Effects of sex chromosomes on the developmental potential of the mouse oocyte

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My long-term objective is to delineate how sex chromosomes affect the developmental potential of mouse oocytes. In mammalian development, germ cells undergo sexual differentiation according to their gonadal environment, testis or ovary, which is determined by the presence or absence of the Y-linked Sry gene. Therefore, spermatogenesis and oogenesis take place in the presence of XY and XX chromosomes, respectively. The gonadal sex also dictates the phenotypic sex by secretion of sex steroid hormones. When the gonadal sex is reversed, however, the germ cell sex becomes discordant with the chromosomal sex. Both XX males and XY females in humans are infertile, while XY females are fertile in certain rodent species. In Mus musculus, XY females are variably fertile depending on the cause of sex-reversal and genetic background. The B6.YTIR mouse, which develops into an infertile female with an intact Y chromosome, provides an excellent model for studying the effects of sex chromosomes on female fertility. We previously reported that the cause of infertility in the B6.YTIR female is intrinsic to its oocytes; they can reach the Metaphase II (MII) but fail to complete the second meiotic division and rarely develop beyond the zytote-stage after fertilization. This developmental incompetence can be attributed to their defective ooplasm; when XY oocyte nuclei have been transferred into enucleated XX oocytes, the reconstructed oocytes generate healthy offspring. Thus, the Y chromosome in the oocyte makes the ooplasm defective, which leaves the first meiotic division largely intact but disrupts the second meiotic division. In the next 5 years, we will investigate the molecular mechanisms underlying the second meiotic defects in the XY oocyte and investigate the involvement of Y-linked gene(s). (1) To characterize the nature of ooplasmic defects, whether it lies in a deficiency in critical components or the presence of detrimental components, we will transfer a small volume of cytoplasm from an XX oocyte into an XY oocyte or vice versa, followed by maturation in vitro, and examine the second meiotic division after parthenogenic activation (or fertilization). (2) To clarify the molecular mechanism underlying the failure in the second meiotic cytokinesis, we will compare the activity of ARP2/3 pathways, which are essential for maintaining the MII-spindle near the cytokinesis furrow, in XX and XY oocytes before and after activation. (3) To evaluate the role of Y-encoded transcription factor ZFY2 in the ooplasmic defects, we will knock-down its transcripts in the XY oocyte by siRNAs expression in vitro or by shRNAs expression from a transgene in vivo. The results of proposed studies will shed light on the mechanism coordinating the sister-chromatid separation and the second polar body extrusion, and evolution of the Y chromosome associated with the loss of fertility in Mus musculus XY females.