QUESTIONS ON THE EVOLUTION OF PRIMATE T-LYMPHOTROPIC VIRUSES RAISED BY MOLECULAR AND EPIDEMIOLOGIC STUDIES OF DIVERGENT STRAINS

In human and simian T-lymphotropic viruses (HTLV and STLV), collective ly refer-red to as primate T-lymphotropic viruses (PTLV), four distinc t clades can be distinguished: PTLV-I, PTLV-II, and the newly discover ed divergent STLVs isolated from hamadryas baboons and from bonobos (p ygmy chimpanzees). The hamadryas STLV is clearly distinct from types I and II. in terms both of sequence divergence and of genomic structure , and would qualify as a separate type, provisionally called PTLV-L. T he bonobo STLV is closer to, although clearly distinct from, PTLV-II, at present known only in humans. While PTLV-II, PTLV-I, and the bonobo STLV appear presently to be species specific, PTLV-I has spread durin g its evolution through repealed interspecies transmissions between pr imates and is now present in many species of Old World monkeys and ape s and in humans. The human subtypes of PTLV-I arose from at least thre e acquisitions from separate simian reservoirs.