Relationship between the characteristics of frozen-thawed ram spermatozoa and in vitro embryo production

QTLs and Candidate Genes Associated with Semen Traits in Merino Sheep

Ram semen traits play a significant role in conception outcomes, which in turn may influence reproductive efficiency and the overall productivity and profitability of sheep enterprises. Since hundreds of ewes may be inseminated from a single ejaculate, it is important to evaluate semen quality prior to use in sheep breeding programs. Given that semen traits have been found to be heritable, genetic variation likely contributes to the variability observed in these traits. Identifying such genetic variants could provide novel insights into the molecular mechanisms underlying variability in semen traits. Therefore, this study aimed to identify quantitative trait loci (QTLs) associated with semen traits in Merino sheep. A genome-wide association study (GWAS) was undertaken using 4506 semen collection records from 246 Merino rams collected between January 2002 and May 2021. The R package RepeatABEL was used to perform a GWAS for semen volume, gross motility, concentration, and percent post-thaw motility. A total of 35 QTLs, located on 16 Ovis aries autosomes (OARs), were significantly associated with either of the four semen traits in this study. A total of 89, 95, 33, and 73 candidate genes were identified, via modified Bonferroni, within the QTLs significantly associated with volume, gross motility, concentration, and percent post-thaw motility, respectively. Among the candidate genes identified, SORD, SH2B1, and NT5E have been previously described to significantly influence spermatogenesis, spermatozoal motility, and high percent post-thaw motility, respectively. Several candidate genes identified could potentially influence ram semen traits based on existing evidence in the literature. As such, validation of these putative candidates may offer the potential to develop future strategies to improve sheep reproductive efficiency. Furthermore, Merino ram semen traits are lowly heritable (0.071-0.139), and thus may be improved by selective breeding.

Heritability and Genetic Parameters for Semen Traits in Australian Sheep

Semen characteristics including volume, gross motility, spermatozoal concentration, and percent post-thaw motility are routinely assessed to determine the quality and quantity of an ejaculate prior to use in artificial breeding programs. Currently, artificial breeding programs in sheep place relatively little emphasis on ram-side factors, such as the fertilising potential of an ejaculate, which may contribute to variability in conception outcomes. Estimating genetic parameters for ejaculate quality and quantity traits could provide insights into whether selective breeding can be used to improve such ram-side traits, improving ovine reproductive performance and farm profitability. Therefore, in this study, a total of 11,470 ejaculate records, including data for ejaculate volume, gross motility, spermatozoal concentration, and percent post-thaw motility, collected over a 20-year period was used to estimate genetic parameters in sheep. Univariate and bivariate mixed model analysis was performed including a variety of fixed effects such as breed, age at collection, centre of collection, collection number, season of collection, and method of collection; and the permanent environmental effects associated with each ram, stud and year of collection, and the breeding value of rams included as random effects. The heritability for ejaculate volume, gross motility, concentration, and percent post-thaw motility was estimated to be 0.161, 0.170, 0.089, and 0.081. Repeatability estimates were moderate, ranging between 0.4126 and 0.5265. Overall, results indicate that semen traits are lowly heritable and moderately repeatable, indicating that these traits are significantly influenced by environmental variables.

Prion protein 2 (dublet) gene (PRND): role in ovine semen capacitation, cryopreservation and fertility

The aim of the present study was to examine the role of Doppel protein in the capacitation process and fertilising ability of both fresh and frozen–thawed (FT) spermatozoa from rams carrying different prion protein 2 (dublet) (PRND) gene polymorphisms. The detection efficacy of new anti-Doppel monoclonal antibodies and PRND mRNA quantification were also explored in ovine spermatozoa. Three different genotypes (AA, GA, GG) were identified for codon 26 of ovine PRND-c.78G>A. Using flow cytometry, a higher fluorescence was detected in fresh compared with FT sperm samples incubated with anti-Doppel primary and fluorescein isothiocyanate-conjugated secondary antibodies (P < 0.05). Capacitation was affected by semen treatment (fresh and FT) and male PRND genotype (P < 0.05). After IVF, the use of fresh semen resulted in a higher cleavage rate than the use of FT spermatozoa (P = 0.004). IVF using spermatozoa from individuals classified as carriers of the AA or GA PRND genotypes resulted in higher cleavage rates than seen using spermatozoa from GG carriers (P ≤ 0.0006). Finally, using semen from rams with the AA PRND genotype resulted in the highest Day 6 and Day 8 embryo rates (P ≤ 0.04). In conclusion, the results of the present study confirm that the identification of different PRND genotypes is important for studying the sperm capacitation process and for improving sperm cryoresistance and embryo production. Furthermore, the detection of Doppel in ejaculated ovine spermatozoa, along with its low expression after cryopreservation, strongly suggests an important physiological function of this protein in male fertility.

Relationship between the characteristics of epididymal red deer spermatozoa and penetrability into zona-free hamster ova

A heterologous (zona-free hamster oocytes) in vitro fertilization (IVF) system was used to evaluate the relationship between sperm factors and penetration capacity of epididymal red deer spermatozoa. The sperm parameters evaluated in 36 sperm samples obtained postmortem from stags selectively shot during the rutting season were sperm motility, functional integrity of plasma membrane by means of the hypo-osmotic swelling test (HOST), and, simultaneously, viability and acrosomal status via a triple-stain technique. Zona-free hamster oocytes were used to evaluate the capacity of the different sperm assays to predict in vitro penetration. In order to increase the variability in sperm quality, we recovered samples from stags at different intervals between the death of the male and the collection of the genitalia. All measures of sperm quality declined progressively (P <.001) with increasing intervals between death and sample collection. In addition, many sperm parameters were related to penetration ability in vitro. Subsequently, sperm samples were rearranged in 2 categories according to the interval that had elapsed between death and the collection of the genitalia (group 1, short interval = 0-12 h; group 2, large interval = 18-40 hours). When samples were grouped, less correlation achieved significance, especially for group 1, than when samples were not divided. Also, correlation between the number of sperm per oocyte and sperm parameters for group 1, which had the highest values of sperm quality, failed to reach significance. It is concluded that the classical parameters accepted in assessing the viability of deer spermatozoa can be good predictors of the penetrating ability of the spermatozoa when satisfactory in vitro conditions are used for the development of the IVF system. Also, this study demonstrates that compatible heterologous gamete interaction allows thorough assessment of sperm function in a wild deer.

The contribution of the male to ovine embryogenesis in an in vitro embryo production system

The differences in the embryo production potential of four rams used in a commercial embryo transfer program were examined in both in vivo and in vitro embryo production systems. Processing frozen-thawed spermatozoa through Percoll density gradients prior to in vitro insemination eliminated differences in the estimates of sperm viability between the four rams, and yet, differences in embryo production persisted throughout the in vitro culture period. However, there was no effect of ejaculate within ram on embryo production rates. In addition, the timing of the onset of the differences between the rams at each stage of in vitro embryo development were revealed. Ram 2 differed from ram 4 in the proportion of fertilized oocytes at 17 h post-insemination (pi) and by 52 h, ram 3 differed from ram 4 in the proportion of cleaved embryos, and the observed differences between ram 1 and ram 2 in their blastocyst production were initiated prior to activation of the embryonic genome. Once differences in embryo development rates were detected among the four rams, they persisted throughout the in vitro culture period. The reduced in vitro fertilization (IVF) rates from ram 2 compared with the other rams was paralleled in vivo by the significantly lower proportion of embryos recovered from ewes mated to ram 2, and this was further exacerbated by a significantly lower embryo survival rate after transfer. However, the subtle differences observed in the timing of the contribution of each sire to embryo development during in vitro culture were not able to be detected in vivo. However, the higher proportions of transferable quality blastocysts obtained from ewes mated to ram 4 did not result in increased embryo survival throughout the remainder of gestation. Therefore, in this study, the blastocyst production potential for a particular sire, either in vitro or in vivo, does not necessarily reflect the potential for the production of live offspring.