Update on endogenous retroviruses in cattle placenta

Endogenous retroviruses (ERVs) are abundant in the bovine genome and have been repeatedly co-opted for placental function, yet their organization and roles in epitheliochorial species remain less well defined than in primates and mice. Here, genomic, transcriptomic, and functional data are combined to provide an updated view of ERVs in the cattle placenta and to frame their contributions in terms of three interacting roles (Roles 1-3). Role 1 comprises structural fusogens, including syncytin-Rum1, BERV-K1/Fematrin-1, and additional trophoblast-biased loci, that support binucleate trophoblast cell fusion and establishment of the synepitheliochorial interface. Genome-wide, BERV-focused UNION intervals cover 3.21% of the ARS-UCD2.0 assembly, whereas RepeatMasker-defined LTR/ERV elements span 4.71%, and these strata are organized into ancient ERVL/ERV1 components and younger ERVK lineages accompanied by long-term decay and soloization. Role 2 encompasses immune-inducible ERV/LTR enhancers-exemplified by MER41_BT and Bov-A2-that recruit STAT1 upon IFNT/IFNG stimulation and can control nearby interferon-pathway genes, together with age-stratified LTR strata enriched for ETS, IRF, GAS, and ISRE motifs, often assembled into multi-factor IFN×ETS modules. Role 3 consists of epigenetic gating and signaling context, illustrated by developmental changes in BERV-K1 promoter methylation, WNT/EMT-responsive BERV-K3, and widespread chromatin-level control of LTR activity in trophoblast and uterine compartments. Together, these roles provide "hardware" (fusogens) and "software" (enhancers and epigenetic switches) for bovine pregnancy. Outstanding questions regarding receptors, lineage-specific motif architectures, and breed-level variation are highlighted, and experimental strategies-integrating genome editing, epigenome profiling, and single-cell approaches-are outlined to test how ERV-derived elements shape implantation, placentome development, and reproductive success in cattle.