Scientists have made significant strides in understanding the intricate development process of Australia’s egg-laying mammals, the platypus and echidna. Although it has been established for decades that these monotremes employ a unique genetic system to determine sex, the precise mechanics of this system were largely unknown—until now.
A groundbreaking study published in Genome Biology sheds light on the genetic underpinnings of sex determination in these ancient mammals. The research reveals that the determination of male and female sex in these species hinges on a single gene, known as anti-Müllerian hormone (AMH). This gene shows closer ties to similar sex determination mechanisms found in certain fish and amphibians than to the sex-determining systems seen in most other mammals.
Humans and a majority of mammals utilize two sex-determining chromosomes: X and Y. Typically, embryos with an XX chromosome configuration develop into females, while XY correlates with male embryos. For mammals, the SRY gene located on the Y chromosome instructs the embryo to develop as male; however, this gene has never been identified within the genetic architecture of monotremes.
Research dating back to approximately 20 years suggests that monotremes function with a distinct sex determination system that incorporates multiple X and Y chromosomes. Scientists initially suspected that the Y chromosomes contained a gene responsible for determining sex, but details surrounding it remained scarce until recent advancements.
In 2008, researchers released a full genome sequence for the platypus, revealing considerable information but lacking insights into Y chromosomes, as the data derived from a female specimen. The past couple of years saw significant progress with the completion of updated platypus and echidna genomes, which included sequences for multiple Y chromosomes. From this research, the AMH gene emerged as the leading candidate for the role of sex determination.
The new study elucidates how an adapted version of AMH, termed AMHY, plays a crucial role in sex determination. The researchers demonstrate that this gene, originating from a common ancestor nearly 100 million years ago, evolved along a different path compared to its counterpart on the X chromosome (AMHX). Through this evolution, AMHY assumed its sexual development role in males, providing the required genetic information to direct testicular development at the appropriate time and location.
In contrast to other mammalian sex determination genes that interact directly with DNA to activate male developmental pathways, AMHY functions as a hormone that triggers cellular responses rather than engaging directly with the DNA. This remarkable discovery marks AMHY as the first known case of a hormone influencing sex determination in mammals, a role previously observed in various fish and amphibians.
As further research unfolds, scientists aim to explore the functional differences between the AMHX and AMHY genes, deepening our understanding of the unique reproductive strategies of monotremes, offering a glimpse into the evolutionary intricacies that distinguish these remarkable creatures from other mammals. The collaborative research involving teams from the University of Adelaide, the University of Melbourne, the University of Queensland, Monash University, and Currumbin Wildlife Sanctuary marks a pivotal moment in the study of evolutionary biology.
