Among vertebrates, fishes show an exceptional range of reproductive strategies regarding the expression of their sexuality. Fish sexualities were categorized into gonochorism, synchronous/sequential hermaphrodite, or unisexual reproduction. In gonochoristic fishes, sex is determined genetically or by environmental factors. After sex determination, the gonads are differentiated into ovary or testis, with the sex remaining fixed for the entire life cycle. In contrast, some sequential hermaphrodite fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. In many cases, sex change is cued by social factors such as the disappearance of a male or female from a group. This unique diversity in fishes provides an ideal animal model to investigate sex determination and differentiation in vertebrates. This review first discusses genetic-orientated sex determination mechanisms. Then, we address the gonadal sex differentiation process in a gonochoristic fish, using an example of the Nile tilapia. Finally, we discuss various types of sex change that occur in hermaphrodite fishes.
A multiplex analysis for profiling the expression of candidate genes along with epigenetic modification may lead to a better understanding of the complex machinery of neuropathic pain. In the present study, we found that partial sciatic nerve ligation most remarkably increased the expression of monocyte chemotactic protein 3 (MCP-3, known as CCL7) a total of 33 541 genes in the spinal cord, which lasted for 4 weeks. This increase in MCP-3 gene transcription was accompanied by the decreased trimethylation of histone H3 at Lys27 at the MCP-3 promoter. The increased MCP-3 expression associated with its epigenetic modification observed in the spinal cord was almost abolished in interleukin 6 knockout mice with partial sciatic nerve ligation. Consistent with these findings, a single intrathecal injection of recombinant proteins of interleukin 6 significantly increased MCP-3 messenger RNA with a decrease in the level of Lys27 trimethylation of histone H3 at the MCP-3 promoter in the spinal cord of mice. Furthermore, deletion of the C-C chemokine receptor type 2 (CCR2) gene, which encodes a receptor for MCP-3, failed to affect the acceleration of MCP-3 expression in the spinal cord after partial sciatic nerve ligation. A robust increase in MCP-3 protein, which lasted for up to 2 weeks after surgery, in the dorsal horn of the spinal cord of mice with partial sciatic nerve ligation was seen mostly in astrocytes, but not microglia or neurons. On the other hand, the increases in both microglia and astrocytes in the spinal cord by partial sciatic nerve ligation were mostly abolished in interleukin 6 knockout mice. Moreover, this increase in microglia was almost abolished by CCR2 gene deletion, whereas the increase in astrocytes was not affected in nerve-ligated mice that lacked the CCR2 gene. We also found that either in vivo or in vitro treatment with MCP-3 caused robust microglia activation. Under these conditions, intrathecal administration of MCP-3 antibody suppressed the increase in microglia within the mouse spinal cord and neuropathic pain-like behaviours after nerve injury. With the use of a functional magnetic resonance imaging analysis, we demonstrated that a single intrathecal injection of MCP-3 induced dramatic increases in signal intensity in pain-related brain regions. These findings suggest that increased MCP-3 expression associated with interleukin 6 dependent epigenetic modification at the MCP-3 promoter after nerve injury, mostly in spinal astrocytes, may serve to facilitate astrocyte-microglia interaction in the spinal cord and could play a critical role in the neuropathic pain-like state.
The sexual plasticity of the gonads is not retained after the completion of sex differentiation in vertebrates, except in some hermaphroditic species. Here, we report that the depletion of estradiol-17β (E2) by aromatase inhibitors (AI) for up to six months resulted in a functional female-to-male sex reversal in sexually-mature adults of two gonochoristic fish species, Nile tilapia and medaka. The sex-reversed fish showed a typical male pattern of E2 and androgen levels, secondary sexual characteristics, and male-like sex behavior, producing fertile sperm. Conversely, co-treatment of E2 inhibited AI-induced sex reversal. In situ hybridization of medaka gonads during AI-induced sex reversal indicated that cysts on the dorsal side of the adult ovaries are the origin of germ cells and Sertoli cells in the newly formed testicular tissue. Gonochoristic fish maintain their sexual plasticity until adulthood and E2 plays a critical role in maintaining the female phenotype.
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