Abstract
The genomes of eukaryotes preserve a striking diversity of ancient viruses in the form of endogenous viral elements (EVEs). Study of this genomic fossil record provides insights into the diversity, origin and evolution of viruses across geological timescales. In particular, Mavericks have emerged as one of the oldest groups of viruses infecting vertebrates (≥419 My). They have been found in the genomes of fish, amphibians and non-avian reptiles but had been overlooked in mammals. Thus, their evolutionary history and the causes of their demise in mammals remain puzzling questions. Here, we conduct a detailed evolutionary study of two Maverick-like integrations found on human chromosomes 7 and 8. We performed a comparative analysis of the integrations and determined their orthology across placental mammals (Eutheria) via the syntenic arrangement of neighbouring genes. The integrations were absent at the orthologous sites in the genomes of marsupials and monotremes. These observations allowed us to reconstruct a time-calibrated phylogeny and infer the age of their most recent common ancestor at 268.61 (199.70–344.54) My. In addition, we estimate the age of the individual integrations at ~105 My which represent the oldest non-retroviral EVEs found in the human genome. Our findings suggest that active Mavericks existed in the ancestors of modern mammals ~172 My ago (Jurassic Period) and potentially to the end of the Early Cretaceous. We hypothesise Mavericks could have gone extinct in mammals from the evolution of an antiviral defence system or from reduced opportunities for transmission in terrestrial hosts.
Importance The genomes of vertebrates preserve an enormous diversity of endogenous viral elements (remnants of ancient viruses that accumulate in host genomes over evolutionary time). Although retroviruses account for the vast majority of these elements, diverse DNA viruses have also been found and novel lineages are being described. Here we analyse two elements found in the human genome belonging to an ancient group of DNA viruses called Mavericks. We study their evolutionary history, finding that the elements are shared between humans and many different species of placental mammals. These observations suggest the elements inserted at least ~105 Mya in the most recent common ancestor of placentals. We further estimate the age of the viral ancestor around 268 My. Our results provide evidence for some of the oldest viral integrations in the human genome and insights into the ancient interactions of viruses with the ancestors of modern-day mammals.
