An Opinion paper released in Cell Press Reviews yesterday has provided an alternative hypothesis to why the Y chromosome still persists – the persistent Y hypothesis.
The shrinking Y chromosome
Understanding the origin and evolution of the eukaryotic Y chromosome has been one of the most fascinating questions in biology. Since its origin more than 165 million years ago, the Y chromosome has continued to shrink and become much smaller than its counterpart the X chromosome. Despite its reference as being ‘wimpy’ in this review, the Y chromosome has continued to persist in almost all mammals. In contrast, other vertebrate lineages see a high turnover of sex chromosomes. A mechanistic solution is necessary in solving why the therian Y sees such persistence.
Throughout evolution, the therian Y lost functional genes and degraded. The only region of homology that remains between the X and Y chromosome is a small pseudoautosomal region (PAR) that synapses during male meiosis. An obligatory recombination event occurs during meiosis and therefore requires this region.
The “wimpy” theory
The ‘wimpy’ theory states that the Y chromosome is a degraded relic of the X, which will eventually be lost in approximately 10 million years at a linear rate of decay. However, Y gene loss occurs in waves, with the current Y chromosome relatively stable and enriched with genes that may function in male viability. This theory is complemented with the ‘fragile’ Y hypothesis which suggests that Y chromosome aneuploidy and the size of the recombining region have a negative correlation.
The collaboration between Professor Paul Waters, from the University of New South Wales, and Professor Aurora Ruiz-Herrera, from the Autonomous University of Barcelona, outlined a new theory. The persistent Y hypothesis proposes that Y-linked meiotic executioner genes are key in preventing Y loss.
The persistent Y hypothesis
During gametogenesis, meiotic silencing is an important checkpoint to avoid ploidy. In therian mammals, this involves X chromosome inactivation during female meiosis. However, during male meiosis, both sex chromosomes are silenced. This phenomenon is called meiotic sex chromosome inactivation (MSCI). Maintaining MSCI in males is essential to avoid ectopic expression of so-called executioner genes.
Executioner genes must be silenced during MSCI for meiosis to progress. If translocated to an autosome, these genes will be ectopically expressed, which in turn will induce apoptosis. The only region executioner genes can be heritably translocated to is the X chromosome. Here, the genes are subjected to obligatory meiotic silencing and; therefore, meiosis can progress. No other scenario is tolerated by germ cells. This poses strong evolutionary constraint for the Y to persist because only rare transposition events to the X chromosome can allow subsequent Y loss.
Aurora Ruiz-Herrera stated:
“We believe that bearing these genes is what protects the Y chromosome from extinction. The genes that regulate the silencing process, the Zfy genes, are called ‘executioner’ 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.”
Several details remain unclear, including the origin of these executioner genes and the intricacies of meiotic silencing. Nonetheless, this hypothesis provides a potential mechanism to explain how the Y chromosome continues to escape its fatal fate.