Endogenous Jaagsiekte sheep retrovirus envelope protein promotes sheep trophoblast cell fusion by activating PKA/MEK/ERK1/2 signaling

A genome-wide study of ruminants uncovers two endogenous retrovirus families recently active in goats

BACKGROUND

Endogenous retroviruses (ERV) are traces of ancestral retroviral germline infections that constitute a significant portion of mammalian genomes and are classified as LTR-retrotransposons. The exploration of their dynamics and evolutionary history in ruminants remains limited, highlighting the need for a comprehensive and thorough investigation of the ERV landscape in the genomes of cattle, sheep and goat.

RESULTS

Through a de novo bioinformatic analysis, we characterized 24 Class I and II ERV families across four reference assemblies of domestic and wild sheep and goats, and one assembly of cattle. Among these families, 13 are represented by consensus sequences identified in the five analyzed species, while eight are exclusive to small ruminants and three to cattle. The similarity-based approach used to search for the presence of these families in other ruminant species revealed multiple endogenization events over the last 40 million years and distinct evolutionary dynamics among species. The ERV annotation resulted in a high-resolution dataset of 100,534 ERV insertions across the five genomes, representing between 0.5 and 1% of their genomes. Solo-LTRs account for 83.2% of the annotated insertions demonstrating that most of the ERVs are relics of past events. Two Class II families showed higher abundance and copy conservation in small ruminants. One of them is closely related to circulating exogenous retroviruses and is represented by 22 copies sharing identical LTRs and 12 with complete coding capacities in the domestic goat.

CONCLUSIONS

Our results suggest the presence of two ERV families with recent transpositional activity in ruminant genomes, particularly in the domestic goat, illustrating distinct evolutionary dynamics among the analyzed species. This work highlights the ongoing influence of ERVs on genomic landscapes and call for further investigation of their evolutionary trajectories in these genomes.

Placental Evolution: Innovating How to Feed Babies

The evolution of the placenta was transformative. It changed how offspring are fed during gestation from depositing all the resources into an egg to continually supplying resources throughout gestation. Placental evolution is infinitely complex, with many moving parts, but at the core it is driven by a conflict over resources between the mother and the baby, which sets up a Red Queen race, fueling rapid diversification of morphological, cellular, and genetic forms. Placentas from even closely related species are highly divergent in form and function, and many cellular processes are distinct. If we could extract the entirety of genomic information for placentas across all species, including the many hundreds that have evolved in fish and reptiles, we could find their shared commonality, and that would tell us which of the many pieces really matter. We do not have this information, but we do have clues. Convergent evolution mechanisms were repeatedly used in the placenta, including the intense selective pressure to co-opt an envelope protein to build a multinucleated syncytium, the use of the same hormones and structural proteins in placentas derived from separate embryonic origins that arose hundreds of millions of years apart, and the co-option of endogenous retroviruses to form capsids as a way of transport and as mutagens to form new enhancers. As a result, the placental genome is the Wild West of biology, set up to rapidly change, adapt, and innovate. This ability to adapt facilitated the evolution of big babies with big brains and will continue to support offspring and their mothers in our ever-changing global environment.

Progressive Exaptation of Endogenous Retroviruses in Placental Evolution in Cattle

Viviparity is made possible by the placenta, a structure acquired relatively recently in the evolutionary history of eutherian mammals. Compared to oviparity, it increases the survival rate of the fetus, owing to the eutherian placenta. Questions such as "How was the placenta acquired?" and "Why is there diversity in placental morphology among mammalian species?" remain largely unsolved. Our present understanding of the molecules regulating placental development remains unclear, owing in no small part to the persistent obscurity surrounding the molecular mechanisms underlying placental acquisition. Numerous genes associated with the development of eutherian placental morphology likely evolved to function at the fetal-maternal interface in conjunction with those participating in embryogenesis. Therefore, identifying these genes, how they were acquired, and how they came to be expressed specifically at the fetal-maternal interface will shed light on some crucial molecular mechanisms underlying placental evolution. Exhaustive studies support the hypothesis that endogenous retroviruses (ERVs) could be evolutional driving forces for trophoblast cell fusion and placental structure in mammalian placentas including those of the bovine species. This review focuses on bovine ERVs (BERVs) and their expression and function in the placenta.