I places at added risk these Australian Aboriginal and Melanesian pop ulations, some of which are in imminent threat of infection with human immunodeficiency virus type 1.