Why the 'Wimpy' Y Chromosome Hasn't Disappeared
Minuscule. Misshapen. Puny.
Each of these words could aptly describe the Y chromosome, the sex chromosome that – in mammals – typically makes a male a male. Scientists seem to have settled on one adjective, however: "wimpy".
It's easy to see why. The Y chromosome is about one-third the size of the X chromosome and contains about 55 genes compared to more than 900 on the X. Over time, this imbalance has actually grown. At the Y's present rate of decay in humans, it could disappear entirely in about ten million years.
But in a new article published to the journal Trends in Genetics, biologists Paul D. Waters from the University of New South Wales and Aurora Ruiz-Herrera from the Autonomous University of Barcelona argue otherwise. While the Y chromosome is undeniably wimpy, it will continue limping on in the vast majority of mammals, they say.
The reason why essentially amounts to a biological technicality.
"We propose that the future of the Y chromosome is secure because it carries executioner genes that are critical for successful progression of male meiosis--and unlike other genes on the Y, these executioners self-regulate," Waters said in a statement.
Meiosis is a multi-step process in which cells divide to produce sperm or eggs. One step requires the silencing of both the X and Y chromosomes during a specific window. The "executioner" genes that regulate the silencing process on the Y chromosome are called Zfy genes.
"When these genes are turned on at the wrong time and at the wrong place during meiosis, they are toxic and execute the developing sperm cell. They essentially act as their own judge, jury, and executioner, and in doing so, protect the Y from being lost," Ruiz-Herrera added.
Essentially, if Zfy moved to any of the non-sex chromosomes, termed autosomes, male meiosis could not occur, the authors contend.
While Ruiz-Herrera and Water's hypothesis suggests that the end of the Y chromosome in mammals is not inevitable, it doesn't mean it can't happen. The Amami spiny rat, native to Amami Ōshima island of the Ryukyu archipelago of Japan, has no Y chromosome, and yes, there are still males. A gene called SRY, unique to the Y chromosome, serves as an "on-off" switch for all of the other genes that suppress female differentiation. Even though the Amami spiny rats don't have it, it seems that another unidentified gene acts as a substitute.
Source: Waters and Ruiz-Herrera. "Meiotic executioner genes protect the Y from extinction." Trends in Genetics. 6 Aug 2020. DOI: http://dx.doi.org/10.1016/j.tig.2020.06.008