SEX SORTED SEMEN; AN INSTANT REVOLUTION AND A LONG TERM CONCERN
By-Dr. Nandani Kumari*
(Assistant Professor cum Junior Scientist, Department of Animal Breeding and Genetics, Ranchi Veterinary College)
The technology called as “ Sexed Semen / Sex sorting Technology” was developed by USDS(United States Department of Agriculture ) researches in Livermon, California, and Beltsvile, Meryland. This technology was patented as Beltsvile Sperm sexing technology “ and was commercialized in 2001 in U.S.
Definition : In simple language sexed semen is the semen which has been selected based on its capability to influence the gender of the foetus. Technically it can be defined as semen having X or Y bearing sperm to produce progenies of a desired sex either male or female with almost 80-90% accuracy.
PRINCIPLE-The sex of mammalian offspring is decided by the fact known for long i.e whether X/Y chromosome from male segregates with X Chromosomes in female. So if these was a technology available which would be to separate X and Y chromosome or help identify and segregate the two types of sex chromosomes or help them identify and segregate the two types of sex chromosomes in male sperm, the purpose was solved. Unsuccessful attempts to differentiate between mammalian sperm producing males and females met with success in 1981 when the precise measurement of DNA content was found out to be the best way to differentiate semen on the basis of sex chromosome and finally led to sperm sorting system. Subsequently flow cytometry was developed. The basic principle behind sex sorting is that in mammals, the DNA content in X Chromosome differs from that in Y chromosome and the difference is variable from species to species and amount to approximately 2.9% in human sperm and 3.8% in cattle. Other difference which can be exploited (Athough the difference in DNA content is being used maximum ) is size, surface charges on sperm, and cell surface antigens etc.
Method of sperm sexing and its principles-
First method of sperm sexing is Quinacrine Mustard Staining in which staining is produced in the fluorescence in certain regions of chromosomes. Another method named Ram Microspectroscopy is the technique in which spectral component in sperm i.e. DNA, Protein, Lipids etc are used to identify difference via Raman Spectroscopy. Centrifugal Counter Current Distribution is the next method in which the basic principal explored is difference in density between X and Y. Further in Albumin gradient method, Y chromosome being smaller than X chromosome is separated . In the next method called as Sperm sorting based on Volumetric Difference, as the name suggests the difference in sperm volume of X and Y is used to separate the two. Swimming pattern under Laminar flow is yet another method for Sexed Semen in which Y bearing spermatozoa swim quickly than X in a column of Flowing media. Other methods are Free flow electrophoresis, Counter Current Galvanic Seperator, Immunological sexing of semen, PCR flow cytometry.
Out of all the above mentioned methods of semen sexing, flow cytometry is the best and most widely used in Animal Field.
Flow cytometry – There are number of commercial flow cytometry commercially grouped as Analytical, Sorter type, etc. But at present flow cytometric method of sorting of spermatozoa based on the difference in DNA content of X and Y bearing spermatozoa is the only confirmed method of sperm sexing . Although the method is expensive, time consuming and doubts of potential harmful effect on spermatozoa have been raised.
The main principal behind Flow cytometry is that the autosomes carried by X and Y spermatozoa have identical DNA content and the difference in DNA mass of X and Y chromosomes has formed the basis upon which flow cytometric analysis of DNA in X and Y spermatozoa has been carried out. Flow cytometry is now a recognized methodology within animal spermatology. There are a type of advanced sorters having Laser which are used to excite the fluorescent dye that binds to the DNA in Spermatozoa. Spermatozoa are treated with dye e.g. Hoechst 3342. According to Nikita et.al.(2018), these dyes are permeable to live and intact sperm membrane and binds to the DNA. Stained spermatozoa are transported to a point where they are exposed individually to a UV Laser Beam ( Wavelength of 351-364) and the bright blue fluorescence emitted is detected and analysed. Due to the more DNA contents in the X chromosome being spermatozoa, it takes more stain than Y sperm. On the basis of this fluorescence, spermatozoa are classified as X or Y chromosome being sorted. She further points out that another dye commonly called “red quencher food colouring dye” , selectively penetrates into damaged dead and non-intact sperm membranes giving a red colour.
Advantages of Sperm Sexing-
- Increased reproductive rates in animals and subsequently increased rates of genetic gain through possible higher selection intensity.
- More use of Important Dairy breed
- Economic benefit to the dairy Farmers who are saved from rearing uneconomic male calf without much gains
Limitations-
- Slow Process
- Half of the sperm are unsexable and go waste
- Only 30% of sperm are sexable to which only 15% is responsible for female offspring.
- High cost of sexed semen
- Damage to spermatozoa
- Low conception rate with sexed semen
- Skilled manpower is required.
- Long term impact on the breed or population w.r.t its conservation and extinction-
Efective population size (Ne) is defined as the size of an idealized population undergoing same rate of genetic drift as the population under study. It is a central concept in population genetics and used extensively in design and monitoring of conservation and breeding programme. Effective population size is an important indicator of prediction of rate of inbreeding and loss of genetic variation in a population. According to Claus Wedekind(2012), genetic problems are only indirectly linked to the census size(Ne). Instead they are directly dependent on the genetically effective population size ( Ne) that is defined as the size of an ideal model population that looses genetic variability at the same rate as observed population. Usually Ne is deviation from a 1:1 operational sex ratio, and other reasons. Risk of extinction are therefore increased if the population sex ratio deviated from 1:1. He further says that 1:1 sex ratio is expected in natural populations because of strong frequency dependent selection and are biased by non-random harvest.
I/Ne=1/Nm+1/Nf ; With the decrease in number of Males in all the important breeds by sperm sorting, the next decade is sure to witness conservation and other genetic issues emanating from sexed semen in all the concerned breeds due to altered 1:1 sex ratio resulting from biased selection via sperm sorting. Choosing one sex over the normal H.W. equilibrium population will be benifitting us today for sure but at the same time is bound to give us the associate genetic pitfalls.
Use of sperm sexing is almost inevitably going to be more widespread in Diary Industry in future in order to meet the economic as well as social pressure to increase the milk productivity of Dairy animals in India. This article is an effort to present a genetic aspect of sexed semen (hitherto undiscussed perhaps ) through the use of concept of effective population size in dairy animals. The technique is indeed a boon but a useful, meaningful discussion from scholars of Animal Genetics and Breeding is awaited to reach a strong, valid and important conclusion.
REFERENCES-
- Sexed Semen an overview, www.dairyknowledge.in.
- Garner D.L., Seidel G E Jr.2008.History of commercializing sexed semen for cattle. Theriogenology.
3.Bhalakiya N., Haque N., Patel D., Chaudhari A., Patel G., Madhavkar M., Patel P., Hossain S.K.A., Kumar R. ( 2018). Int. J. Curr. Microbiol. App. Sci. Special Issue-7, 259-272.
4.Claus Wedekin, 2012. Managing sex ratio in conservation practice. How and Why? Topics in Conservation Biology. DO-10.5772/3760/
