Divergent PXR Function in Seals: Endocrine Adaptation or Functional Loss?

Seals accumulate xenobiotics through dietary biomagnification and exposure to polluted marine environments, with contaminants concentrating in their blubber. Biotransformation mitigates xenobiotic toxicity by converting lipophilic compounds into excretable hydrophilic metabolites, a process coordinated by nuclear receptors including the Pregnane X Receptor (PXR), whose plastic ligand-binding domain enables broad xenobiotic sensing. By examining PXR in pinnipeds, we investigated the evolutionary conservation and functional characterization of PXR using genomic sequence analysis, protein structural prediction, and transactivation assays, revealing broadly conserved structural features alongside species-specific functional divergence in receptor responsiveness to environmental stressors. Specifically, the obtained results highlight divergent gene and functional landscapes with ORF-disrupting mutations identified in Monachus monachus and Neomonachus schauinslandi that abolish receptor activation toward known PXR ligands. In contrast, Leptonychotes weddelli retained an intact PXR ORF but showed reduced receptor activity, revealing functional divergence in PXR among pinnipeds.