Monotreme Sex Gene Discovery: Insights from Platypuses and Echidnas

In a groundbreaking study published in the *Genome Biology* journal on June 12, 2025, researchers have unveiled a significant discovery regarding the sex determination mechanisms in monotremes, specifically the platypus and echidna. Unlike typical mammals that rely on the SRY gene located on the Y chromosome, these unique egg-laying mammals utilize a distinct gene, AMHY, indicating a divergent evolutionary path that has puzzled scientists for decades.

Monotremes, which include the platypus and the echidna, are known as the most ancient living group of mammals, characterized by their egg-laying reproductive strategy and reptile-like features. Traditionally, mammalian sex determination is facilitated by the presence of two sex chromosomes, X and Y, where typically an XX combination results in female offspring, while an XY combination leads to male offspring. The SRY gene, located on the Y chromosome, acts as the primary driver for male development in most mammals. However, the absence of this gene in monotremes has raised questions about their reproductive genetics.

According to Dr. Linda Shearwin from the Comparative Genome Biology Laboratory at the University of Adelaide, the newly identified gene AMHY, a variant of the anti-Muellerian hormone (AMH), plays a crucial role in sex determination for these species. "Our research indicates that AMHY is pivotal in directing male sexual development, functioning distinctly from other mammalian sex determination genes," Dr. Shearwin stated.

The research team, which included experts from the University of Melbourne and the University of Queensland, discovered that the AMHY gene is activated in the appropriate tissues at the right developmental stage to facilitate the formation of testes in male embryos. This revelation offers a new perspective on the evolutionary adaptations that have occurred over the last 100 million years. The divergence in the genetic pathways for sex determination between monotremes and other mammals suggests a complex evolutionary history that may provide insights into the biology of early mammals.

The 2021 publication of an improved platypus genome and the first echidna genome provided essential sequences of multiple Y chromosomes, which were critical for this study. The researchers hypothesized that the AMHY gene emerged following significant evolutionary changes in the AMH gene, which occurred early in monotreme evolution. This discovery not only establishes AMHY as a definitive sex-determining gene in monotremes but also marks it as the first known instance of a hormone playing a direct role in sex determination among mammals.

Dr. Frank Grützner, a professor at the School of Biological Sciences, University of Adelaide, emphasized the importance of these findings, stating, "Understanding how AMHY operates differently from other mammalian sex genes could deepen our comprehension of sexual development across species. Our ongoing research will focus on the functional differences between AMHX and AMHY in monotremes compared to other mammals."

The implications of this research extend beyond academic interest, potentially influencing conservation strategies for monotremes, which are threatened by habitat loss and climate change. Furthermore, these findings could contribute to broader discussions on reproductive biology and evolutionary processes across different animal groups.

In conclusion, the identification of AMHY as a sex-determining gene in monotremes not only sheds light on the unique reproductive strategies of platypuses and echidnas but also opens avenues for future research into the evolutionary biology of sex determination. As scientists continue to explore the genetic underpinnings of these fascinating creatures, the monotreme reproductive system may serve as a critical model for understanding the complexities of sexual development in the animal kingdom.