Development and organisation of gonadal steroidogenesis in bony fishes - A review

Gonadal steroidogenesis is pivotal to synchronize various reproductive stages including sexual development,growth and maturation.In all vertebrates including teleost,steroidogenesis is triggered by the mobilization of cholesterol by steroidogenic acute regulatory protein from outer to inner mitochondrial membrane.Thereafter,the entire process occurs in endoplasmic reticulum or mitochondria wherein various steroidogenic enzyme genes play a crucial role.The onset of steroidogenesis during sexual development in teleost is essentially regulated by differential expression of several transcription as well as steroidogenesis-related factors.More specifically,the role of dmrt,sox9,sox3,other sox forms,ad4bp/sf-1,wt-1,foxl2,ftz-f1,gata4,gsdf,Activator protein-1,fgfs and growth factors and steroidogenic enzymes such as cytochrome P450aromatase(cyp19a1),hydroxysteroid 3β-dehydrogenase(hsd3b),hydroxysteroid 17β-dehydrogenase(hsd17b)have been well characterised.Recently,the role of pax2,THO complex(thoc),pentraxin(ptx)and few signalling molecules like wnt4/5 regulating teleostean steroidogenesis has been reported.In females,cyp19a1 appears to be critical as it converts androgens to estrogens during ovarian differentiation which suggests that estradiol-17βis indispensable for the event.Unlike females,males do not depend on testosterone for testicular determination,yet has a major role along with 11-ketotestosterone in testicular development and growth when compared to early testicular differentiation.In males,onset of sex determining or testis-related genes seems most essential.Considering these,the regulation of steroidogenesis is virtually critical at later stages.In view of this,several steroidogenic motifs pertaining to transcriptional regulation have been analysed in teleost,yet far below than the reports on mammals.In this context,the regulatory influence on HHG axis plays a critical role in teleost.Further,multiple gonadotropin-releasing hormones and duality of gonadotropins play a differential role in gonadal function.As for steroidogenic pathway,the synthesis of sex steroids predominantly usesΔ4,however,Δ5 pathway was also evident.The next aspect is shift in steroidogenesis that usually occurs in the maturing follicles during final oocyte maturation,yet such a mechanism is not clear in males.The review highlights the interactions of steroidogenic enzyme gene regulation in terms of HHG axis and other important transcription factors that are involved in the regulatory pathways as well as the influence of environmental and dietary factors by comparing both sexes to present a holistic view on steroidogenesis onset and organisation during gametogenesis in teleost.