**1. Introduction**

Olfaction is a sense for detecting environmental odorants that plays essential roles in many aspects of animal activities, such as foraging, migration, prey avoidance and mating [1]. In most terrestrial vertebrates, there are two distinct chemosensory organs: the olfactory epithelium (OE) and the vomeronasal organ (VNO) [2]. In general, the OE is assumed to detect environmental odorants, while the VNO senses pheromones, although some exceptions were also reported in recent studies [3,4]. Di fferent set of chemosensory receptors is thought to be expressed in each organ. For the OE, odorant receptors (ORs) and trace amine-associated receptors (TAARs) are known to be expressed, while for the VNO, vomeronasal receptors type 1 (V1Rs) and type 2 (V2Rs) are considered to be expressed [5]. Therefore, in terrestrial mammals, ORs and TAARs are suggested to detect "ordinary" odorant molecules, while V1Rs and V2Rs are employed to detect pheromones [6–8].

However, unlike the terrestrial counterparts, fish have only the OE to sense their olfactory environment, due to an absence of the VNO [9]. Without directly functional studies of the chemosensory receptors in fish, it is unclear whether the di fferent types of chemosensory receptors in fish respond to di fferent classes of odorants—and which family of the chemosensory receptors is used for recognizing odorants or pheromones. Interestingly, there are also some studies using electrophysiological experiments that provide indirect evidence of putative ligands for chemosensory receptors [10–16]. For example, members of ORs from goldfish can recognize F-prostaglandins, while both ORs and V2Rs can perceive amino acids [17,18].

Several studies have found that sex-specific behaviors can be due to sex differences in responses to external stimuli, including courtship songs, colors and chemosensory cues [19]. Indeed, chemosensory cues in mice, such as pheromone molecules, have been shown to be detected specifically by their vomeronasal receptors [20]. Therefore, sex-specific behaviors maybe initiated by sex-specific pheromones [21–23] or by sex-differential expression of chemosensory receptors in response to the same pheromones [24]—or even by sex-specific alternative splicing [25]. For instance, Darcin (the MUP20 encoded by Mup20)—a male-specific pheromone in mouse urine—only attracts females to affect their memory [24]. ESP1 (exocrine-secreted peptide 1) is detected by both male and female mice, but only stimulated female-specific mating behaviors, suggesting the existence of sex-specific neuronal circuits [23,26]. Male fruit flies require the protein encoded by the fruitless (fru) gene to complete courtship, which is produced in different sex-specific isoforms via alternative splicing [25]. However, to date, few studies have reported whether sexual differences in odorant-sensing exists between males and females, especially in fish.

To determine the full repertoire of chemosensory receptors expressed in the zebrafish olfactory epithelium and evaluate the extent of sexual differentiation in odorant-sensing between sexes, we used RNAseq to profile their transcriptomes in male and female zebrafish. We found that a very high percentage of chemosensory receptors are indeed expressed in the zebrafish olfactory epithelium, with a higher expression level in males than females. However, the olfactory transcriptomes between males and females are highly similar, with limited genes showing differentially expressed or alternatively splicing, which may be associated with odorant-sensing between sexes. Collectively, our results provide a comprehensive catalog of the genes mediating olfactory perception and pheromone-evoked behavior in fish.

#### **2. Materials and Methods**
