Vertebrates can distinguish numerous odorants in the environment using chemosensory receptors that are expressed in the olfactory epithelium [1–3]. Four types of chemosensory receptors have been found in the vertebrate olfactory epithelium, including the main odorant receptors (ORs) , vomeronasal receptors (VRs) [5–7], trace-amine associated receptors (TAARs)  and formyl peptide receptor-like proteins . The OR genes were initially identified in mouse olfactory organ by Linda Buck and Richard Axel , who found that each olfactory sensory neuron expressed a single OR allele [4, 10, 11]. The ORs, located on the surface of dendrites of sensory neurons on the olfactory epithelia, are the most important chemosensory receptors in the detection and perception of common odorants in the environment. Two types of VRs (V1R and V2R) that are located in the vomeronasal organ in mammals are mainly responsible for detection of pheromones. In fishes, ORs and VRs are both distributed in the one olfactory organ (there is no vomeronasal organ) [12, 13]. The TAARs, expressed in the olfactory epithelium, are responsible for recognition of trace amines and related compounds [8, 14]. The formyl peptide receptor-like proteins that are found in the vomeronasal organs in mammals have an olfactory function associated with the identification of pathogenic states.
The ORs are the most important chemosensory receptors in detecting environmental chemicals in daily life, and they detect a wide range of compounds. A large number of OR genes have now been isolated from various species. Approximately 1,068 putative functional OR genes and ~334 pseudogenes in mouse [15, 16], ~340 putative functional OR genes and ~388 pseudogenes in human [17–19] and ~100 OR genes in fishes [20–22] have been identified from genome databases. The vertebrate OR genes have recently been divided into two major types, type 1 and type 2. The type 1 genes were subdivided into five groups, α, β, γ, δ, ε and ζ, and the type 2 genes into four groups, η, θ, κ and λ, but the groups θ, κ and λ are considered to likely be non-OR genes because they were identified from genome databases and found not to be expressed in the olfactory epithelium [20, 21]. Mammalian OR genes are clearly classified into class I and class II ; here, groups α and β correspond to class I and the group γ to class II [20, 21].
Currently, the most fish OR genes have been identified from genome databases. However, we do not know whether these genes are really expressed in the fish olfactory epithelia. So far, few experiments have been carried out to validate the expression status of OR genes in the fish olfactory epithelium. In addition, it is necessary to expand the knowledge of fish ORs, especially for marine fishes, as the teleost fishes from which OR genes have been reported so far are mostly not strict marine fishes but freshwater, brackish or amphidromous.
The large yellow croaker (Larimichthys crocea), an economically important fish in China, belongs to the family Sciaenidae of the order Perciformes and dwells on the coast of the temperate zone in China. To clarify the evolution of the L. crocea OR genes and to discover whether they are expressed in the olfactory epithelium, we cloned OR cDNAs from large yellow croaker by RT-PCR on the olfactory organ and isolated full-length cDNA using rapid amplification of cDNA ends (RACE). We then conducted phylogenetic analysis using these OR genes and others from 11 vertebrate species and determined the expression levels of the different subfamilies in wild-type fishes using quantitative real-time PCR.