The defect in the XY oocyte remains to be clarified. Hunt and LeMaire have reported a high frequency of precocious dissociation between the X and Y chromosomes in oocytes isolated from the B6.Ypos female mouse at the diakinesis/metaphase I stage. Accordingly, it is conceivable that the B6YTIR female may lose its embryos because of aneuploidy. However, the XO female mouse is invariably fertile although it lacks the pairing partner for the single X chromosome and suffers from a high incidence of aneuploidy and embryo death. Furthermore, some strains of XY sex-reversed female mice demonstrate high frequencies of aneuploidy and yet produce litters, albeit in small numbers. Therefore, the rate of aneuploidy must be very high to be the sole cause of the complete absence of progeny in the Вб.У™ female. Actual aneuploidy in matured or fertilized eggs from the B6.YTIR female remains to be examined.
Reproductive Life Length of XX BALB/c XY B6. V™ Chimeric Females
We have previously reported that the ovaries of the B6.YTIR female consistently lose their oocytes by 2 mo of age . In contrast, the XX BALB/c XY B6.YTIR chimeric females that were dominated by XY cells conceived up to over 4 mo of age in the present study. Therefore, it appears that introduction of normal XX cells prolonged the reproductive life of the B6.YT1R ovary to some extent. Nonetheless, the chimeric ovary had been deprived of all but a few oocytes by 5 mo of age, much earlier than the normal XX ovary. It remains possible that XY somatic cells exert long-term effects on reproductive functions. Alternatively, the number of XX oocytes may limit the reproduction life length.
In conclusion, the XY chromosomal composition of the oocyte appears to be responsible for the infertility of the B6.YTIR female mouse. On the other hand, the presence of XY somatic cells allows ovarian differentiation to support the XX oocyte for producing live-boms.