And that knowledge allows them to (likely subconsciously) influence the sex of their offspring, a new study reveals. Their reproductive systems appear to know whether a sperm will produce a boy or a girl -- even before the sperm reaches the egg.
The sex allocation of offspring is usually considered a matter of chance. It simply depends on which sperm gets there first: an X-chromosome bearing (female) sperm or a Y-chromosome bearing (male) sperm. In nature, the ratio is usually about 50:50, but these findings suggest that females might be able to change the environment of their oviducts (or fallopian tubes) to favor one sex over the other. This has been seen in humans and other animals, especially during famines, but how it happens has been a mystery.
To figure out how some females can sense the sex of sperm, a team led by Alireza Fazeli from the University of Sheffield injected four female pigs with all X sperm into one of their oviducts, and all Y sperm into their other oviduct. Then they analyzed the gene expression in those tubes.
They found that under these experimental conditions, different genes are active in the cells of female pigs’ reproductive systems depending on if all X or all Y sperm are present. In particular, 501 genes -- that’s about 2 percent of the pig genome -- consistently produced different protein amounts depending on whether it was an X or a Y oviduct. These included genes involved in hormone, immune system, and signal transduction functions.
"What this shows is that mothers are able to differentiate between the sperm that makes boys and girls. That on its own is amazing,” Fazeli says in a news release. “It's also of great scientific and evolutionary importance. If we understand how they can do that, this can revolutionize the field.” The team is now working on figuring out what it is exactly that the female system is sensing when they’re screening the sperm.
The work was published in BMC Genomics this week.
Image: University Sheffield