Expression pattern of the maternal factor zygote arrest 1 (Zar1) in bovine tissues, oocytes, and embryos

Zygote arrest 1 (Zar1) is an ovary-specific maternal factor that plays an essential role during the oocyte-to-embryo transition in mouse. In this species, Zar1 expression is strictly limited to the oocyte, the zygote and, at a lower level, the 2-cell embryo. Aim of the present study was to analyze the presence and the expression pattern of the Zar1 ortholog in bovine tissues and embryos. Reverse transcription (RT)-polymerase chain reaction (PCR) analysis was performed in a panel of bovine tissues, in oocytes and pre-implantation in vitro produced embryos. The results demonstrated that a Zar1 ortholog is present in cattle. In the adult, the gene is expressed in ovary, testis, muscle, and myocardium. The gene is also expressed in the oocyte, the zygote, and in all the stages of embryonic development until blastocyst formation. A semi-quantitative RT-PCR analysis revealed that Zar1 levels are constant through in vitro development with the exception of the 4-cell stage, when a significant increase is observed. The exposure of fertilized oocytes to the RNA polymerase II inhibitor alpha-amanitin was able to suppress this Zar1 increase indicating that transcription of this gene occurs at the 4-cell stage. Zar1 is conserved in cattle but has an expression pattern different from the mouse. In particular, Zar1 expression in the adult is not limited to the ovary and in the embryo is expressed well beyond the oocyte to embryo transition. Moreover, the identification of Zar1 transcription at the 4-cell stage represents the first characterization of one of the genes expressed in cattle embryos before the major onset of embryonic transcription.