Postmeiotic transcription of X and Y chromosomal genes during spermatogenesis in the mouse

During the meiotic prophase of spermatogenesis, the X and Y chromosomes form the heterochromatic sex body, showing little transcriptional activity. It has been suggested that transcription of the Xist gene is involved in this inactivation. After completion of the meiotic divisions, at least two Y chromosomal genes, Zfy and Sry, are transcribed in haploid spermatids. In contrast, postmeiotic transcription of X chromosomal genes has not been demonstrated. Using highly purified preparations of mouse pachytene spermatocytes, round spermatids, and cytoplasmic fragments from elongated spermatids, the present experiments show differential postmeiotic expression of the Y chromosomal genes Ubely and Sry, with highest mRNA levels in round spermatids and cytoplasmic fragments, respectively. Postmeiotic transcription of the X chromosomal gene Ube1x is indicated by an increased level of Ube1x mRNA in round spermatids and cytoplasmic fragments. The X chromosomal gene MHR6A shows a marked temporary postmeiotic expression in round spermatids. This postmeiotic activity of the X chromosome is a novel finding, which may have implications for our understanding of X chromosome inactivation during spermatogenesis and paternal genome imprinting.