X Chromosomes Linked to Sperm Production

The X chromosome might not be as ‘female’ as researchers once thought.

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Like most people, you probably learned in school that the X chromosome is the female counterpart to the male-associated Y chromosome. But researchers at MIT’s Whitehead Institute for Biomedical Research just proved that your high school biology teacher might have been wrong.

After what a report called the “painstaking new analysis” of the genetic sequence of the X chromosome, researchers have discovered that the X chromosome plays a role in sperm production. Yep, that’s right— the X chromosome is not strictly female.

The study was published last week in the journal Nature Genetics, and will likely cause quite a stir in the scientific community because of the X chromosome’s reputation for being a stable, well-known chromosome. In a report, Whitehead Institute Director David Page said:

“We view this as the double life of the X chromosome. The X is the most famous, most intensely studied chromosome in all of human genetics. And the story of the X has been the story of X-linked recessive diseases, such as color-blindness, hemophilia, and Duchenne’s muscular dystrophy. But there’s another side to the X, a side that is rapidly evolving and seems to be attuned to the reproductive needs of males.”

Page’s lab came upon this discovery by mistake. In an effort to compare mouse X chromosomes with human X chromosomes, the researchers had to first upgrade the human X reference sequence. That known sequence, which was compiled by the findings of more than 16 people, had possible errors and gaps which were so tiny that they likely weren’t found until now, according to the report. The researchers in Page’s lab used a new sequencing method to confirm the older sequence, finding errors and new information. The report explained:

The lab greatly improved the human X reference sequence, accurately assembling three large amplicons, identifying previously unknown palindromes, and ultimately shortening the entire length of the sequence by eliminating four major gaps. These important updates will now be incorporated into the reference sequence of the human X for use by the greater scientific community.

With this information in hand, the researchers went back to their original project, finding that 95 percent of mouse and human X chromosomes are the same. However, according to the report, 340 genes are not shared between the two species. These human genes are most active in the testicular cells, where they likely contribute to sperm production, which was previously thought to be a Y-linked task. Page says in the report:

“These genes are more likely to have roles in diseases that are related to reproduction, infertility, perhaps even testis cancer. There’s a whole book to be written about this aspect of the X.”

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