**4. Discussion**

During the last few decades, chemosensory receptor gene families have received extensive attention across various organisms, including humans [45,47–49], mice [40,46,50], fish [28,51–53] and insects [21,54–57]. However, it remains still unclear whether male and female individual possesses the same ability for odorant-sensing. In the present study, we reported the transcriptional profiles of the olfactory epithelium from the male and female zebrafish obtained by RNA-seq. By comparing the expression levels of genes from three di fferent biologic replicate samples for each sex, we obtained highly correlated samples, which suggested that the subsequent di fferential expression analyses were reliable.

Sexual dimorphisms in behaviors have been widely observed in the olfactory systems in both mice [50,58] and flies [59]. Although previous studies have suggested that sexually dimorphic behaviors between sexes could be influenced by sensory input such as olfactory cues [60], it is unclear whether di fferences in gene expression in olfactory epithelia can underlie sexually dimorphic behaviors in fish [40]. Our results showed that the overall transcriptional profiles of zebrafish olfactory epithelium between sexes are highly similar. However, we indeed detected a few genes di fferentially expressed between male and female zebrafish, whose roles may be associated with odorant-sensing. Moreover, the functional enrichment analyses of the di fferentially expressed genes were also involved in response to external biotic stimulus. Therefore, all these results from our study suggested that di fferences in gene expression in the olfactory epithelium between sexes in zebrafish may play a role in odorant sensing to some extent.

However, when we focused on the chemosensory receptor genes that are directly binding to the odorants in surrounding environment, our results sugges<sup>t</sup> that the olfactory receptor genes may not all be involved in behavioral di fferences between males and females. Although almost all of the chemosensory receptor genes can be detected to be expressed in the olfactory epithelium of zebrafish, most them displayed similar expression levels between sexes, with only six *or* genes showing di fferentially expressed between the two sexes. Considering the existence of nearly 358 chemosensory receptor genes in zebrafish genome, the six di fferentially expressed *or* genes should play minor roles in the dimorphic behaviors between male and female zebrafish [61,62]. Therefore, the sexually dimorphic behavioral responses to odorants in zebrafish are unlikely to be solely accounted for by transcriptional di fferences at the level of detection [40].

Interestingly, several functional studies have identified several olfactory receptor ligands in fish [10–16]. For example, Yabukiet al. reported that *or114-1* and *or114-2* in the group β *or* genes are the key olfactory receptors for the sex pheromone prostaglandin F2α to mediate male courtship behavior in zebrafish [16]. However, none of these olfactory receptor genes were found to be significantly di fferentially expressed in the olfactory epithelium between male and female in our RNA-seq results. It should be noted that di fferential expression of peripheral odorant receptors is not the only way to generate sex-specific behavior, as higher neuronal circuits could be di fferent between males and females. Therefore, whether the six di fferentially expressed *or* genes can play crucial roles in sexually dimorphic behaviors is still uncertain and need to be confirmed by further functional studies in future.
