Mating Pattern and Chromosome Analysis of a Mule and Her Offspring

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Abstract
The aim of this report is to describe the case of a fertile mule. The chromosomes of the mule and her offspring, and of six foals sired by two of these offspring, were analyzed. Gonadotropin and progestogen production were examined during two pregnancies. One conceptus was a horse and the other was a mule. Chromosome analysis of the dam showed a diploid number of 63 chromosomes with one horse-like and one donkey-like X chromosome. 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 karyotypes of foals sired by donkeys showed a pattern identical to that of a mule. All these karyotypes argue in favor of chromosome segregation during the proband meiosis, in which the horse chromosomes are preferentially kept in the ootide nuclei. Peak concentrations of plasma eCG levels of the dam when carrying a horse conceptus were at least 3.5 times higher than those produced when she was carrying a mule conceptus, showing that influence of fetal genotype on eCG levels also occurred in this unusual model. A difference in progesterone plasma levels was detected between the two pregnancies.