Evolution of dimorphisms of the proteasome subunit beta type 8 gene (PSMB8) in basal ray-finned fish

The proteasome subunit beta type 8 (PSMB8) gene encodes a catalytic subunit of immunoproteasome that plays a central role in the processing of antigenic peptides presented by major histocompatibility complex class I molecules. The A- and F-type alleles defined by the 31st amino acid residue determining cleaving specificity have been identified from ray-finned fish, amphibia, and reptiles. These two types show extremely long-term trans-species polymorphism in Polypteriformes, Cypriniformes, and Salmoniformes, suggesting the presence of very ancient lineages termed A and F. To elucidate the evolution of the PSMB8 dimorphism in basal ray-finned fish, we analyzed Pantodon buchholzi (Osteoglossiformes), seven species of Anguilliformes, and Hypomesus nipponensis (Osmeriformes). Both A and F lineage sequences were identified from P. buchholzi and H. nipponensis, confirming that these two lineages have been conserved by basal ray-finned fish. However, both the A- and F-type alleles found in Anguilliformes species belonged to the F lineage irrespective of their types. This apparently suggests that the A lineage was lost in the common ancestor of Anguilliformes, and recovery of the A type within the F lineage occurred in Anguilliformes. The apparent loss of the F lineage and recovery of the F type within the A lineage have already been reported from tetrapods and higher teleosts. However, this is the first report on the reverse situation and reveals the dynamic evolution of the PSMB8 dimorphism.