Comparative genomic analysis of oxytocin and oxytocin receptor genes in the family Equidae.

BACKGROUND: The oxytocin (OXT) - oxytocin receptor (OXTR) system regulates diverse biological processes including social behavior, stress responses, and processes related to domestication. While OXT is highly conserved, OXTR shows greater variability, which may contribute to species-specific adaptations. METHODS: Comparative genomic analyses of OXT and OXTR coding sequences were performed across equid species using available genome assemblies and newly generated sequences obtained by next-generation sequencing. Phylogenetic relationships were inferred using maximum likelihood methods. Selection was assessed using codon-based approaches (FEL, SLAC, FUBAR). The potential effects of amino acid substitutions were predicted using in silico tools, including SIFT, Missense3D, and NetPhos. RESULTS: The OXT gene was conserved across equids, with no variation in the amino acid sequence. The OXTR gene was more variable, with most nonsynonymous substitutions located in the C-terminal region. Predominantly purifying selection was detected, with limited diversifying selection. OXTR gene variability differed among species, being lower in horses and higher in zebras and wild asses. Differences between domesticated and nondomesticated groups were observed. Trans-species haplotype sharing and distinct patterns at amino acid positions 182 and 344 were identified. CONCLUSIONS: The OXTR gene shows genetic variability in equids, with differences between domesticated and nondomesticated species. Some variants may represent candidate markers for future studies of domestication, behavior, and comparisons between ancient and modern DNA.