Sperm Sex Ratio (X: Y Ratio) and its Variations

Research Article

Austin J Reprod Med Infertil. 2014;1(1): 7.

Sperm Sex Ratio (X: Y Ratio) and its Variations

Chaudhury I, Jain M and Halder A*

Department of Reproductive Biology, All India Institute of Medical Sciences, India

*Corresponding author: Halder A, Department of Reproductive Biology, All India Institute of Medical Sciences, New Delhi 110029, India

Received: October 24, 2014; Accepted: November 28, 2014; Published: December 01, 2014

Abstract

Sex ratio can be studied at various levels viz., at the time of spermatogenesis i.e. ratio of X and Y bearing sperm (pre-zygotic sex ratio) or at the time of fertilization/early preimplantation embryo (close to primary sex ratio or post-zygotic sex ratio) or at the time of birth (secondary sex ratio). The natural sex ratio at the time of spermatogenesis is expected to be 1:1. In this study we have examined sex ratio in ejaculated spermatozoa (human) as well as epididymal sperm (mouse) to determine proportion of X and Y bearing sperm i.e., pre-zygotic sex ratio. We also examined effects of seasons (temperature; summer vs. winter), diet (vegetarian vs. non vegetarian), profession (professionals vs. laborer) on sperm (pre-zygotic) sex (X: Y) ratio of ejaculated sperm.

The sperm sex ratio was carried out on 813066 human spermatozoa and 10390 mouse spermatozoa. In human, we have found more (52%) X than Y (48%) bearing sperms (421531X: 391535Y or 1.07X: 1Y). In mouse also we observed preponderance of X (55.5%) as compared to Y (44.5%) bearing sperms (1.24X: 1Y). In all sub-groups (season, diet, profession) we have observed more X bearing sperm. Our observations at pre-zygotic sex ratio have shown skewed sex ratio towards female. A probable reason for this could be preferential elimination of Y bearing sperm. This is also supported by the evidence of more aneuploidy with Y-bearing spermatozoa (~1.5 times more with Y bearing sperms).

Keywords: Spermatozoa; Prezygotic sex ratio (X: Y ratio); Season-Diet- Profession Influence

Introduction

Sex ratio is defined as ratio of number of males to females. Worldwide, the human sex ratio at birth is fairly constant with male excess (51.4%) [1]. The natural sex ratio at the time of spermatogenesis is expected to be 1:1 (accordance with Mendelian segregation principle). Theoretically, offspring sex ratio may be attributed to events that occur before fertilization viz., sperm sex ratio or favor selection of Y or X chromosome bearing spermatozoa or events that occur after fertilization such as preferential survival of embryos of one sex or a combination. Sex ratio in sperm was studied initially by Y-body analysis or analysis of chromosome complements derived from fusion of human sperm with hamster oocytes. With this technique Barry Bean [2] demonstrated an excess of X-bearing sperm. However, Chernos and Martin [3] reported that the sex ratios did not differ significantly on fresh as well as cryopreserved sperm. With the recent advances in molecular technologies sexing of sperm is easily possible thus allowing us to study sperm sex ratio in large numbers of sperms rapidly and made possible to test the hypothesis that a sperm sex ratio underlies a live birth sex ratio bias.

Polymerase chain reaction (PCR) technique for sexing sperm was used initially by many [4,5]. Lobel et al. [4] had found a variation in Y chromosome bearing spermatozoa in humans from 41.9% to 56.7%. Chandler et al. [5] studied the variation in sex ratio in ejaculates in bulls and found that X & Y bearing spermatozoa are unequally distributed per ejaculate in a wave pattern. However with the application of X and Y chromosome FISH to sperm allowed us a more accurate assessment of the sex chromosomal complements [6]. In a sperm fluorescent in situ hybridization (FISH) study by Martin et al. [7] mean frequencies of X and Y bearing sperm was found to be 50.1% and 49.0%, respectively. In a study by Mercier et al. [8] on XYY male, an X: Y ratio of 0.78:1 was found in sperm with normal sex chromosome constitution. Similarly Griffin et al. [9] also reported equal Y and X bearing sperm. However, in contrast Halder and Tutscheck [10] was observed unequal sperm sex ratio, with an excess of X bearing sperm. They also observed higher segregation error of Y chromosome in comparison to X chromosome, thus reducing the number of normal Y bearing sperm further. Similarly, Spriggs et al. [11] identified a small but significant excess of X bearing sperm in a study with 50,000 sperm. Sex ratio distortion is also observed in bovine sperm [12], an excess of X bearing sperm (53%) than Y bearing sperm (46%). However, in a recent [13] Rhesus monkeys study did not find any significant difference between X and Y bearing sperm.

Due to above contradictory reports this study was undertaken to examine sex ratio in ejaculated spermatozoa (human) as well as epididymal sperm (mouse) to determine proportion of X and Y bearing sperm i.e., pre-zygotic sex ratio in a very large number of sperms (over 0.8 millions). We have also examined effects of seasons (temperature), diet & profession on sperm (pre-zygotic) sex ratio.

Material and Methods

Men with normal sperm count [14] were enrolled into the study between February 2008 and March 2011. Study group comprised of 50 subjects (80 samples) among which, 10 subjects were enrolled in season based study (provided semen samples 4 times). In season’s category, semen sample was taken twice in summers (May and August) as well as in winters (December and February). This was done to evaluate the change in sperm sex ratio at the beginning and end of a season and also to find variation in sperm sex ratio according to seasons (temperature). Twenty subjects were included in diet based category; out of them 10 were strict vegetarians and 10 were non-vegetarians (who eat meat, fish etc at least thrice in a week). Another twenty subjects were included for profession based category; out of them 10 were professionals and 10 were daily wage laborer. Age of the volunteers ranged from 21-45 years. Information & biological samples were collected from all cases as per prescribed proforma. In addition we also obtained epididymal sperms from 10 adult male mice.

Institute Animal Ethics Committee & Institute Human Ethics Committee clearance for human & mouse study were obtained (IHEC & IAEC letter nos. T-10/30.01.09 & 485/IAEC/09). Written consent was also obtained from all donors before obtaining semen samples for the study.

Preparation of sperm cells (human) for FISH

Liquefied semen sample was transferred to the 1.5ml micro centrifuge tube and is centrifuged to separate sperm cells and seminal plasma. In the sperm cells pellet 1 ml of phosphate buffer saline (PBS) was added. It was then centrifuged at 8000 rpm for 3mins. The supernatant was removed. This step was repeated 2 more times or when sperm pellet appears clean. Then 1 ml 50mM hypotonic solution (KCl) was added to the sperm pellet, mixed well and incubated at 37°C incubator for 45mins. It was then centrifuged again at 8000rpm for 3mins. About 1 ml fixative (3:1, methanol: acetic acid) was added to the pellet and vortexed and centrifuged at 8000 rpm for 3mins. The supernatant was removed. This procedure was repeated until pellet was whitish and then sperm cells were suspended in fixative and stored at -80°C.

Extraction of mouse spermatozoa

Male mouse (6-8 weeks old) issued from the animal experimental facility, AIIMS. Within 15-20mins of euthanasia epididymis was extracted and placed in petridish containing PBS. Under a zoom dissecting microscope cauda epididymis was gently incised at 2-3 points and left for 3-4mins. This caused release of all live sperms into PBS (Figure 1). PBS along with sperm was collected into 1.5 ml tube & centrifuged at 5000 rpm for 3mins. Supernatant discarded and pellet was given hypotonic treatment for 1 hour. This was followed by 2 washes of fixative and finally stored at -80°C till further use.