Mammalian ovarian germ cells initiate meiosis earlier than testicular germ cells. Stra8 (Stimulated by Retinoic Acid Gene 8) is required for pre-meiotic DNA replication and entry into meiosis. In the mouse, retinoic acid (RA) diffuses from the adjoining mesonephros into the ovary, promoting Stra8 expression and entry of germ cells into meiosis, whereas in the testis Cyp26b1 expression within the Sertoli cells inhibits pre-natal Stra8 expression. Premature initiation of meiosis can be induced in mice by culturing fetal testes with exogenous RA or a Cyp26 inhibitor; conversely, culture of prenatal ovaries in the presence of a RA receptor antagonist prevents entry into meiosis. There have been conflicting data on several aspects of the RA/Stra8/Cyp26b1 pathway in mice and data from human and chicken suggest that the mouse may not be a truly representative model for the control of entry into meiosis in vertebrates.
Using a marsupial, the tammar wallaby, we have examined aspects of the RA/STRA8/CYP26 pathway on entry into meiosis. In the tammar, meiosis starts at day 25 post-partum (pp) in females and from 19 months of age in males. Tammar gonads express aldehyde dehydrogenase and RA receptor proteins and so have the capacity to synthesise and respond to RA before and after the onset of meiosis. The timing of tammar STRA8 expression relative to germ cell sexual differentiation is similar to the mouse but tammar STRA8 expression is not affected by culturing Day 22 pp ovaries with an inhibitor of RA synthesis (citral) or Day 22 pp testes with either exogenous RA or a CYP26 inhibitor.
Our data on the tammar supports the idea that although the RA/STRA8/CYP26 pathway has a role in the control of meiosis in mouse, human and chicken, there is considerable variation in several aspects of this pathway among species.