Molecular evolution of human T-cell leukemia virus

Phylogenetic trees for the human T-cell leukemia virus type I (HTLV-I) and its related viruses were constructed by use of nucleotide sequences of the long terminal repeat (LTR) and the tax gene. The trees showed that the viruses diverged from a common ancestral virus and that they are classified into two groups whose hosts are either primates or bovines. However, the topology of the trees for the viruses differed from that for the hosts. This suggests that HTLV-I and HTLV-I-related viruses evolved independently of host-species divergence and that interspecies transmission between human and monkeys occurejd in the past. The nucleotide diversity of the tax genes of HTLV-I was estimated to be 0.025. This value is more than 10 times larger than that of human globin genes, but it is about 20 times smaller than that of hemagglutinin genes of influenza A viruses. Thus, the genetic variability of the HTLV-I genes seems to be higher than that of nuclear genes but much lower than the genes of typical RNA viruses. Furthermore, we examined functional constraints on the overlapping region of the rex and tax genes. The results obtained imply that for the overlapping region, the tax gene has much stronger constraints against amino acid changes than the rex gene.