Short R V
J Reprod Fertil Suppl. 1975 Oct(23):359-64.
The infertility of the mule has proved a continuing challenge to scientific thought. Since the chromosomal differences between the two parental species are so great as to render normal meiosis impossible, it is postulated that all mules and hinnies are sterile. The problem now is to explain how mules and hinnies can occasionally produce spermatozoa or ova. The appearance of the mule was sufficient to persuade the ancients that both parents, not just the male, must contribute to the make-up of the offspring. The mule has also taught us that, when the number of oocytes in the ovary is reduced, the ovary becomes the time-clock that regulates the length of the oestrous cycle. A study of gonadotrophin production in horses and donkeys bearing hybrid foals has yielded fascinating results about the immunology of pregnancy. The behaviour of the sex chromosomes of female mules and hinnies has helped to confirm the Lyon hypothesis about X-chromosome inactivation. The mule has also provided spectacular proof of the doctrine of hybrid vigour, and the marked deficiency of males at birth confirms the general truth of Haldane's Law. The mule and the hinny remain man's only successful attempt at the production of a commercially viable interspecific mammalian hybrid.
骡子的不育问题一直是科学思维面临的挑战。由于两个亲本物种之间的染色体差异极大，使得正常减数分裂无法进行，因此据推测所有骡子和驴骡都是不育的。现在的问题是要解释骡子和驴骡为何偶尔能产生精子或卵子。骡子的出现足以让古人相信父母双方，而不仅仅是雄性，都对后代的构成有贡献。骡子还让我们明白，当卵巢中的卵母细胞数量减少时，卵巢就成为调节发情周期长度的生物钟。对怀有杂交驹的马和驴的促性腺激素产生情况进行的研究，得出了有关妊娠免疫学的有趣结果。雌性骡子和驴骡的性染色体行为有助于证实关于X染色体失活的莱昂假说。骡子还为杂种优势学说提供了惊人的证据，而出生时雄性明显不足则证实了霍尔丹法则的普遍正确性。骡子和驴骡仍然是人类在生产具有商业可行性的种间哺乳动物杂种方面唯一成功的尝试。
