New genes from domesticated genome parasites. The example shown illustrates the origin of a new placenta gene from an endogenous retrovirus sequence (the scenario illustrates the origin of one of the several syncytin genes that evolved important placenta functions in mammals [Heidmann et al. 2009]; see main text for details). The domestication event involved the decay of two of the human endogenous retrovirus ORFs (gag and pol) and the selective preservation of the ORF encoding the virus envelope protein. (Empty box) Loss of function/decay, (gold boxes) ORFs. The newly formed syncytin gene (transcript structure indicated by thin black line) became transcribed from the retrovirus' long terminal repeat (LTR; green) promoter (TSS shown as red right-angled arrow) and evolved a placenta-specific expression pattern and (fusogenic) function (Heidmann et al. 2009).
