Two Foxo1 homologues in the orange-spotted grouper Epinephelus coioides: sequences, expression, and possible involvement in the activation of cyp19a1a expression in the ovary

Understanding Genetic Regulation of Sex Differentiation in Hermaphroditic Fish

As a fundamental taxonomic group within vertebrates, fish represent an invaluable resource for investigating the mechanisms underlying sex determination and differentiation owing to their extensive geographical distribution and rich biodiversity. Within this biological cohort, the processes of sex determination and differentiation are intricately governed by both genetic factors and the complex interplay of environmental cues. While variations in external environmental factors, particularly temperature, can exert a modulatory influence on sex differentiation in fish to a limited degree, genetic factors remain the primary determinants of sexual traits. Hermaphroditic fish display three distinct types of sexual transitions: protandry (male to female), protogyny (female-to-male), bidirectional sex change (both directions serially). These fish, characterized by their unique reproductive strategies and sexual plasticity, serve as exemplary natural models for elucidating the mechanisms of sex differentiation and sexual transitions in fish. The present review delves into the histological dynamics during gonadal development across three types of sequential hermaphroditic fish, meticulously delineating the pivotal characteristics at each stage, from the inception of primordial gonads to sexual specialization. Furthermore, it examines the regulatory genes and associated signaling pathways that orchestrate sex determination and differentiation. By systematically synthesizing these research advancements, this paper endeavors to offer a comprehensive and profound insight into the intricate mechanisms governing sex differentiation in sequential hermaphroditic fish.

FoxH1 Represses the Promoter Activity of cyp19a1a in the Ricefield Eel (Monopterus albus)

Forkhead box H1 (FoxH1) is a sexually dimorphic gene in Oreochromis niloticus, Oplegnathus fasciatus, and Acanthopagrus latus, indicating that it is essential for gonadal development. In the present study, the molecular characteristics and potential function of FoxH1 and the activation of the cyp19a1a promoter in vitro were evaluated in Monopterus albus. The levels of foxh1 in the ovaries were three times higher than those in the testes and were regulated by gonadotropins (Follicle-Stimulating Hormone and Human Chorionic Gonadotropin). FoxH1 colocalized with Cyp19a1a in the oocytes and granulosa cells of middle and late vitellogenic follicles. In addition, three FoxH1 binding sites were identified in the proximal promoter of cyp19a1a, namely, FH1 (-871/-860), FH2 (-535/-524), and FH3 (-218/-207). FoxH1 overexpression significantly attenuated the activity of the cyp19a1a promoter in CHO cells, and FH1/2 mutation increased promoter activity. Taken together, these results suggest that FoxH1 may act as an important regulator in the ovarian development of M. albus by repressing cyp19a1a promoter activity, which provides a foundation for the study of FoxH1 function in bony fish reproductive processes.

Transcription factors Dmrt1a, Foxl2, and Nr5a1a potentially interact to regulate cyp19a1a transcription in ovarian follicles of ricefield eel (Monopterus albus)

Aromatase (encoded by Cyp19a1) in the ovarian follicular cells catalyzes the production of estradiol from testosterone, which plays important roles in the ovarian development of vertebrates. In the present study, the interaction of Dmrt1, Foxl2, and Nr5a1a on the regulation of cyp19a1a transcription in ovarian follicles was examined in a teleost, the ricefield eel Monopterus albus. The expression of dmrt1a, foxl2, and nr5a1a was detected in ovarian follicular cells together with cyp19a1a at the mRNA and/or protein levels. Sequence analysis identified one conserved Foxo binding site in the proximal promoter region of ricefield eel cyp19a1a. Transient transfection assay showed that Foxl2 may bind to the conserved Foxo site to activate cyp19a1a transcription and act synergistically with Nr5a1a. Mutation of either the conserved Nr5a1 site or Foxo site abolished or significantly decreased the synergistic effects of Nr5a1a and Foxl2 on cyp19a1a transcription. The sequence between Region III and I-box of Nr5a1a was critical to this synergistic effect. Dmrt1a modulated the Foxl2- and Nr5a1a-induced activation of cyp19a1a transcription and their synergistic effects in a biphasic manner, with inhibitory roles observed at lower doses (10 to 50ng) but release of the inhibition or even potentiating effects observed at higher doses (100 to 200ng). Collectively, data of the present study suggest that the interaction of Dmrt1a, Foxl2, and Nr5a1a in the ovarian follicular cells may facilitate the adequate expression of cyp19a1a and the production of estradiol, and contribute to the development and maturation of ovarian follicles in ricefield eels and other vertebrates as well.