Phylogenetic analysis of a retroposon family as represented on the human X chromosome.

SINE-R elements constitute a class of retroposons derived from the long terminal repeat (LTR) of the human endogenous retrovirus HERV-K family that are present in hominoid primates and active in the human genome. In an investigation of the X chromosome, we identified twenty-five SINE-R elements with between 89.6 and 97.7% homology with the SINE-R.C2 element that is human specific, originally identified in the gene for the C2 component of complement. SINE-R.C2 and a sequence HS307 that we previously identified in a region of Xq21.3 that has a recently created homology with a 4 Mb block in Yp11.2 are amongst the group of elements that have diverged furthest from the parent HERV-K10 sequence. The sequence on the X chromosome resemble those that we previously described on chromosomes 7 and 17 and the Y chromosome, with a similar range of variation. Phylogenetic analysis from the retroposon family including those of African great apes using the neighbor-joining method suggests that the SINE-R retroposon family have evolved independently during primate evolution. Further investigation of SINE-R elements on the sex chromosomes, particularly in recently created regions of X-Y homology, may cast light on the timing of the retroposition process and its possible relevance to recent evolutionary change.