Sperm can be segregated easily into male and female, scientists have shown for the first time, in a breakthrough which experts said could have ‘disruptive social consequences’ if applied to humans.
Japanese researchers discovered that sperm bearing the ‘X’ chromosome - which generates a female when it joins with the ‘X’ chromosome of an egg - carry molecules which when activated slow down its movement.
When a chemical to trigger those receptors is added to sperm, the male ‘Y’ chromosomes power ahead, separating themselves from the tardy ‘X’s.
In mice sperm, when the fast swimmers were gathered and used in IVF, they produced litters that were 90 per cent male. Similarly, when the slower sperm was used, the litters were 81 per cent female.
The team from Hiroshima University also told The Telegraph they had successfully used the technique in cattle and produced sex ratios of 90 per cent, although those experiments have not yet been published.
Scientists believe the method could help farmers to produce more valuable female dairy cattle or male beef cattle.
Dr Masayuki Shimada said: “We have already adapted this method to cattle production by in vitro fertilization and to pig production by artificial insemination. The successful efficient ratio by this method in cattle in IVF is more than 90 per cent in both male and female. Hiroshima University has applied for a patent.
“I think that this technique would be available in human IVF. We have to discuss the ethics issue before this technique is adapted in human.”
British experts said the implications could be ‘colossal’ but warned that it could be used for sex selection of humans, which although banned in the UK is legal in other countries, including the US.
Dr Peter Ellis, lecturer in Molecular Biology and Reproduction at the University of Kent said: “If this study were to be replicated – and in particular if it holds true in species other than mice – then the implications could be colossal for both animal and human artificial insemination and assisted reproduction.
“Routine sex skewing in livestock animals would be a major boon with dramatic benefits for animal welfare in many species. Sex skewing in humans would be an ethical minefield with the potential for unpredictable and disruptive social consequences.”
Usually in nature sperm bearing an X or a Y are present in equal numbers and so have an equal chance of fertilising an egg, allowing the sex ratio of most populations to be about 50:50.
Some techniques already exist to separate sperm by sex. Farmers use a special dye to stain DNA so that the genetic material can be measured. X-bearing sperm have slightly more DNA.
But the method is laborious, expensive and can damage the sperm. To find a better method scientists looked for receptor molecules that were only on the outside of ‘X’ chromosomes. Receptors act like the eyes and ears of the cell, and can receive information from the outside so they act as a letter-box for scientists trying to get a new message inside.
They found a pair or receptors called Toll-like receptor 7 and 8 (TLR7/8) and showed that sending a chemical message inside slowed down sperm speed by limiting energy production, like activating a dimmer switch. Crucially it could be done without causing them harm or impairing fertilisation.
Prof Robin Lovell-Badge of The Francis Crick Institute, said: “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, for example in beef cattle, or females in dairy cattle.
“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.”
Experts also said they were baffled about why the X chromosome would have evolved a mechanism that would harm its own chance of reproducing. But they said it could explain how sex ratios sometimes differ from 50:50 with environmental conditions, for example male kangaroos are selected more than females after periods of drought.
The research was published in the journal Plos Biology.