Evidence for RA-dependent meiosis onset in a turtle embryo.

Meiotic entry is one of the earliest sex determination events of the germ cell in higher vertebrates. Although advances in meiosis onset have been achieved in mammals, birds and fish, how this process functions in reptiles is largely unknown. In this study, we present the molecular analysis of meiosis onset and the role of retinoic acid (RA) in this process in the red-eared slider turtle. Our results using Stra8 as a pre-meiosis indicator show that in the female embryonic gonad, meiosis commitment starts around stage 19. Additionally, signals of the meiosis marker Sycp3 could be detected at stage 19 and become highly expressed by stage 23. No expression of these genes was detected in male embryonic gonads, suggesting the entry into meiosis prophase I was restricted to female embryonic germ cells. Notably, RA activity in fetal gonads is likely to be elevated in females than that in males, as evidenced by the higher expression of RA synthase Aldh1a1 and lower expression of RA-degrading enzyme Cyp26a1 in female gonads prior to meiotic entry. In addition, exogenous RA treatment induced the expression of Stra8 and Sycp3 in both sexes, whether in vivo or in vitro. Together, these results indicate that high levels of RA in the embryonic female gonads can lead to the initiation of meiosis in the turtle.