Retinoic acid, steroids and bird meiosis

It is assumed that meiosis of female mammals is induced by retinoic acid (RA), which is inhibited in the males by CYP26B1. Unlike mammals, the molecular mechanisms of sex determination in birds remain largely unknown. In particular, the questions remain about the reasons why the sex cells of males and females enter meiosis during different stages of development, and what is the role of sex hormones in this process. To confirm the possible role of retinoic acid (RA) in the induction of meiosis in birds, we investigated the effect of the RA on meiosis of primordial germ cells (PGCs) when it was administered to chicken embryos at different stages of development. Obtained data permits us to suggest that at early stages of embryonic development RA in small dozes (12.5mkg) allows to overcome first meiotic block in spermatocytes I and permits the entry into a stage of zygotene for oocytes I when the synaptonemal complexes begin to form. There is evidence that birds having heterogametic female sex have the ability to adjust their offspring's sex ratios before fertilization occurs. Although there is evidence that adjustment occurs at both the primary and secondary levels (before and after fertilization, respectively), most research focuses on the potential hormonal mechanisms underlying adjustment of primary sex ratios, and thus the determination of the genetic sex. Also we tested the hypothesis that increased progesterone can bias sex ratios in the domestic hen. Progesterone treatment at 9.0 day of incubations can accelerate germ cell meiosis entry in the female chicken embryos and the common meiosis inducer RA was not involved in this process. We did not find a significant shift of the sex ratio after these preliminary experiments. These results suggest the existence of progesterone as additional inducer of meiosis in embryonic germ cell and independently of RA signaling in the chicken.