PHYLOGENETIC ASSOCIATIONS OF HUMAN AND SIMIAN T-CELL LEUKEMIA LYMPHOTROPIC VIRUS TYPE-I STRAINS - EVIDENCE FOR INTERSPECIES TRANSMISSION/

Homologous env sequences from 17 human T-leukemia/lymphotropic virus t ype I (HTLV-I) strains from throughout the world and from 25 simian T- leukemia/lymphotropic virus type I (STLV-I) strains from 12 simian spe cies in Asia and Africa were analyzed in a phylogenetic context as an approach to resolving the natural history of these related retroviruse s. STLV-I exhibited greater overall sequence variation between strains (1 to 18% compared with 0 to 9% for HTLV-I), supporting the simian or igin of the modern viruses in all species. Three HTLV-I phylogenetic c lusters or clades (cosmopolitan, Zaire, and Melanesia) were resolved w ith phenetic, parsimony, and likelihood analytical procedures. Seven p hylogenetic clusters of STLV-I were resolved with the most primitive ( deeply rooted) divergence involving several STLV-I strains from Asian primate species. Combined analysis of HTLV-I and STLV-I revealed that neither STLV-I clusters nor HTLV-I clusters recapitulated host species specificity; rather, multiple clades from the same species were close r to clades from other species than to each other. We interpret these evolutionary associations as support for the occurrence of multiple di screte interspecies transmissions of ancestral viruses, between primat e species (including human) that led to recognizable phylogenetic clad es that persist in modern species. Geographic concordance of divergent host species that harbor closely related viruses reinforces that phys ical feasibility for hypothesized interspecies virus transmission in t he past and in the present.