First report of mule-in-mule pregnancies with live births following embryo transfer

• Published in: Journal of equine veterinary science 2022-06, Vol.113, p.103999, Article 103999
• Main Authors: Fanelli, D, Losinno, L, Castañeira, C, Alonso, C, Bragulat, AF, Panzani, D, Bocci, C, Degl'Innocenti, A, Moroni, R, Camillo, F, Wilsher, S, (Twink) Allen, WR
• Format: Article
• Language: English
• ISSN: 0737-0806; 1542-7412
• DOI: 10.1016/j.jevs.2022.103999

Many of the equid species can interbreed to produce viable but usually infertile offspring, with the mule (♂donkey [2n=62] × ♀mare [2n=64]) being the commonest of these hybrids. Despite its reduced ability to breed successfully, the mule has been shown capable of carrying and rearing horse and donkey foals using between-species embryo transfer (ET). These pregnancies have provided insight into the early development of the conceptus by highlighting the role of the uterine environment on endometrial cup development and lifespan (Allen et al., Reproduction, 1993); 98(1), 55-60). The aim of this preliminary study was to see if a mule (2n=63) was capable of carrying a mule pregnancy to term, which to the authors' knowledge has not previously been reported. Mule embryos were produced in vivo, by inseminating donor horse mares with fresh donkey semen. The uteri of donor mares were flushed on either Day 7.5 or 8 post-ovulation and three grade 1 expanded blastocysts and one grade 1 early blastocyst were recovered. In Argentina, two embryos were transferred using the Wilsher ET technique into each of two anestrous recipient muleson day 8 after initiating treatment with IM estradiol 17β for 3 days followed by long-acting progesterone every 7 days until day 100 of gestation. In Italy, two embryos were simultaneously transferred by a conventional ET technique into a cyclic recipient mule on day 5 after ovulation. The transfers resulted in two singleton pregnancies, in Argentina, and a twin pregnancy, which was manually reduced to a singleton, in Italy. Repeated ultrasound examinations showed normal conceptus and, subsequently, fetal development to term. Spontaneous delivery of three female mule foals occurred on Days 355, 342, and 335 of gestation, respectively and no placental abnormalities were recorded. No complications occurred during either parturition or peripartum, and the resulting mule fillies developed normally to weaning. To the best of the authors’ knowledge, this is the first report of mules foaling mules after embryo transfer. More studies are warranted to describe the hormonal profile of mule-in-mule pregnancy, especially equine Chorionic Gonadotrophin (eCG) which is known to be influenced by both maternal and fetal factors.