Some ‘white’ oocytes undergo atresia and fail to mature during the reproductive cycle in female genetically improved farmed tilapia (Oreochromis niloticus)
2021
Qiang, Jun | Duan, Xue-Jun | Zhu, Hao-Jun | He, Jie | Tao, Yi-Fan | Bao, Jin-Wen | Zhu, Xiao-Wen | Xu, Pao
Low fecundity and unsynchronized spawning cycles in tilapia contribute to low broodstock productivity, a major factor limiting tilapia culture production and expansion. Previous studies have reported that ‘white’ oocytes in the ovarian tissue of tilapia continue to develop to maturity during the next spawning period. To test this hypothesis we use hematoxylin-eosin staining, terminal deoxynucleotidyl nick-end labeling, and electron microscopy to analyze development of ‘white’ oocytes in genetically improved farmed tilapia (GIFT: Oreochromis niloticus) females over time (1, 7, 13, 19, and 25 days) during one reproductive cycle, and real-time PCR to identify expression of autophagy- and apoptosis-related genes in gonad tissue. Only some of these ‘white’ oocytes developed to maturity and discharge, while many underwent atresia during development. Atretic follicles were detected at various stages of oocyte development, and the number of ‘white’ oocytes continued to increase during the reproductive cycle. On days 19 and 25 during one reproductive cycle, many apoptotic granulosa cells were distributed around stage IV and V oocytes; the integrity of granulosa cells was poor, organelles were deformed and ruptured, and the number of autophagic lysosomes was significantly increased. We detect increased transcript levels of Caspase-3 and Becline-1 and a reduced Bcl-2/Bax ratio in ovarian tissues on day 25 of culture. Our results show that autophagy and apoptosis of granulosa cells participate in the atresia of GIFT follicles. These findings provide a theoretical basis for further improving the reproductive efficiency of tilapia and for reducing follicular atresia.
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