The mule: An alternative as an equine embryo recipient

Abstract

Interest in interspecific pregnancies within the Equus genus has recently increased, due to the use of reproductive biotechnologies such as embryo transfer (ET). Among them, the mule (Equus mulus) in particular, due to its biological, ethological, and reproductive characteristics; has proven to be potentially advantageous for commercial ET programs. Infertility inmulesresults from the incompatibility between paternal (Equus asinus2n = 62) and maternal (Equus caballus2n = 64) chromosomes. However, cases of cyclical (ovulatory) mules, and even fertile females with offspring have been documented. It has been verified that mules are capable of forming functional endometrial cups compared to other xenogeneic transfers, where there is a failure in the formation and invasion of the endometrial cups. These pregnancies often lead to a natural interruption, due to the maternal immune response to the fetal genotype. Estrous cycle, follicular dynamics and embryonic-fetal development are similar to those in mares. A suitable maternal instinct has been described in recipient mules towards the foals, such as sufficient milk production, great neonatal care and protection. It is essential to know the differential hormonal protocols tested, both for cyclic and acyclic mules to be used as embryo recipients. In summary, the scientific and empirical evidence accumulated to date shows that mules can be utilized as an alternative for equine embryo recipients in commercial programs, especially in marginal and semi-arid ecosystems.