Morphological and histological changes in the brains of turbot(Scophthalmus maximus)with gonadal development

The brain plays a critical role in controlling reproduction through the hypothalamus-pituitary-gonadal(HPG)axis in vertebrates.Turbot(Scophthalmus maximus)has become an economically important marine fish in Europe and North China.Previous research investigating turbot reproduction has focused on the role of the HPG axis in regulating egg and sperm production.However,the morphology and histology of the organs in the HPG axis have not been studied.In this study,we investigated the morphology and histology of brains in female and male turbot at different stages of gonadal development.The results showed that the brains of both female and male turbot were composed of seven parts that are typical of advanced teleosts:the telencephalon,diencephalon,cerebellum,hypothalamus,pituitary gland,myelencephalon,and olfactory bulbs.The telencephalon was well-developed and contained five distinct lobes,with the contiguous diencephalon at the caudal portion.The torus longitudinales and rostral torus semicircularis of the mesencephalon flattened along the dorsal surface,and the rostral corpus cerebellum was located in the dorsal portion.The actual total brain volume in mature males was significantly greater(p<0.05)than that of females with gonadal development.Notably,the pituitary volume in male turbot significantly increased(p<0.05)from immature to mature stage,but this difference did not occur in females.The data together illustrate a distinct sex difference in the turbot brain during gonadal development,providing insight into their HPG axes.

Changes of melatonin and its receptors in synchronizing turbot(Scophthalmus maximus)seasonal reproduction and maturation rhythm

In most fish,reproduction is seasonal or periodic under the suitable conditions.In turbot(Scophthalmus maximus)farms,one of the most economically important marine flatfish species,changes in daylength could cause changes in the spawning time.In this study,to characterize the regulation of reproductive physiology following light signals,three melatonin receptors(Mtnr)investigated in turbot were named sm Mtnr1,sm Mtnr2,and sm Mtnr1 c.Distinct expression profiles demonstrated that Mtnr m RNAs were concentrated in the brain(as detected in the hypothalamus(Hy)and mesencephalon(Me)),gonad and eye.The most abundant Mtnr1 and Mtnr2 m RNA expression levels were detected in the central nervous system at the beginning of the breeding season,suggesting that Mtnr1 and Mtnr2 may play vital roles in the regulation of turbot gonadal development.In addition,the melatonin profiles gradually increased and reached to the highest level at the spawning stage,indicating that melatonin is a potent hormone in the regulation of fish oocyte growth and maturation.The results of this study suggested that melatonin is the primary factor that transduces the light signal and regulates the physiological functions of turbot seasonal reproduction.Moreover,the results of this study may establish a foundation for further research seeking to identify fish melatonin receptors involved in the gonadal development and gamete maturation.