RA mediates meiosis onset in germ cells of Nile tilapia, Oreochromis niloticus

Abstract： Recent studies have implicated that retinoic acid (RA) signal pathway played a critical role in meiosis initiation in mammals, birds and amphibians. However, the role of RA in meiosis initiation of teleosts remains to be elucidated. In the present study, RA synthetase (aldh1a2) and catabolic enzymes (cyp26a1 and cyp26b1) were cloned from the Nile tilapia. Tissue distribution analysis showed that aldh1a2, cyp26a1 and cyp26b1 were expressed both in gonadal and non-gonadal tissues (heart, liver, spleen, intestine, kidney, muscle, brain, gill, testis and ovary). Expression of aldh1d2 and cyp26a1 showed significant sexual dimorphism in gonads, while cyp26b1 showed no difference between ovary and testis. Moreover, ontogeny study revealed that aldh1a2 was up-regulated just before meiosis initiation of germ cells in XX and XY gonads. Expression level of cyp26a1 was very low and showed no significant difference between XX and XY gonads at 20 and 30 dah (day after hatching). Nevertheless, it was consistently low in XY gonads while up-regulated in XX gonads from 90 to 180 dah. Expression of cyp26b1 was very low in both XX and XY gonads at all sampling points except for 30 dah when it was significantly up-regulated in XY gonads. Treatment of XX fish by citral (RA synthetase inhibitor) alone or together with RA were performed from 5 to 70 dah. Citral treatement resulted in delayed meiosis initiation and partial or complete sex reversal. On the contrary, RA rescued citral treated fish and induced normal meiosis initiation and oogenesis. Immunohistochemistry analysis in the ovotestis showed that aromatase signals were detected in interstitial cells in ovarian tissues while Dmrt1 signals were confined to Sertoli cells in testicular tissues and around degenerating oocytes. Furthermore, aldh1a2 and meiosis marker scp3 were up-regulated in premeiotic phase of germ cells in sex reversed gonads. In contrast, cyp26a1 was down-regulated while cyp26b1 remained relatively stable throughout the examined time points in citral treated fish. These results indicated that RA is indispensable for meiosis initiation, and probably Cyp26a1, instead of Cyp26b1, was involved in meiosis initiation of germ cells in fish. In conclusion, the role for RA in meiosis initiation may be conserved in vertebrates.