Random expression of main and vomeronasal olfactory receptor genes in immature and mature olfactory epithelia of Fugu rubripes

Main olfactory receptor genes were isolated from a seawater fish, Fugu rubripes (puffer-fish), and characterized. Two subfamilies of genes encoding seven transmembrane re-ceptors were identified; one consists of five or more members, termed FOR1-1 to 5 of FOR1 subfamily, and the other appears to be a single copy gene, termed the FOR2 sub-family. FOR1 members show extremely high amino acid sequence similarities of about 95 % to one another, and are distantly related to catfish-1 with the highest similarity of 37%. FOR2 shows 43 % similarity to goldfish-A28. Phylogenically, both FOR members are categorized among pedigrees of the fish main olfactory receptor family outside the mammalian receptor family, although similarities between Fugu receptors and those of fresh-water fishes are lower than those among fresh-water fishes. In situ hybridization shows that both subfamilies of receptor genes are expressed randomly over the olfac-tory epithelium throughout all developmental stages, and no segregation of the signals was found. On the other hand, when three members of a vomeronasal olfactory receptor gene family, related to the Caa+-sensing receptor, were used as probes, they were also randomly expressed over the same epithelium as the main olfactory receptors. This is i