Ability of donkey sperm to tolerate cooling: Effect of extender base and removal of seminal plasma on sperm parameters and fertility rates in mares

Current and Emerging Advanced Techniques for Breeding Donkeys and Mules

Donkeys and mules have historically played an important role in agriculture and are now gaining recognition for their contributions to animal conservation, milk production, tourism, and equid-assisted services. However, their distinctive reproductive challenges pose obstacles to breeding management. As a result, the application of assisted reproductive technologies (ARTs) could help address these challenges, enhancing their roles in both traditional and emerging industries. This review examines the current and emerging in vitro techniques for breeding donkeys and mules. Key methodologies such as sperm cryopreservation, innovative sperm preservation technologies, embryo transfer, ovum pick-up (OPU), oocyte maturation, and vitrification are discussed, emphasizing their importance in optimizing ARTs. Advances in in vitro embryo production technologies such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), and somatic cell nuclear transfer (SCNT) are reviewed, with particular attention to its success in producing the first donkey and mule blastocysts or foals. Despite significant progress in the last decade, standardization of protocols for gamete conservation and embryo transfer are still required in long-ear equids. Advancing these technologies in combination with established in vitro embryo production could significantly improve reproductive outcomes and enhance the genetic management of donkey and mule populations.

Post-cooling sperm processing can rescue sperm quality of cooled-stored stallion semen

Poor sperm quality in cooled-shipped semen has been related to subpar fertility in horses. Therefore, this study aimed to evaluate the ability of post-cooling sperm processing to improve sperm parameters of cooled-stored stallion semen for artificial insemination. For all experiments, ejaculates were collected, processed, and diluted in skimmed milk-based (SM) medium and stored at 5 °C/24h. In all experiments an aliquot of unprocessed cooled semen was used as a control. In the first experiment (Exp 1.), cooled-stored semen from 16 stallions (n = 32) was processed by SpermFilter or centrifugation (600×g/10min) and resuspended in an egg yolk-based freezing medium containing permeating cryoprotectants (EY-C) for cryopreservation. Sperm recovery and motility parameters were immediately assessed after sperm resuspension in both groups and compared with unprocessed (Unp) samples. In Exp 2., cooled semen samples from six stallions (n = 18) were processed using SpermFilter and resuspended in SM or EY-C. Motility parameters and plasma membrane integrity were assessed in all groups (Unp, SM, and EY-C). In Exp 3, cooled semen from four stallions (n = 20) was processed by SpermFilter, resuspended in SM, EY-C, or egg yolk-based medium without cryoprotectants (EY-nC); and submitted to a thermoresistance test (37 °C/3h). Motility parameters, plasma membrane integrity and stability, mitochondrial membrane potential, mitochondrial superoxide generation, and DNA fragmentation index were evaluated in all groups. Finally, in Exp 4, 39 estrous cycles of 11 mares were inseminated with unprocessed (n = 6) cooled-stored semen or semen cooled at 5 °C/24h and then processed by SpermFilter and resuspended in SM (n = 5), EY-C (n = 11), EY-nC (n = 11), or centrifuged and resuspended in EY-C (n = 6). Overall, semen processing and resuspension in EY mediums (EY-C and EY-nC) improved sperm parameters compared with those of unprocessed semen (P < 0.05). Centrifugation (91 ± 5 %) recovered more sperm than SpermFilter (84 ± 9 %; P < 0.05). Sperm resuspended in EY-nC maintained better sperm parameters throughout the thermoresistance test than those in the other groups (P < 0.05). The fertility rates were similar between all groups (P > 0.05). In conclusion, processing and resuspension in EY medium can improve sperm parameters in post-cooled-stored stallion semen.

Post-cooling semen processing and sperm re-suspension as an alternative method to circumvent poor semen cooling in stallions

BACKGROUND

Artificial insemination with cooled-shipped semen is the primary method used in the equine breeding industry; yet, sperm quality and fertility can be suboptimal for some stallions when standard techniques are used. Therefore, there is a critical need to develop alternative approaches for these stallions.

OBJECTIVE

To assess sperm quality parameters and fertility of cooled-stored stallion semen processed by SpermFilter® or centrifugation and resuspended in three extenders.

STUDY DESIGN

Controlled and field study.

METHODS

In Experiment 1, semen was collected from 21 stallions classified as having good ('Good-coolers', n = 8) or poor ('Bad-coolers', n = 13) semen cooling. The semen was extended at 30 million spermatozoa/mL in a skimmed milk-based (SM) diluent, and refrigerated for 24 h. Then, the cooled-stored semen was processed through SpermFilter® or centrifugation, and the resulting sperm pellets were resuspended in SM, SM containing pentoxifylline (SM-P), or an egg yolk-based (EY) extender. Unprocessed cooled-stored semen served as control. Sperm motility parameters, plasma membrane integrity (PMI), and mitochondrial membrane potential (HMMP) were assessed in cooled-semen pre- and post-processing. Experiment 2, cooled semen from 9 stallions classified as Bad-coolers was used to inseminate 18 embryo donor mares at 66 cycles (Unprocessed, n = 22; SpermFilter®/SM-P, n = 16; or SpermFilter®/EY, n = 28). Data were analysed with a mixed model and Tukey's as posthoc, and logistic regression.

RESULTS

Processed semen resuspended in EY had superior sperm motility compared to unprocessed, SM and SM-P (p < 0.0001). Semen processed by SpermFilter® resuspended in SM-P was similar to EY (p > 0.05). Pellet resuspension with EY and SM-P improved the HMMP of Bad-cooler stallions (p = 0.0010). Semen processed by SpermFilter® had superior PMI to centrifuged semen (p < 0.0001). Mares inseminated with SpermFilter®/SM-P (50%, 8/16) or SpermFilter®/-EY (68%, 9/28) had higher pregnancy rates than mares bred with unprocessed semen (14%, 3/22) (p < 0.001).

MAIN LIMITATIONS

Low number of mares in the fertility trial.

CONCLUSION

Sperm quality and fertility of Bad-cooler stallions can be enhanced by SpermFilter® and pellet resuspension with either EY or SM-P.

Mule embryos share identical morphological features to horse embryos

This study aimed to compare the morphometry of horse and mule embryos. The study's hypothesis was that the micronuclei and nuclear fragmentation indexes are higher in mule embryos than in horse embryos. Twenty-two mares were randomly assigned in a crossover design to receive semen from a horse and a donkey; thirteen horse and thirteen mule embryos were obtained. Embryos were recovered eight days post-ovulation and classified according to the stage of development and quality with a score from 1 (excellent) to 4 (degenerate). Embryos were stained with Hoechst33342, and images were acquired with a fluorescence microscope. Nuclei were categorized as compact, mitotic, or fragmented; the fragmented and mitotic indexes were calculated based on their proportion over the total amount of nuclei counted. Embryo size and nuclear morphometry were assessed through ImageJ. Data analyses were carried out with GraphPad using ANOVA and T-test; significance was set at P < 0.05. The number of positive flushes in cycles bred with donkey or stallion semen did not differ when compared per cycle or per ovulation (13 vs. 12) (P > 0.05). One set of twins was recovered from a mare bred to the stallion that had a double ovulation; a mule and horse embryos were both recovered from eight mares. There was no difference in size between mule and horse embryos (915.5 ± 288 μm vs. 575.8 ± 69.6 μm) (P > 0.05) size of the study. The mule embryos scored between grade 1 (n = 9) and grade 2 (n = 4); similarly, the horse embryos scored between grade 1 (n = 6) and grade 2 (n = 7). The evaluation of the nuclear morphometry revealed that horse and mule embryos have a similar number of compact nuclei per sector (148.7 ± 6.8 nuclei/sector in mule embryos vs. 156.5 ± 8.5 nuclei/sector in horse embryos) (P > 0.05); however, the number of mitotic nuclei tended to be higher in mule embryos (5.2 ± 0.82) than in horse embryos (3.3 ± 0.3) (P = 0.08). The fragmented nuclei index was similar between mule (0.25 ± 0.1%) and horse (0.22 ± 0.1%) embryos (P = 0.4); the mitotic nuclei index was higher in mule embryos (3.2 ± 0.4%) than in horse embryos (2.2 ± 0.2%) (P = 0.02). In conclusion, embryo morphology of mares bred to a donkey and a horse shares similar nuclear ultrastructure features, except that mule embryos have a higher mitotic index.

Comparison of the metabolite profile of donkey and horse seminal plasma and its relationship with sperm viability and motility

Previous research revealed that several seminal plasma (SP) metabolites are related to sperm functionality, fertility, and preservation. While it is understood that variations between species exist, whether the SP metabolome differs between donkeys and horses has not been previously investigated. The aim of this work, therefore, was to characterize and compare donkey and horse SP metabolites using nuclear magnetic resonance (NMR) spectroscopy, and relate them to sperm viability and motility. For this purpose, ejaculates from 18 different donkeys and 18 different horses were collected and separated into two aliquots: one for harvesting the SP by centrifugation and obtaining the metabolic profile through NMR, and the other for evaluating sperm viability and motility. Based on total motility and sperm viability, samples were classified as with good (GQ) or poor (PQ) quality. The metabolomic profile of donkey and horse SP revealed the presence of 28 metabolites, which coincided in the two species. Yet, differences between horses and donkeys were observed in the concentration of 18 of these 28 metabolites, as well as between ejaculates classified as GQ or PQ and in the relationship of metabolites with sperm motility and viability. These findings suggest that sperm from donkeys and horses differ in their metabolism and energetic requirements, and that the concentration of specific SP metabolites may be related to sperm functionality. Further research should shed light on the metabolic needs of donkey and horse sperm, and evaluate how the knowledge collected from the contribution of these metabolites can help improve semen preservation in the two species.