Transcription of the sex-determining region genes Sry and Zfy in the mouse preimplantation embryo.

We have confirmed the faster growth of male preimplantation mouse embryos. We have also studied the transcription of Y chromosomal genes postulated to have a role in sex determination, using the highly sensitive technique of reverse-transcription polymerase chain reaction at these early stages. We find that two sex-determining region genes, Sry and Zfy, are transcribed during mouse preimplantation development, while the Zfy homologs Zfx and Zfa and a sex-determining region gene originally called A1s9 (now called Ube1y-1) are not. We also show that the anti-Müllerian hormone gene, which contains a Sry consensus binding element in its 5' promoter region, is not transcribed at this time. Developmental curves show that Sry and Zfy are expressed commencing at the two-cell stage. These results suggest that mammalian sex determination starts prior to gonad differentiation.