Collection frequency affects percent Y-chromosome bearing sperm, sperm head area and quality of bovine ejaculates

High-Efficiency Bovine Sperm Sexing Used Magnetic-Activated Cell Sorting by Coupling scFv Antibodies Specific to Y-Chromosome-Bearing Sperm on Magnetic Microbeads

Sperm sexing technique is favored in the dairy industry. This research focuses on the efficiency of bovine sperm sexing using magnetic-activated cell sorting (MACS) by scFv antibody against Y-chromosome-bearing sperm (Y-scFv) coupled to magnetic microbeads and its effects on kinematic variables, sperm quality, and X/Y-sperm ratio. In this study, the optimal concentration of Y-scFv antibody coupling to the surface of magnetic microbeads was 2–4 mg/mL. PY-microbeads revealed significantly enriched Y-chromosome-bearing sperm (Y-sperm) in the eluted fraction (78.01–81.43%) and X-chromosome-bearing sperm (X-sperm) in the supernatant fraction (79.04–82.65%). The quality of frozen–thawed sexed sperm was analyzed by CASA and imaging flow cytometer, which showed that PY-microbeads did not have a negative effect on X-sperm motility, viability, or acrosome integrity. However, sexed Y-sperm had significantly decreased motility and viability. The X/Y-sperm ratio was determined using an imaging flow cytometer and real-time PCR. PY-microbeads produced sperm with up to 82.65% X-sperm in the X-enriched fraction and up to 81.43% Y-sperm in the Y-enriched fraction. Bovine sperm sexing by PY-microbeads showed high efficiency in separating Y-sperm from X-sperm and acceptable sperm quality. This initial technique is feasible for bovine sperm sexing, which increases the number of heifers in dairy herds while lowering production expenses.

Males can adjust offspring sex ratio in an adaptive fashion through different mechanisms

Sex allocation research has primarily focused on offspring sex-ratio adjustment by mothers. Yet, fathers also benefit from producing more of the sex with greater fitness returns. Here, we review the state-of-the art in the study of male-driven sex allocation and, counter to the current paradigm, we propose that males can adaptively influence offspring sex ratio through a wide variety of mechanisms. This includes differential production and motility of X- versus Y-bearing sperms in mammals, variation in seminal fluid composition in haplo-diploid invertebrates, and epigenetic mechanisms in some fish and lizards exhibiting environmental sex determination. Conflicts of interest between mothers and fathers over offspring sex ratios can emerge, although many more studies are needed in this area. While many studies of sex allocation have focused on adaptive explanations with little attention to mechanisms, and vice versa, the integration of these two topics is essential for understanding male-driven sex allocation.

Sexual conflict in action: An antagonistic relationship between maternal and paternal sex allocation in the tammar wallaby, Notamacropus eugenii

Sex ratio biases are often inconsistent, both among and within species and populations. While some of these inconsistencies may be due to experimental design, much of the variation remains inexplicable. Recent research suggests that an exclusive focus on mothers may account for some of the inconsistency, with an increasing number of studies showing variation in sperm sex ratios and seminal fluids. Using fluorescent in-situ hybridization, we show a significant population-level Y-chromosome bias in the spermatozoa of wild tammar wallabies, but with significant intraindividual variation between males. We also show a population-level birth sex ratio trend in the same direction toward male offspring, but a weaning sex ratio that is significantly female-biased, indicating that males are disproportionately lost during lactation. We hypothesize that sexual conflict between parents may cause mothers to adjust offspring sex ratios after birth, through abandonment of male pouch young and reactivation of diapaused embryos. Further research is required in a captive, controlled setting to understand what is driving and mechanistically controlling sperm sex ratio and offspring sex ratio biases and to understand the sexually antagonistic relationship between mothers and fathers over offspring sex. These results extend beyond sex allocation, as they question studies of population processes that assume equal input of sex chromosomes from fathers, and will also assist with future reproduction studies for management and conservation of marsupials.

A father effect explains sex-ratio bias

Sex ratio allocation has important fitness consequences, and theory predicts that parents should adjust offspring sex ratio in cases where the fitness returns of producing male and female offspring vary. The ability of fathers to bias offspring sex ratios has traditionally been dismissed given the expectation of an equal proportion of X- and Y-chromosome-bearing sperm (CBS) in ejaculates due to segregation of sex chromosomes at meiosis. This expectation has been recently refuted. Here we used Peromyscus leucopus to demonstrate that sex ratio is explained by an exclusive effect of the father, and suggest a likely mechanism by which male-driven sex-ratio bias is attained. We identified a male sperm morphological marker that is associated with the mechanism leading to sex ratio bias; differences among males in the sperm nucleus area (a proxy for the sex chromosome that the sperm contains) explain 22% variation in litter sex ratio. We further show the role played by the sperm nucleus area as a mediator in the relationship between individual genetic variation and sex-ratio bias. Fathers with high levels of genetic variation had ejaculates with a higher proportion of sperm with small nuclei area. This, in turn, led to siring a higher proportion of sons (25% increase in sons per 0.1 decrease in the inbreeding coefficient). Our results reveal a plausible mechanism underlying unexplored male-driven sex-ratio biases. We also discuss why this pattern of paternal bias can be adaptive. This research puts to rest the idea that father contribution to sex ratio variation should be disregarded in vertebrates, and will stimulate research on evolutionary constraints to sex ratios-for example, whether fathers and mothers have divergent, coinciding, or neutral sex allocation interests. Finally, these results offer a potential explanation for those intriguing cases in which there are sex ratio biases, such as in humans.

Male pygmy hippopotamus influence offspring sex ratio

Pre-determining fetal sex is against the random and equal opportunity that both conceptus sexes have by nature. Yet, under a wide variety of circumstances, populations shift their birth sex ratio from the expected unity. Here we show, using fluorescence in situ hybridization, that in a population of pygmy hippopotamus (Choeropsis liberiensis) with 42.5% male offspring, males bias the ratio of X- and Y-chromosome-bearing spermatozoa in their ejaculates, resulting in a 0.4337±0.0094 (mean±s.d.) proportion of Y-chromosome-bearing spermatozoa. Three alternative hypotheses for the shifted population sex ratio were compared: female counteract male, female indifferent, or male and female in agreement. We conclude that there appears little or no antagonistic sexual conflict, unexpected by prevailing theories. Our results indicate that males possess a mechanism to adjust the ratio of X- and Y-chromosome-bearing spermatozoa in the ejaculate, thereby substantially expanding currently known male options in sexual conflict.

Some animal populations are able to shift their birth sex ratio from the expected unity. This study shows, using fluorescence in situ hybridization, that in a captive population of pygmy hippopotamus the males appear to be able to adjust the ratio of X- and Y-chromosome-bearing spermatozoa in their ejaculates.

Rainfall-driven sex-ratio genes in African buffalo suggested by correlations between Y-chromosomal haplotype frequencies and foetal sex ratio

Background

The Y-chromosomal diversity in the African buffalo (Syncerus caffer) population of Kruger National Park (KNP) is characterized by rainfall-driven haplotype frequency shifts between year cohorts. Stable Y-chromosomal polymorphism is difficult to reconcile with haplotype frequency variations without assuming frequency-dependent selection or specific interactions in the population dynamics of X- and Y-chromosomal genes, since otherwise the fittest haplotype would inevitably sweep to fixation. Stable Y-chromosomal polymorphism due one of these factors only seems possible when there are Y-chromosomal distorters of an equal sex ratio, which act by negatively affecting X-gametes, or Y-chromosomal suppressors of a female-biased sex ratio. These sex-ratio (SR) genes modify (suppress) gamete transmission in their own favour at a fitness cost, allowing for stable polymorphism.

Results

Here we show temporal correlations between Y-chromosomal haplotype frequencies and foetal sex ratios in the KNP buffalo population, suggesting SR genes. Frequencies varied by a factor of five; too high to be alternatively explained by Y-chromosomal effects on pregnancy loss. Sex ratios were male-biased during wet and female-biased during dry periods (male proportion: 0.47-0.53), seasonally and annually. Both wet and dry periods were associated with a specific haplotype indicating a SR distorter and SR suppressor, respectively.

Conclusions

The distinctive properties suggested for explaining Y-chromosomal polymorphism in African buffalo may not be restricted to this species alone. SR genes may play a broader and largely overlooked role in mammalian sex-ratio variation.

Calving sex ratio as related to the predicted Y-chromosome-bearing spermatozoa ratio in bull ejaculates

The first objective was to correlate calving sex-ratio data from semen lots with the semen sex ratio obtained by two duplex polymerase chain reaction (PCR)/gel electrophoresis techniques. The two techniques involved different starting DNA amounts, PCR conditions, agarose gel concentrations, sample placement on the gels, lane size, number of lanes per gel, and duration of electrophoresis. The second objective was to sequence the duplex PCR products to verify their match to genes and chromosomes for which they were designed. Thirty-six ejaculates (lots) from eight Holstein sires were collected. Semen straws were distributed among dairies in three states. Ten straws per lot were used for the different PCR techniques. Sperm DNA was extracted and PCR analysis was done using one primer set to amplify a single copy section of the factor IX precursor (X-chromosome only) and another primer set to amplify a single copy section the sex determining region (Y-chromosome only). The glyceraldehyde phosphate dehydrogenase gene was amplified as an internal control. Standard curves were designed using PCR products in known ratios. Gel electrophoresis and image analysis were used to determine predicted %Y-chromosome-bearing spermatozoa (PredPtY). Sex (male=1, female=0) was reported on 526 calves and the ratio of the number of male to total calves (proportion of male calves (PMC)) was determined between sire and lot within sire. The PredPtY and PMC were significantly correlated (r=0.82, P<0.0002). No significant variance between sires was found in PredPtY or PMC, but lots within sires was a significant variance source for both. The two PCR technologies adequately determined semen sex ratio. The technology-by-lot-within-sire interaction was a significant variance source for PredPtY. Acrosomal integrity (after a 2-h) incubation, was correlated with both PMC and PredPtY; other semen quality characteristics had no significant correlations with PMC or PredPtY. Therefore, calf crop sex ratio skewness could be controlled by screening semen for PredPtY through the use of PCR.

Sex ratio determination in bovine semen: A new approach by quantitative real time PCR

Sex preselection of livestock offspring in cattle represents, nowadays, a big potential for genetic improvement and market demand satisfaction. Sperm sorting by flow cytometer provides a powerful tool for artificial insemination and production of predefined sexed embryos but, an accurate verification of the yield of sperm separation remains essential for a field application of this technique or for improvement and validation of other related semen sexing technologies. In this work a new method for the determination of the proportion of X- and Y-bearing spermatozoa in bovine semen sample was developed by real time PCR. Two sets of primers and internal TaqMan probes were designed on specific X- and Y-chromosome genes. To allow a direct quantification, a standard reference was established using two plasmid cDNA clones (ratio 1:1) for the specific gene targets. The method was validated by a series of accuracy, repeatability and reproducibility assays and by testing two sets of sorted and unsorted semen samples. A high degree of accuracy (98.9%), repeatability (CV=2.58%) and reproducibility (CV=2.57%) was shown. The results of X- and Y-sorted semen samples analysed by real time PCR and by flow cytometric reanalysis showed no significant difference (P>0.05). The evaluation of X-chromosome bearing sperms content in unsorted samples showed an average of 51.11+/-0.56% for ejaculates and 50.17+/-0.58% for the commercial semen. This new method for quantification of the sexual chromosome content in spermatozoa demonstrated to be rapid and reliable, providing a valid support to the sperm sexing technologies.

Male Fertility and Sex Ratio at Birth in Red Deer

Efforts to test sex ratio theory have focused mostly on females. However, when males possess traits that could enhance the reproductive success of sons, males would also benefit from the manipulation of the offspring sex ratio. We tested the prediction that more-fertile red deer males produce more sons. Our findings reveal that male fertility is positively related to the proportion of male offspring. We also show that there is a positive correlation between the percentage of morphologically normal spermatozoa (a main determinant of male fertility) and the proportion of male offspring. Thus, males may contribute significantly to biases in sex ratio at birth among mammals, creating the potential for conflicts of interest between males and females.

Effect of the interval between estrus onset and artificial insemination on sex ratio and fertility in cattle: a field study

We have carried out a field trial in cattle to study the effect of the interval between the onset of estrus and AI on sex ratio and fertility. Data were obtained from 716 cows that had been inseminated at different times between 8 and 44 h from the visual detection of estrus. Before analyzing the data, it was grouped in three intervals considering the time between estrus onset and AI (8-18, 18-30, and > or = 30 h). Our results show that the percentage of calved females (73.05%) is significantly superior for early inseminations (8-18 h), and it decreases 1.85% per hour from the onset of estrus. Delayed AIs (> or = 30 h) produce a significant deviation of the sex ratio towards the males (72.06%); nevertheless, fertility (percentage of successful pregnancies) diminishes significantly, from 66.19% (8-18 h) to 45.35% (> or = 30 h). In conclusion, variations in the interval between the onset of estrus and AI modify sex ratio. However, we must consider its effect on fertility.

High resolution DNA flow cytometry of boar sperm cells in identification of boars carrying cytogenetic aberrations

The cytogenetic quality of boars used for breeding determines the litter outcome and thus has large economical consequences. Traditionally, quality controls based on the examination of simple karyograms are time consuming and sometimes give uncertain results. As an alternative, the use of high-resolution DNA flow cytometry on DAPI-stained sperm cell nuclei (CV </= 1.3%) was investigated as a screening method for the cytogenetic quality of boars. By analyzing a series of 25 animals judged normal by their fertility statistics and a series of seven animals with known reciprocal translocations, a model for identifying sperm cells from cytogenetically aberrant animals was proposed. This model was applied to a series of 50 uncharacterized animals. The model successfully identified a mosaic or chimaeric carrier of an aberrant X chromosome. However, implementation of this technique for screening purposes would necessitate essential improvements in standardization and measurement precision.

Effect of processing and measuring procedures on estimated sizes of bull sperm heads

Reported estimates of sperm head size within a species vary considerably, partly due to procedural effects. A simple India ink method was developed that provided good contrast without inducing artifacts. Semen from five fertile bulls was smeared on replicate slides and left unfixed or fixed in Carnoys solution, with ink added for background. Other slides were fixed, and sperm were stained by the Feulgen procedure. Sperm head area was measured four ways. These were linear measurements made with the aid of an ocular micrometer and an oil immersion objective, plus three methods of measuring sperm heads projected at magnification 5,000 x. The areas of unfixed and fixed sperm heads did not differ (41.5 microm(2) versus 41.6 microm(2), respectively, P>0.05). The Feulgen-stained head area was smaller (26.2 microm(2), P<0.05). Sperm head areas calculated from ocular micrometer measurements were slightly smaller (P<0.05) than areas measured using projection. Identical results obtained by two technicians were treated as duplicates and approximately half of the variation was biological, due to source of semen. There was an interaction (P<0.05) between the sample source and fixation procedures. Thus, preparative techniques must be carefully controlled, and experiments designed to partition possible interactions between the biological material sampled and procedures used.