Morphological and Molecular Gonadal Sex Differentiation in the Wild Japanese eel Anguilla japonica.

Most cultured Japanese eels (Anguilla japonica) show male sex differentiation; however, natural gonadal sex differentiation has not been evaluated. In this study, this process was characterized in wild eels. Differentiated ovaries and testes were observed after the eels grew to 320 and 300 mm in total length, respectively. The youngest ovary and testis appeared at 3 and 4 years old, respectively; however, undifferentiated gonads were found up to 7 years, suggesting that sex differentiation was triggered by growth rather than aging. gsdf, amh, foxl2b and foxl3b were highly expressed in the testes, whereas figla, sox3, foxn5, zar1, and zp3 were highly expressed in the ovaries. The expression of cyp19a1a and foxl2a did not differ significantly between the testis and ovary. In the ovaries, the cyp19a1a and foxl2a levels were highest in the early stages, suggesting that their function is limited to early ovarian differentiation. The foxn5, zar1 and zp3 levels tended to increase in the later stages, suggesting that they function after the initiation of ovarian differentiation. In undifferentiated gonads, dimorphic gene expression was not observed, suggesting that the molecular sex differentiation phase is short and difficult to detect. These findings provide the first demonstration of the whole course of natural gonadal sex differentiation in eels at molecular and morphological levels.

Soy Isoflavones Induce Feminization of Japanese Eel (Anguilla japonica).

Under aquaculture conditions, Japanese eels (Anguilla japonica) produce a high percentage of males. However, females gain higher body weight and have better commercial value than males, and, therefore, a high female ratio is required in eel aquaculture. In this study, we examined the effects of isoflavones, genistein, and daidzein on sex differentiation and sex-specific genes of eels. To investigate the effects of these phytoestrogens on the gonadal sex, we explored the feminizing effects of soy isoflavones, genistein, and daidzein in a dose-dependent manner. The results showed that genistein induced feminization more efficiently than daidzein. To identify the molecular mechanisms of sex-specific genes, we performed a comprehensive expression analysis by quantitative real-time PCR and RNA sequencing. Phenotypic males and females were produced by feeding elvers a normal diet or an estradiol-17ß- or genistein-treated diet for 45 days. The results showed that female-specific genes were up-regulated and male-specific genes were down-regulated in the gonads, suggesting that genistein induces feminization by altering the molecular pathways responsible for eel sex differentiation.

Maternal transcripts in good and poor quality eggs from Japanese eel, Anguilla japonica-their identification by large-scale quantitative analysis.

Our understanding of maternal control of development in vertebrates remains incomplete. In this study, we investigated levels of maternal transcripts in good and poor quality eggs from artificially matured Japanese eel, using RNA-Seq and quantitative polymerase chain reaction (qPCR), to identify candidate maternal transcripts related to development. De novo assembly or mapping of reads to the eel draft genome yielded 619,029 contigs and 85,906 transcripts, respectively; normalized read counts to these assemblies were calculated using reads (RPKM) or fragments (FPKM) per kilobase of transcript per million mapped reads. In silico screening identified 1,594 contigs and 150 transcripts with lower RPKM or FPKM in poor than in good quality eggs, 245 contigs, and 85 transcripts of which could be annotated by BLASTx, respectively. From selected contigs or transcripts, six genes (dnajb4, gnpat, card14, pdp1, fcgbp, ttn) had significantly lower messenger RNA levels in poor than in good quality eggs by qPCR. Multiple regression analysis showed that five genes (gnpat, b4galnt1, acsl6, rtkn, trim24) significantly correlated with hatchability. Taken together, 10 genes were identified as candidate maternal transcripts, regulating development in Japanese eel. Our results contribute to understanding the molecular basis for maternal control of development in vertebrates.