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Background  Most human endogenous retroviruses (HERVs) invaded our genome at least 25 million years ago. The majority of the viral genes are degenerated, since no selection preserves them within the genome. However, a few intact and very old HERV genes exist, and likely are beneficial for the host. We here address evolutionary aspects of two HERV-V envelope genes,  ENVV1  and  ENVV2 , located in tandem and containing a long open reading frame.    Results  The  ENVV2  gene is preserved with an intact reading frame during simian evolution, but none of the  ENVV  genes are found in the prosimian species tested. While we observe many transposon insertions in the  gag  and  pol  regions of the ERV-V2 provirus, the  ENVV2  genes have escaped transposon crossfire in all species tested. Additional analysis of nucleotide substitutions provides further strong evidence of purifying selection on the  ENVV2 gene  during primate evolution. The other copy,  ENVV1 , seems to be involved in gene conversion of the major part of the envelope. Furthermore,  ENVV1  and  ENVV2  show placenta-specific expression in human and a baboon species.    Conclusion  Our analyses show that ERV-V entered our genome after the split between simian and prosimian primates. Subsequent purifying selection and gene conversion have preserved two copies of the  ENVV  envelope gene in most species. This is the first case of gene conversion involving long open reading frames in HERVs. Together with the placenta-specific expression of the human and baboon  ENVV1  and  ENVV2  envelope genes, these data provide strong evidence of a beneficial role for the host.

Gene conversion and purifying selection of a placenta-specific ERV-V envelope gene during simian evolution