Scientists at Hiroshima University have discovered a new sperm separation technique that could be used for sex selection in future. 

The study, published in PLOS biology, showed for the first time that it is possible to selectively slow down the motility of X-bearing sperm, distorting the normal 50:50 offspring ratio. The researchers say they have also used the method to selectively produce male and females in cattle and pigs, but they have not yet published their research on this.

'This is the first study to scientifically [show] the functional differences, [i.e.] fertilisation ability, between X-sperm and Y-sperm,' Masayuki Shimada, co-author of the research from Hiroshima University, told the Guardian.

The mouse X chromosome codes for over 3,000 genes, whereas the Y chromosome codes for less than 700 genes. These differences in gene expression can be used as a basis to distinguish between X- and Y-bearing sperm. The researchers focussed on genes expressed exclusively on the X chromosome that code for two receptors, TLR7 and TLR8, which play a role in mediating immune responses. They found that treating mouse sperm with Resiquimod, an antiviral drug that activates these receptors, selectively slows down the movement of X sperm without damaging them.

Sperm cells usually swim upwards when placed in a test tube; however, following treatment with Resiquimod, there were fewer X-bearing sperm in the upper nutrient-rich layer. Further investigations revealed this was due to the drug impeding energy production in the X-bearing cells.

The researchers then collected drug-treated mouse sperm from the upper and lower layer of the test tube and used it for in IVF in mice. They found that fertilising mouse eggs with sperm from the lower layer resulted in litters that were 81% female. Whereas sperm from the upper layer produced litters that were 83% male.

Professor Robin Lovell-Badge, group leader at The Francis Crick Institute, said: 'The authors of this very nice paper show that this [Resiquimod] can be used to separate the X- and Y-bearing sperm… This could be very useful if it was shown to work well with a number of farm animals where it is beneficial to have an excess of males (e.g. beef cattle) or females (e.g. dairy cattle).'

Since this technique is faster and cheaper than current methods for sexing mammalian sperm, it may be relevant for the livestock sector and for human sperm selection in IVF to prevent sex-linked disorders. However, the long-term effects of this method and whether these receptors are present on human sperm still need to be determined in further studies. 

'There is a long way to go to prove exactly how the activity of these receptors has an effect on sperm motility, whether the human equivalents show the same expression and would have the same effect,' said Professor Lovell-Badge. 'While the mice born after the sperm sorting apparently appeared normal, it would be essential to verify that there were no long-term effects of activating these receptors prior to fertilisation. In other words, do not try this at home in attempts to bias the likelihood of having a boy or a girl.'

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Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.