Why does a mule have 63 chromosomes

Blood samples of six foals of mares, sired either to the horse-like first or second offspring of the mule, were also taken for somatic karyotyping and chromosome analysis. Chromosome analysis of the dam confirmed that she was a mule, with a diploid number of 63 chromosomes and two X chromosomes, one of horse and the other of donkey origin, according to C band patterns. The karyotypes of the foals sired by stallions gave a diploid number of 64, and the individual chromosomes were typically horse-like in appearance.

The foals sired by jack donkeys were a male and a female, phenotypically mule-like and having 63 chromosomes 63 XX and 63 XY , of which the karyotypes were similar to that of the mule. Finally, the six offspring sired by two sons of the original mule had karyotypes similar to that of Equus equinus 64 paired chromosomes. The first foal was suckling in the paddock when discovered, but the second and third pregnancies were monitored closely and the second parturition was assisted.

Other pregnancies were monitored sporadically. The three foals sired by a horse stallion two male [ and ] and one female [late ] were phenotypically horse-like, and all had a dark stripe along the midline of their backs; the second foal male also showed a trace of a cross on his shoulders and transversal dark lines on the distal parts of his legs.

The sire of the and foals did not show either of these coat markings, but as his ancestors were unknown, it cannot be ruled out that one of them belonged to a breed that normally displays these characteristics.

The female born late in showed long ears proportionally to her body size and transversal dark lines on the distal parts of her legs. The sire of the horse-like female belonged to a breed that normally does not show these characteristics. The offspring of mares sired by the two sons of the mule were all phenotypically like horses.

The scheme of karyotypes of all foals and the mule dam is shown in Figure 1. The karyotype of the mule dam is shown in Figure 2. Karyotype of a mule mated by stallions and jacks and her offspring. Resumption of ovarian activity in the mule dam occurred within 15 days postpartum as indicated by follicular growth and estrous behavior.

She conceived to matings during the foal heat on three occasions and during the second and third postpartum estrous periods on the other occasions. During the last third of both pregnancies, the ovaries were very small and inactive. Strong maternal behavior and adequate udder development and milk production were observed after each birth. However, no samples were available before Day 84, and it is likely that the true peak in eCG levels occurred earlier in pregnancy and was therefore missed.

Progesterone levels see Fig. During this overlapping period between Days and , progesterone concentrations of the two pregnancies were substantially different. The pattern of chromosomic segregation at meiosis in the case of the mule seems to be fully oriented toward horse chromosomes. The three karyotyped offspring sired by a stallion yielded 64 chromosomes, and two foals with 63 unpaired chromosomes were sired by two different jack donkeys.

With regard to chromosomes, there is no doubt that each mule offspring received almost entirely horse chromosomes from the mother. However, with the techniques available today, it is impossible to be sure that the chromosomes from both sets are homologous. Peripheral plasma eCG and progesterone levels of a mule mated by a jack donkey and a stallion.

For a mule to achieve fertility, a great number of biological events must occur simultaneously. Cytoplasmic organelles, most of them maternally inherited, play a crucial role in the maintenance of integrity of germ cells during gametogenesis in a mule. However, they alone are not responsible for the complete process, since very few mules are fertile. Rong et al. That fact argues in favor of an intrinsic involvement of the "crossing-over" process in which sections of the maternally inherited chromosomes may or may not be exchanged, thus altering their identity.

In this regard it is interesting that the horse-like Fl hybrid exhibited some donkey characteristics. How many chromosomes are fully identical in the two haploid sets Equus asinus and Equus cabalus? If none are, the possibility of random production of a euploid gamete could be estimated as just 1 in 2 However, as demonstrated by Gustavsson [] , there may be more than six totally identical pairs of chromosomes and an unknown number of partially identical ones.

These facts could increase the chance of a distinctive euploidy in particular cases. A pronounced influence of fetal genotype on eCG production has been shown in mares and donkeys carrying mule and hinny conceptuses. In an elegant experiment, D. Antczak and his colleagues used embryo transfer to create three horse-in-mule pregnancies and one donkey-in-mule pregnancy and then compared eCG secretion rates. These secretion rates were appreciably higher in the horse-in-mule pregnancies, although gonadotropic activity persisted in the blood to approximately the same stage of gestation in both types of pregnancy [ Davies et al.

In more recent studies, W. Allen and co-workers highlighted major differences in the width and overall development of the progenitor chorionic girdle as the underlying cause of the profound differences in the size of the endometrial cups and their secretion of eCG between equine pregnancies of different genotypes [ Allen, ].

From a comparison of the eCG profiles measured in the female mule while she was carrying conceptuses of different genotypes, it is clear that peak concentrations produced when she was carrying the horse conceptus were at least 3.

Thus, while the eCG profile for the mule carrying the horse conceptus was essentially similar to that in a mare carrying a normal intraspecies horse conceptus, a striking difference was apparent between the eCG profile in the mule carrying a mule conceptus and the profile displayed normally by a mare carrying a mule conceptus. This is a much longer persistence of eCG production than that occurring normally in mares carrying mule conceptuses [ Allen, b ], and it no doubt reflects a longer life span of the endometrial cups in the mule mother.

Once in a great while, all the possible errors are avoided, and a molly foals. Though even hybrids from species with the same number of chromosomes, like tigers and lions whose offspring are called ligers or tigons , have trouble with fertility.

Historic livestock and draft animals, Poitou donkeys are endangered but being revived by Texas ranchers Christopher Jones and Patrick Archer. Donkeys served as major contributors towards building the country, providing labor in areas such as farming, logging, and mining. Why Are Mules Sterile? Published on Dec 8, Tagged with: chromosomes donkeys jennys mollies mules.

A mule's cells can divide and make new cells just fine. Which is important considering a mule went from 1 cell to trillions of them! Chromosomes sort differently in regular cells than they do in sperm and eggs. Regular cells called somatic cells use a process called mitosis.

Mitosis is like the first step of meiosis. The chromosomes all make copies of themselves. But instead of matching up, they just sort into two new cells. So for the mule, each cell ends up with 63 chromosomes. No matching needs to happen. And our lone horse chromosome is fine. The other reason a mule is alive is that nothing on the extra or missing chromosome causes it any harm.

This seems obvious at first except that usually having extra DNA causes severe problems. In people, extra chromosomes usually result in miscarriages. Sometimes though, a child can survive with an extra chromosome. For example, people with an extra chromosome 21 have Down syndrome.

Having all of the extra genes on that extra copy of chromosome 21 cause the symptoms associated with Down syndrome. The extra genes must not be that big a deal for the mule.

In other words, the extra genes on the horse chromosome do not cause problems for the every day life of a mule. So mules are sterile because horse and donkey chromosomes are just too different. But they are alive because horse and donkey chromosomes are similar enough to mate. Simple meiosis animation Chromosomal changes in new species Mitosis vs.

A mule can not make sperm or eggs. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Summary 1. References Anderson, W. Google Scholar Makino, S. Google Scholar -, and M. Google Scholar Rothfels, K. Google Scholar —, and L.

Google Scholar Yamashina, Y. Google Scholar Download references. Trujillo View author publications. View author publications. Rights and permissions Reprints and Permissions.