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  • Introduction Fish commonly employ pheromones to regulate

    2021-08-02

    Introduction Fish commonly employ pheromones to regulate a variety of functions including reproductive communication [1]. Reproductive pheromones induce both primer effects, such as changes in the endocrine or physiological state of conspecifics, and releaser effects such as rapid behavioral responses [2,3]. For example, in male goldfish (Carassius auratus), the pheromone 4-pregnen-17α,20β-dihydroxy- progesterone (DHP) triggers both primer increases in GnRH-II mRNA level in the telencephalon, luteinizing hormone (LH) in the plasma and milt volume [[4], [5], [6]] and releaser effects on socio-sexual behaviors [43]. Prostaglandin F2α (PGF2α), a key regulator of ovulation in many teleost (see review [7]), is another well-known teleost reproductive pheromone inducing both primer effects on milt production and releaser effects on sexual behavior in male fish [8,9]. However, in some teleost species including medaka (Oryzias latipes), prostaglandin E2 (PGE2) is involved in the ovulatory process [10,11]. In these fish species, PGE2 instead of PGF2α might serve as a sex pheromone. Numerous studies showed that pheromonal responsiveness, including electro-olfactogram (EOG) responses, vary with reproductive status ([[12], [13], [14], [15], [16]]). It is believed that this effect of reproductive status on pheromonal responsiveness is regulated by endocrine factors [1,15,17]. Androgens are important gonadal AVE 0991 mediating the reproductive activity in male fish [18,19] and also an endocrine factor modulating pheromonal responsiveness ([[12], [13], [14], [15], [16]]). In Epalzeorhynchos bicolor, Carassius auratus, Danio rerio and juvenile Puntius schwanenfeldi, androgen treatment increases the magnitude and sensitivity of the EOG response to pheromones prostaglandins [5,15]. However, the molecular mechanism for the regulatory effects of androgens on odorant receptors is unclear. One possibility is that androgens bind receptors found in the olfactory epithelium [20], leading to increases in the number of pheromone receptors and/or varying sensitivity of the receptors [5]. The biological activity of androgens is mediated by the androgen receptor (AR) which belongs to the nuclear receptor family, and exerts its function as a ligand-activated transcription factor to regulate the transcription of androgen target genes. The main ligand for the Ar in teleost fishes is 11-KT, while in humans testosterone (T) and the even more potent hormone 5α-dihydrotestosterone are the active androgens [3,21,22]. Unlike most vertebrates, which express a single AR gene, two distinct Ar genes have been identified in many teleosts, e.g. Arα and Arβ [[23], [24], [25]]. The subtype designated Arβ is more closely related to AR from mammals, birds and reptiles, while Arα exhibits sequence divergence [[23], [24], [25]]. Arα and Arβ have different transactivation potencies in response to androgens [24,25], and they exhibit unique expression patterns in some species [26,27]. Moreover, the sequence similarity between the DNA-binding domain (DBD) of Arα and Arβ is only around 70%, which suggests different biological functions of Arα and Arβ in teleosts. The Chinese black sleeper, Bostrychus sinensis, belongs to the family Eleotridae, suborder Gobioidei. This species is a burrowing animal and inhabits intertidal zones [28]. As a seasonal breeding fish, females and males live in individual burrows during the non-spawning season, but spawn as a pair inside the same burrow during the spawning season [29]. This spawning behavior and burrow-living habit suggest that mature males and females may employ sex pheromones to synchronize gametogenesis, spawning and fertility, since visual communication seems less efficient inside the burrow. Our previous studies showed that the levels of PGE2 were obviously higher than those of PGF2α in ovarian extract and urine at sex maturation stage [32], which suggested that PGE2 may be involved in the ovulatory process, and serves as a sex pheromone in B. sinensis. This hypothesis was further proved by our study which showed that nests with a PGE2-releasing tube attract more males and females and have in a higher spawning rate than control nests [29]. In addition, both mature male and female B. sinensis display greater EOG response to PGE2, compared to immature fish [30], which indicates that PGE2 is a putative sex pheromone in B. sinensis and that the olfactory sensitivity to PGE2 depends on the reproductive status. According to transcriptomic data, three subtypes of PGE2 receptor mRNAs (ep1, ep2 and ep4) were expressed in the B. sinensis olfactory rosette. The ep1 mRNA levels in the olfactory rosette of mature fish were significantly higher than those in immature fish [31], suggesting that the distinct olfactory sensitivities to PGE2 might be due to the changes of Ep1 levels [31]. However, it is not clear if androgens contribute to the up-regulation of ep1 expression in male fish.