Transcriptome analysis provides new insights into ovarian follicle growth in common carp (Cyprinus carpio)

Ovarian growth is a critical phase of oogenesis that involves several events such as steroidogenesis, lipid deposition, and vitellogenesis. Although many studies have been conducted on fish ovarian growth and development, the molecular mechanisms that contribute to steroidogenesis and lipid deposition in common carp, the world's third largest freshwater aquaculture fish, remain elusive. In this study, transcriptome analyses were performed on ovaries from Yellow River carp at three different developmental stages: primary growth stage (PG), pre-vitellogenesis stage (PV), and mid-vitellogenesis stage (MV). A total of 22,511 genes were detected, and of these 122, 253, and 278 were differentially expressed genes (DEGs) when comparing PG to PV, PV to MV, and PG to MV stages, respectively. Gene ontology (GO) and KEGG enrichment analysis showed that the DEGs were mainly involved in steroidogenesis (e.g., fshr, cyp19a1a, hsd3b and foxl2b), lipid deposition (e.g., lpl, slc27a6, fabp4, acsl1a, agpat3, plpp3 and dgat1b), and vitellogenesis (e.g., lrp13, plb1, dgkaa and pik3r1). We also identified many signalling pathways involved in ovarian growth, including steroid hormone biosynthesis, the phospholipase D signalling pathway, glycerophospholipid metabolism, the Notch signalling pathway, the TGF-beta signalling pathway, and the Wnt signalling pathway. qPCR results of 9 representative genes related to steroidogenesis and lipid deposition verified the reliability of the generated RNA-seq data. Finally, a potential model for the formation of lipid droplets and yolks in the ovaries of common carp was proposed. Overall, the data from this study will contribute to further research on the molecular regulatory mechanisms of ovarian growth and development in common carp and other fish species.