Modulation of chronic hypoxia on ovarian structure, oxidative stress, and apoptosis in female Nile Tilapia (Oreochromis niloticus)

Abstract

Chronic hypoxia in aquaculture impairs gonad growth and reduces the reproductive capacity of farmed fish. The aim of this study was to investigate how prolonged hypoxia affects gonad maturation, the degree of oxidative stress, and apoptosis in female Nile Tilapia. We conducted an 85-day experiment in which female Nile tilapia were cultured with dissolved oxygen (DO) levels of 6.0 ± 0.1, 4.0 ± 0.1, and 2.0 ± 0.1 mg/L, designated as the normal-DO group (NG), the low-DO group (LG), and the hypoxia-stressed group (HG), respectively. The results show that chronic hypoxia reduced the specific growth rate and gonadosomatic index of female Nile tilapia. The chronic hypoxia significantly affects the final body weight, weight gain rate, feed conversion ratio, hepatosomatic index and gonado somatic index, compared with the fish in the NG, those in the HG and LG had significantly lower values for all growth indicators (P < 0.05). The gonads of fish in the LG and HG did not reach maturity, whereas those of fish in the NG developed normally. This was related to their higher levels of follicle-stimulating hormone, luteinizing hormone, and estradiol, causing their oocytes to reach stages IV and V. In contrast, the majority of oocytes in fish in the LG were in stages II and III, and the presence of more atretic follicles in the ovary of fish in the HG indicated that their gonads were still immature. Compared with fish in the NG and LG, those in the HG showed increased malondialdehyde levels in ovarian tissues and decreased activities of superoxide dismutase and catalase. The transcript levels of genes related to the nrf2 signaling pathway (nrf2, keap1) and apoptosis (nfκb, bcl-2, caspase-3, caspase-8) in the ovary differed among the three groups. The nrf2-bcl2 pathways were also positively correlated with GSI. The pathogenic changes were linked to chronic hypoxia, according to the results of the PCA, and the negative consequences were more severe for the fish in the HG (DO 2.0 mg/L) than for those in the NG and LG. Together, our results show that apoptosis and the antioxidant system are dysregulated under chronic hypoxia stress, and this may be responsible for impaired gonad development in hypoxia-stressed female Nile tilapia.