Functional diversification of vitamin D receptor paralogs in teleost fish after a whole genome duplication event

The diversity and success of teleost fishes (Actinopterygii) has been attributed to three successive rounds of whole-genome duplication (WGD). WGDs provide a source of raw genetic material for evolutionary forces to act upon, resulting in the divergence of genes with altered or novel functions. The retention of multiple gene pairs (paralogs) in teleosts provides a unique opportunity to study how genes diversify and evolve after a WGD. This study examines the hypothesis that vitamin D receptor (VDR) paralogs (VDRα and VDRβ) from two distantly related teleost orders have undergone functional divergence subsequent to the teleost-specific WGD. VDRα and VDRβ paralogs were cloned from the Japanese medaka (Beloniformes) and the zebrafish (Cypriniformes). Initial transactivation studies using 1α, 25-dihydroxyvitamin D3 revealed that although VDRα and VDRβ maintain similar ligand potency, the maximum efficacy of VDRβ was significantly attenuated compared with VDRα in both species. Subsequent analyses revealed that VDRα and VDRβ maintain highly similar ligand affinities; however, VDRα demonstrated preferential DNA binding compared with VDRβ. Protein-protein interactions between the VDR paralogs and essential nuclear receptor coactivators were investigated using transactivation and mammalian two-hybrid assays. Our results imply that functional differences between VDRα and VDRβ occurred early in teleost evolution because they are conserved between distantly related species. Our results further suggest that the observed differences may be associated with differential protein-protein interactions between the VDR paralogs and coactivators. We speculate that the observed functional differences are due to subtle ligand-induced conformational differences between the two paralogs, leading to divergent downstream functions.