PubMed2001
An investigation was carried out on a family of Limousin-Jersey crossbreds exhibiting low fertility in the females, to determine the impact of a previously identified X-autosome translocation (X-AT) on the reproductive performance of the carrier cows. Three of the identified translocation carriers, including a cow and two of her daughters, were maintained at our University Research Station and artificially inseminated periodically with semen from different bulls of known fertility. Attempts to breed the X-AT carriers resulted in high rates of return to estrus between days 28 and 60, abortions between days 121 and 235 after insemination, and a total of 13 live births including 4 translocation carrier calves. Results of superovulation and embryo retrieval trials on X-AT carriers revealed significantly higher proportions of unfertilized and uncleaved ova and abnormal embryos compared to those from normal cows, and no pregnancy in the recipients transferred with morphologically normal blastocysts from X-AT carriers. While the higher rates of failed fertilization and cleavage, abnormal embryos and return to estrus in X-AT carriers could be attributed to chromosome imbalance expected in their gametes, the relatively high prevalence of abortion (late in gestation) was unexpected. Our observations on the fetuses expelled by X-AT carriers after 5 months of gestation indicated that a majority (three out of four) of these fetuses were products of abnormal (3:1) segregation in meiosis I and that these chromosomally unbalanced (hyperdiploid) conceptuses were able to survive early embryogenesis and fetal life up to the end of the second trimester. We hypothesize that their relatively long in utero life and the absence of any overt birth defects may be attributable to the type of chromosomes over-represented in these fetuses and that their eventual expulsion may have been the result of selection against the clonal population of cells in which the altered X carrying a segment of chromosome 23 (Xp(+)), remained inactive.