Temporal dynamics of oocyte growth and vitellogenin gene expression in zebrafish (Danio rerio)

Little is known about how hepatic vitellogenin gene (vtg) expression relates to oogenesis in fish, especially among fractional spawners. The objective of this study was to relate hepatic vtg 1A/B expression to stage-specific oocyte development in zebrafish (Danio rerio), an asynchronous spawning fish. Liver samples were collected at seven time points postspawning (1-32 days) and fish were preserved for subsequent histological analyses. Relative vtg 1A/B expression among liver samples was quantified by reverse transcription-quantitative PCR and oogenesis was evaluated following standard hematoxylin and eosin staining of serial ovarian sections. Histological analyses indicate that a subset of previtellogenic oocytes (stages 1-2) transitioned into postvitellogenic oocytes (stages 3-4) within 4 days (96 h) postspawning. By 8 days postspawning (192 h), the majority of the ovary was occupied by mature (stage 4) oocytes, a trend that continued through 32 days postspawning. Hepatic vtg 1A/B gene expression was upregulated 3.89-fold 1-h postspawning relative to the average gene expression across all time points, but was not correlated to stage-specific oogenesis. Follicular atresia among fish sampled 32 days postspawning highlights the importance of regular spawning in zebrafish and suggests that the event of spawning itself may be integral to the regulation of oocyte development.