'Lost' Y chromosomes discovered on autosomes

Chuck Bednar for redOrbit.com – @BednarChuck

Essential genes lost by the Y chromosome in humans and other mammals and previously thought to have been eliminated have actually just relocated to other chromosomes, the authors of a study appearing in the latest edition of the journal Genome Biology have discovered.

In the paper, lead author Dr. Jennifer Hughes of the Whitehead Institute for Biomedical Research and her colleagues explain that, while the Y chromosome has the majority of the 640 genes it had once shared with the X chromosome, those genes live on elsewhere in the genome.

“The Y chromosome,” Dr. Hughes, a research scientist in the lab of David Page, told redOrbit via email, “is particularly vulnerable to gene loss because it does not have a homologous partner for genetic recombination. Our lab’s previous work comparing Y-chromosome gene content across eight mammals revealed that the genes that did survive on the Y are extraordinarily long-lived and likely serve important biological functions.”

“However, there are numerous exceptions where seemingly critical genes were lost from the Y chromosome in certain mammals,” she added. “We discovered that in many cases, these genes were not actually eliminated but have found new homes in the genome. Our new study reveals that gene loss from the Y chromosome has been rescued by gene transposition, or relocation, to another chromosome at least 8 times in diverse mammalian species, including human.”

Mechanism more widespread than previously believed

One example of genes apparently disappearing from the Y chromosome is found in a species of rodent indigenous to an island in Japan, the Ryukyu spiny rat. This creature’s Y chromosome had vanished completely, with most of the genes linked to it moving on to either the X chromosome or to non-sex chromosomes known as autosomes.

Previously, this was believed to be an oddity found only in this one type of creature. However, new data suggests that the phenomenon of gene-relocation is widespread among several different mammal species, including humans. Critical genes thought lost from the Y chromosome have simply changed locations and continued functioning as normal, Dr. Hughes explained.

Dr. Hughes said her team found four genes that had completed this relocation, demonstrating for the first time that this phenomenon takes place in humans as well as in a wide range of other mammal species, including apes, rodents, cattle, and marsupials. These genes have been preserved, she said, because they are “indispensable for normal development.”

“Our work demonstrates that rescue of Y gene decay via gene transposition is common among mammals,” she told redOrbit. “Our findings also provide a new explanation for the relatively high frequency of gene transposition off of the sex chromosomes, which had previously been explained by the drive to escape inactivation during male germ cell development.”

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