Donkey jennies usually make poor embryo transfer recipients because their post-transfer pregnancy rates are low. But the problem might be in the embryo delivery technique itself, which is better adapted for mares’ reproductive tracts, said Brazilian researchers.
In a much-celebrated event among equine breeding specialists, two healthy miniature donkey foals have recently been born to surrogate mother jennies of a different breed following the development of an embryo transfer new technique. That technique involves, mainly, a speculum, lubricants, a flashlight, and specially designed forceps that allow technicians to better navigate the embryo transfer gun through the jennies’ reproductive tracts, which longer and tighter than equine reproductive tracts.
“(Jennies) have a narrower cervix compared to mares, which makes it more difficult to do conventional embryo transfers,” said Lorenzo Segabinazzi, DVM, MS, PhD, at Ross University School of Veterinary Medicine, Basseterre, the West Indies, previously associated with the Department of Veterinary Surgery and Animal Reproduction in the School of Veterinary Medicine and Animal Science at São Paulo State University in Botucatu, Brazil.
“It’s a challenge to manipulate their cervix for embryo transfer,” Segabinazzi said. “I believe that’s why we see such poor results generally with embryo transfers in jennies; I think it’s related to this technique involved in getting through the cervix.”
A New Job for Unemployed Jennies?
The primary motivation for this study arose from a desire to help make a native Brazilian donkey breed more useful again, said Segabinazzi. Brazilian Northeastern jennies long worked in the country’s transportation and agriculture industries, but in recent decades they’ve been cast aside, even becoming a nuisance in urban areas, now that machinery has replaced them on the roads and fields, he said.
However, they could serve as excellent surrogate mothers for other donkey breeds in higher demand, such as miniature donkeys, Segabinazzi explained. But that’s only possible if scientists can perfect the embryo transfer technique in donkeys, which so far has resulted in poor pregnancy rates, often as low as only 16% in small-frame donkeys, he said.
Getting Embryos Through Long, Tight Jenny Cervixes Equals Healthy Baby Donkeys
Segabinazzi’s team suspected the low embryo transfer success rates might have something to do with the long, tight, narrow shape of the donkey cervix compared to that of the horse, he said. Previous researchers had also suggested this was the case, and that too much manipulation of the cervix might sometimes cause the jenny to release prostaglandin F2α (PGF2α), which destroys the corpus luteum and stops progesterone production.
“We believe this is a challenge mainly in small-framed donkeys,” he added.
So Segabinazzi and his fellow researchers designed a new technique in which they lubricated a Polansky equine vaginal speculum and gently stretched the vagina enough to shine a flashlight onto the cervix. Then, using their own, custom-designed forceps, they tugged lightly on the cervix enough to straighten out the canal as they slipped the embryo transfer gun through it, just long enough to release the embryo.
The team collected embryos from a total of 18 fertilized ovulations in two miniature donkeys. They transferred the embryos into six jennies using standard techniques and into five jennies using their new, adapted technique.
None of the jennies receiving embryos via standard (designed for horses) transfer techniques became pregnant, he said. However, two of the five in the alternative method group became pregnant, and they both delivered healthy donkey foals.
“It was frustrating because we just weren’t getting any jennies pregnant with the standard technique,” Segabinazzi said. “But as soon as we adapted the technique with our method, the very first jenny we tried it on got pregnant, and we thought, maybe we’ve found the problem!”
Even so, the technique still wasn’t fool proof, he added, as three of the five jennies did not get pregnant. “It’s not a good result, but it’s promising,” he said. That’s especially true given that Brazilian Northeastern jennies are a particularly small breed, measuring between 96 and 110 cm at the withers, and may have even narrower reproductive channels than larger donkey breeds.
The new technique gives hope that donkey-to-donkey embryo transfer can lead to more success in modern donkey breeding efforts, Segabinazzi said.
The study, “Two successful embryo transfers of mini-donkey embryos in Brazilian Northeastern jennies using an alternative method: Case report,” was published by Reproduction in Domestic Animals on Aug. 10, 2021.