April 9th, 2018
Deltaretroviruses are a genus of retroviruses that have so far only been identified in primates and cattle. They include the Human T-lymphotrophic viruses (HTLVs), which are currently estimated to infect 15-20 million people worldwide.
There are four distinct HTLVs that infect humans. HTLV-1 and HTLV-2 are closely related but have very different geographic distributions. HTLV-1 can cause a rare type of lymphoma called adult T-cell leukaemia/lymphoma (ATLL), while HTLV-2 has been associated with sporadic cases of neurological disorders. HTLV-3 and HTLV-4 have only been found in isolated populations in Central Africa, and are not known to be associated with disease.
The global distribution of HTLV-1 and HTLV-2 seems to reflect to historical patterns of human migration (see figure above), indicating that these viruses have an ancestral presence in the human population.
Among the lines of evidence supporting this are HTLV-1 DNA sequences recovered from 'mummies' excavated in the Atamaca desert Due to the extremely arid conditions that occur in this region, the bodies of people buried there over 1,000 years ago have remained relatively well preserved, such that DNA could be recovered (even in the early 2000s, when techniques for doing this were considerably less advanced than those available today).
The evolutionary relationships between (i) HTLV-1 DNA sequences recovered from an Atacamanian mummy, and (ii) HTLV-1 isolates obtained from infected populations in present day Chile and Japan, were observed to be consistent with a scenario in which HTLV-1 spread to South America along with the human populations that migrated from Asia to the Americas over 10,000 years ago (see figure above).
The HTLVs are closely related to viruses found in african primates, called simian T-lymphotrophic viruses (STLVs). Phylogenetic studies indicate that the various HTLVs (1-4) likely originated in separate primate-to-human transfer events. Accordingly, the deltaretroviruses found in human and non-human primates are now collectively referred to as 'primate T-lymphotrophic viruses' (PTLVs).
A recent study of PTLV sequences obtained from orangutans revealed new insights into the evolutionary history of PTLV-1 (the group of PTLVs that contains HTLV-1) in Asia. This analyses used a molecular clock based approach to estimate the timing of PTLV-1 spread, and indicated that the virus was first introduced into Australia between 3,000 and 4,000 years ago.
The study also inferred that HTLV-1 arrived in Melanesia between 2,300 to 2,700 years ago - approximately corresponding to the time when the Lapita peoples are thought to have migrated into the region.
But while it seems clear that deltaretroviruses have an association with primates that dates back thousands of years, their deeper origins have until recently remained a mystery.