Duplication of the POMC Gene in the Paddlefish (Polyodon spathula): Analysis of γ-MSH, ACTH, and β-Endorphin Regions of Ray-Finned Fish POMC

Abstract

The proopiomelanocortin (POMC) gene, which encodes the common precursor for MSH-related and β-endorphin-related end products, appeared early in chordate evolution and features a variety of lineage-specific modifications. Key among these has been the apparent degeneration and subsequent deletion of the γ-MSH region during the evolution of POMC in the ray-finned fish. A second area of increasing focus has been the role of gene duplication in the evolution of POMC in particular and the opioid/orphanin gene family in general. The cloning and phylogenetic analysis of two POMC cDNAs from the paddlefish (Polyodon spathula) is reported here and biochemical data on their processed end products are presented. Based on conceptual amino acid translations, the paddlefish cDNAs encode all functional domains and, in most cases, the flanking paired-basic amino acid cleavage sites characteristic of gnathostome POMCs (i.e., signal sequence, γ-MSH-like region, ACTH (α-MSH and CLIP), γ-LPH, β-MSH, and β-endorphin). Phylogenetic analysis of the paddlefish POMC sequences in the context of the duplicated POMCs of sturgeon and salmonids suggests that degeneration of the γ-MSH core sequence and its amino-terminal proteolytic cleavage site was initiated prior to divergence of the sturgeon and paddlefish lineages over 150 mya. Finally, a comparison of the relative rates of evolutionary divergence between paralogously related POMC genes within chondrostean and salmonid lineages provides potential support for the hypothesis that some taxa (e.g., the Chondrosteii) represent relic species as a result of an exceptionally slow rate of evolutionary change.