Tsukamoto, K., Miura, F., Fujito, N.T., Yoshizaki, G., and Nonaka, M. (2012) Long-lived dichotomous lineages of the proteasome subunit beta type 8 (PSMB8) gene surviving more than 500 million years as alleles or paralogs. Mol. Biol. Evol.. 29(10):3071-3079.
On an evolutionary time scale, polymorphic alleles are believed to have a short life, persisting at most tens of millions
of years even under long-term balancing selection. Here we report highly diverged trans-species dimorphism of the proteasome
subunit beta type 8 (PSMB8) gene, which encodes a catalytic subunit of the immunoproteasome responsible for the generation of peptides presented by
major histocompatibility complex (MHC) class I molecules, in lower teleosts including Cypriniformes (zebrafish and loach)
and Salmoniformes (trout and salmon), whose last common ancestor dates to 300 MY ago. Moreover, phylogenetic analyses indicated
that these dimorphic alleles share lineages with two shark paralogous genes, suggesting that these two lineages have been
maintained for more than 500 MY either as alleles or as paralogs, and that conversion between alleles and paralogs has occurred
at least once during vertebrate evolution. Two lineages termed PSMB8A and PSMB8F show an A31F substitution that would probably affect their cleaving specificity, and whereas the PSMB8A lineage has been retained by all analyzed jawed vertebrates, the PSMB8F lineage has been lost by most jawed vertebrates except for cartilaginous fish and basal teleosts. However, a possible functional
equivalent of the PSMB8F lineage has been revived as alleles within the PSMB8A lineage at least twice during vertebrate evolution in the amphibian Xenopus and teleostean Oryzias species. Dynamic evolution of the PSMB8 polymorphism through long-term persistence, loss and regaining of dimorphism and conversion between alleles and paralogs
implies the presence of strong selective pressure for functional polymorphism of this gene.