Picking the Sex of a Baby One Step Closer as Scientists Separate X and Y-Chromosome-Carrying Sperm: 'The Implications Would Be Colossal

Scientists have separated the sperm which carry X and Y chromosomes, in a study which could one day have "colossal" implications for choosing the sex of animals including humans.

Sperm carry either an X or Y chromosome, which helps to determine the sex of offspring in most mammals. In general, X and Y-carrying sperm are swimming about in semen in equal numbers, which explains why the human population, for instance, is made up of more or less equal numbers of males and females.

But as both X and Y-carrying sperm share the same proteins so the fetus can develop normally, there are no known markers which differentiate between the two. For years, scientists have been trying find a way to divide up these reproductive cells in different species, as this would help to select the sex of farm animals and humans, but they've had no success.

Now, the authors of a paper published in the journal PLOS Biology say they have found markers which show whether a sperm carries the X or Y chromosome in mice. The scientists found an X-chromosome protein in X-sperm, and used this to separate them from the Y-carrying reproductive cells. They used their technique to create litters made up of mostly one sex.

Study co-author Professor Masayuki Shimada of Hiroshima University told Newsweek of a potential use for their research. "In dairy farms, the value of female cows is much higher than male cows, because milk is only produced by the female cow. In the case of beef meet production, the speed of growing is much higher in male after castration than female. Thus, the value of male calves is higher than female."

Experts who didn't work on the research were excited by the findings, but stressed they need to be replicated in other species before they can be of use.

Peter Ellis, lecturer in molecular genetics and reproduction at the University of Kent, told Newsweek: "If this study can be replicated—and in particular if it holds true in species other than mice—then the implications would be colossal for both animal and human artificial insemination/assisted reproduction."

He asked why the researchers didn't replicate the work in other species, but added: "I doubt it will be long before someone has a look though!"

The work potentially allows for sex selection, but stressed "this is only conjecture at present and remains to be tested."

David Elliott, professor of genetics at Newcastle University who did not work on the study told Newsweek: "This study gives us a wider understanding of how sperm are made. During meiosis—the kind of cell division that makes sperm, the X chromosome has been thought to be 'turned off', with special genes on other chromosomes replacing those on the X, and these other genes would be shared between X and Y bearing sperm. During the later stages of sperm manufacture, many genes are turned off anyway, as the sperm head becomes miniaturized. This study suggests that despite this the X chromosome can still manage to create a distinct kind of sperm."

Elliott said he was surprised "that the two sets of sperm should be so different biochemically, since they develop so closely together."

"If X and Y bearing human sperm have similar differences, then in theory they could also be separated in a similar way. However, the receptors on sperm can be often different between species, so it is not a given that this would work, and there would be a lot of important ethical and safety questions before any application to humans."

James Turner, who leads the Sex Chromosome Biology lab at the Francis Crick Institute, told Newsweek: "The discovery of a protein that marks only X-sperm is really surprising, so the top priority will be to reproduce this finding, and to understand why this protein proves the exception to the rule."

Charlotte Douglas, a PhD student in the Sex Chromosome Biology lab of the Francis Crick Institute, told Newsweek existing methods for sorting bovine sperm are more efficient.

"Furthermore, an extensive assessment of the fertility/viability of the offspring generated after chemical inhibition of the sperm, particularly in agricultural species, would need to be assessed," she said.