Researchers from UNSW Sydney have discovered fundamental differences in biological processes between males and females by studying the unique sex chromosome systems of the platypus and the chicken. These findings, published in Proceedings of the National Academy of Sciences (PNAS), offer a fresh perspective on how sex chromosomes evolved and function, potentially leading to new discoveries in biology.
Mechanisms of Dosage Compensation
Mammals, including humans, exhibit a difference in sex chromosome composition, with females having two X chromosomes and males having one X and one Y chromosome. This imbalance necessitates a mechanism known as sex chromosome dosage compensation. According to Dr. Nicholas Lister from UNSW’s School of Biotechnology and Biomolecular Sciences, “This imbalance is corrected by a process called sex chromosome dosage compensation.”
Traditionally, in female mammals, one of the X chromosomes is silenced to balance the gene products on the sex chromosomes, preventing females from producing double the amount of proteins from the X chromosome compared to males. This silencing ensures that gene expression is balanced between the sexes.
Protein Balance Beyond mRNA Levels
The study highlighted an interesting discovery: despite differences in mRNA levels, protein production remains balanced between the sexes. Associate Professor Paul Waters, also from UNSW’s School of Biotechnology and Biomolecular Sciences, explained, “Being male or female affects mRNA levels of X chromosome genes, which we would then expect to affect protein production.”
However, the study demonstrated that a balance of proteins occurs even when mRNA levels aren’t balanced, suggesting that dosage compensation is essential across all vertebrate species, not just mammals.
Insights from Platypus and Chicken
The researchers focused on the platypus and the chicken, two species with vastly different sex chromosome systems. Lister noted, “Platypus are monotreme mammals with interesting sex chromosome systems. They have five pairs of X chromosomes in females and five X’s and five Y’s in males. Birds, such as chickens, have a ZW system, where males have two copies of a Z chromosome and females have a Z and a W chromosome.”
Waters added, “In birds and monotremes, there is an imbalance of mRNA between the sexes. For the first time, we show that this imbalance is corrected at the protein level, indicating a novel mechanism of dosage compensation different from humans.”
Technological Advances and Their Impact
Co-author Professor Jenny Graves from La Trobe University reflected on past assumptions: “As the genes were silenced by their failure to make RNA, the control of dosage compensation was assumed to be at the level of RNA only—not at the level of making proteins.” However, measuring protein levels has historically been more challenging than measuring mRNA levels due to technological limitations. With advancements in technology, it is now evident that dosage compensation occurs at the protein level in the platypus and the chicken.
Future Research Directions
The authors stress the complexity of genetic regulation and the importance of considering multiple levels of control in gene expression. Co-author Shafagh Waters from UNSW’s School of Biomedical Sciences remarked, “Studying unique species like the platypus provides us with new insights into the cellular and molecular mechanisms that could regulate various aspects of human physiology, or be implicated in disease states.”
Lister also emphasized the potential for future research: “This work will help us discover other dosage compensation systems in nature. We can find out how these evolved and how they work in other species.” Waters concluded, “Understanding these processes in other species can enhance our grasp of gene regulation at a fundamental level.”