Wichian Magtoon

Transspecies dimorphic allelic lineages of the proteasome subunit β-type 8 gene (PSMB8) in the teleost genus Oryzias

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.

Expression Levels of Hormone Receptors and Bone Morphogenic Protein in Fins of Medaka

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) &bgr; expression levels in the dorsal and anal fins were higher in females than in males. AR and ER&bgr; 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 sex-dependent 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.

Description and Karyotype of a New Ricefish, Oryzias mekongensis, from Thailand

A new species of ricefish, Oryzias mekongensis, from the Mekong basin of northeast Thailand is described. It is characterized by a small size (11.5-14.0 mm SL) and 13-16 (modally 14) anal-fin rays. Live males have bright orange longitudinal subdistal stripes along upper and lower margins of caudal fin. The karyotype revealed 2n, 48 chromosomes consisting of 1 meta-, 4 submeta-, 12 subtelo- and 7 acrocentric pairs, the number of major arms (NF) being 58. Estimated DNA value was 1.5 pg per nucleus. This species is distinct from another small species, 0. minutillus distributed in central, north and south Thailand, by morphological, karyotypical and reproductive differences. 0. mekongensis belongs to the biarmed chromosome group which is distributed in east Asia; this species is most closely related to 0. curvinotus from Hainan Island. The new species having the smallest NF, is considered to be karyotypically most primitive among the members of the biarmed chromosome group so far studied.