Cheap, simple technique allows sex selection of sperm before fertilization.

Sperm sorting is a technique that allows the selection of which type of sperm cell to fertilize the egg cell. It can be used to distinguish which sperm are the most healthy, as well as for determination of more specific traits, such as sex selection in which sperm are separated into X- (female) and Y- (male) chromosome bearing populations based on their difference in DNA content. The resultant ‘sex-sorted’ spermatozoa are then used in artificial insemination when farming animals or in-vitro fertilization (IVF) in human medical practice. Newly applied methods such as flow cytometry expand the possibilities of sperm sorting, however, these techniques are cumbersome, expensive, and risk damaging the DNA of the sperm. Now, a study from researchers at Hiroshima University develops a simple, reversible chemical treatment which separates X-bearing sperm from Y-bearing sperm. The team state even though their study was performed in mice, the technique is likely to be widely applicable to other mammals also. The opensource study is published in the journal PLOS.

Previous studies show that during sperm development in male mammals, the X and Y chromosomes are segregated into different cells so that an individual sperm will carry either one or the other, with an X chromosome giving rise to female offspring and a Y chromosome producing male offspring. Unlike the Y chromosome, which carries very few genes, the X chromosome carries many, some of which remain active in the maturing sperm. This difference in gene expression between X- and Y-bearing sperm provides a theoretical basis for distinguishing the two. The current study identifies chemicals which bind to two proteins on the female sperm cell’s surface, coded specifically by the X-bearing sperm’s genes, which slow down the movement of X-carrying sperm without affecting the Y-carrying ones.

The current study identifies almost 500 genes which are active only in X-bearing sperm, of which 18 genes encode receptors on the sperm cell’s surface. Results show a chemical which binds to a pair of receptors on the X-bearing sperm cells’ surface, called Toll-like receptor 7 and 8 (TLR7/8), slowed sperm motility without impairing either sperm fertilization ability or viability. Data findings show the effect was due to impaired energy production within the X-bearing sperm and could be reversed by removal of the chemical from the medium.

Results show treatment of mouse sperm with this retarding chemical, followed by IVF with the fastest swimmers, led to litters that were 90% male. Data findings show when the slower swimmers were used instead, the litters were 81% female. The lab state they have also succeeded in the selective production of male or female offspring in cattle and pigs using this method.

The team surmise they have developed a cheap and simple technique that allows them to separate mouse sperm carrying an X chromosome from those carrying a Y chromosome. For the future, the researchers state their technique has the potential to greatly simplify sex selection for either IVF or artificial insemination in animals.

Source: Hiroshima University

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