Two subfamilies of olfactory receptor genes in medaka fish, Oryzias latipes: Genomic organization and differential expression in olfactory epithelium

We cloned and characterized two subfamilies of olfatory receptor (OR) genes from medaka fish (Oryzias latipes). Southern blot analysis showed that eac h of the two subfamilies, designated as subfamilies Y and E, consists of ab out five members, as usually observed for other vertebrate ORs. Analyses of the genomic clones encoding these members revealed that two members of sub family Y and four members of subfamily E are tandemly reiterated in 15 and 22 kbp regions of the medaka genome, respectively. The members of each subf amily show very similar amino acid sequences, with similarities greater tha n 70%. Hoc-ever, the similarities to the sequences of other vertebrate ORs are lower. Members of subfamily Y show amino acid sequence similarities of ca. 30% to other fish ORs, including subfamily E members, as well as to mam malian ORs. On the other hand, members of subfamily E show sequence similar ities of ca. 50% to other fish ORs and ca. 30% to mammalian ORs. Phylogenic analyses of various fish ORs, including medaka, catfish, and zebrafish ORs , indicate that the primary structures of fish ORs are diverse compared wit h those of mammalian ORs, which consist of much larger numbers of members. The expression patterns of subfamilies Y and E genes in the olfactory epith elium of adult medaka fish were examined by in situ hybridization, showing that the frequency of positive signals is different between the two subfami lies: about 2% of the olfactory neurons are positive to probes for members of subfamily Y, while less than 1% are positive to probes for members of su bfamily E. These results indicate that each subfamily is under different tr anscriptional control.