No abstract

The proteasome subunit β-type 8 (PSMB8) gene in the jawed vertebrate MHC genomic region encodes a catalytic subunit of the immunoproteasome involved in the generation of peptides to be presented by the MHC class I molecules. A teleost, the medaka (Oryzias latipes), has highly diverged dimorphic allelic lineages of the PSMB8 gene with only about 80% amino acid identity, termed "PSMB8d" and "PSMB8N," which have been retained by most wild populations analyzed. To elucidate the evolutionary origin of these two allelic lineages, seven species of the genus Oryzias were analyzed for their PSMB8 allelic sequences using a large number of individuals from wild populations. All the PSMB8 alleles of these species were classified into one of these two allelic lineages based on their nucleotide sequences of exons and introns, indicating that the Oryzias PSMB8 gene has a truly dichotomous allelic lineage. Retention of both allelic lineages was confirmed except for one species. The PSMB8d lineage showed a higher frequency than the PSMB8N lineage in all seven species. The two allelic lineages showed curious substitutions at the 31st and 53rd residues of the mature peptide, probably involved in formation of the S1 pocket, suggesting that these allelic lineages show a functional difference in cleavage specificity. These results indicate that the PSMB8 dimorphism was established before speciation within the genus Oryzias and has been maintained for more than 30-60 million years under a strict and asymmetric balancing selection through several speciation events.long-term balancing selection | antigen processing | transspecies polymorphism T ransspecies polymorphism (TSP) is the passage of allelic lineages from ancestral to descendent species (1), and the underlying selective mechanism is referred to as balancing selection caused by "overdominant selection," "frequency-dependent selection," or "selection that varies in time and space" (2). The vertebrate MHC class I and II genes with a large number of alleles and wide allelic differences provide a classical example of TSP (3, 4), most probably arising by overdominant selection (5). TSP of the MHC genes sometimes persists for very long periods. Certain HLA-DRB alleles are reported to have persisted for 50-60 million years (6). Another well-characterized TSP has been reported for the rabbit Ig heavy-chain variable-region genes; its persistence has been estimated at 50 million years based on molecular phylogenetic analysis (7,8). Except for these genes encoding molecules showing high degree of binding specificity, information for long-term balancing selection resulting in TSP is limited (9-11). Moreover, all TSP reported thus far involve only a few species, and the possible presence of a long-lasting TSP surviving through many speciation events is still to be clarified by systematic phylogenetic analysis.The MHC class I molecules deliver the peptides derived from cytosolic proteins to the cell surface for recognition by cytotoxic T cells. The proteins are degraded proteolytically into s...

No abstract

In the genus Oryzias, the morphologies of the dorsal and anal fins are typical secondary sex characters. In the Japanese medaka (Oryzias latipes) and Thai medaka (Oryzias minutillus), androgen receptor (AR) expression levels in the dorsal, anal, and pectoral fins were higher in males than in females. Conversely, in both species estrogen receptor (ER) β expression levels in the dorsal and anal fins were higher in females than in males. AR and ERβ expression levels in the dorsal and anal fins of sex-undeterminable individuals of Thai medaka were intermediate between those in normal male and female Thai medaka. There was no difference in the bone morphogenic protein (Bmp) 2b expression level between male and female Japanese medaka. In contrast, the Bmp2b expression level in the dorsal fin of sex-undeterminable individuals was lower than in normal male and female Thai medaka. It is thus clear that androgen and estrogen regulate the sexdependent characters of fin morphology in both Oryzias species. In sex-undeterminable individuals of Thai medaka, the low levels of Bmp2b expression in the dorsal fin are evidence that androgen and estrogen are necessary for adequate expression of Bmp2b in the normal development of at least the dorsal fin.

No abstract