Dynamic transcriptome analysis of ovarian follicles in artificial maturing Japanese eel (Anguilla japonica)

Inducing maturation of the ovaries to enable the production of good-quality eggs is critical for the successful artificial breeding of Anguilla japonica. During the spawning season, however, the ovaries of A. japonica have been found to develop into asynchronous clutches, impeding the success of artificial breeding on a commercial scale. The dynamic molecular regulation of follicular development in the same individual was assessed by transcriptome analysis of the five stages of follicles, the pre-vitellogenic, early vitellogenic, midvitellogenic, late vitellogenic, and migratory nucleus stages in artificial maturing A. japonica. Comparisons across these developmental stages identified a total of 19,298 differentially expressed transcripts (DETs). Short time-series expression miner analysis across these DETs revealed four significant expression profiles. Gene Ontology function and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses found that some of the significantly enriched biological processes and metabolic pathways included those related to steroid hormone biosynthesis (cyp11a1, cyp17a1, cyp17a2, hsd17b1, and hsd17b12), cargo receptor activity (vtgr and vldlr), meiosis and ovulation (pgrs and mPRγ), hydration (cts and aqp1), and egg coat formation (zp). These genes and pathways were associated with serum 17β-estradiol concentrations and morphological changes. The levels of hsd17b12 and mPRγ mRNAs were much higher during the migratory nucleus stage, suggesting their respective involvement in the biosynthesis and functional pathway of the maturation-inducing steroid 17α,20β-dihydroxy-4-pregnen-3-one. The gene subtypes aqp1b and ctsd may regulate water influx into oocytes and yolk protein proteolysis, respectively. To our knowledge, the present study is the first to describe combined transcriptome profiling of asynchronously developing follicles in the same individual. The findings suggest that steroid hormone synthesis and nutrient absorption in follicular somatic cells play important roles during follicular development and maturation, despite the same external physiological surroundings.