2025-06-12T05:11:17Z

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For many years, scientists have understood that Australia’s remarkable egg-laying mammals, the platypus and echidna, possess a distinctive feature in their reproductive biology: they do not utilize the same genetic mechanisms as other mammals to develop male and female embryos. This intriguing characteristic has baffled researchers, leaving them to wonder how these unique creatures determine their sex.

However, a recent study published in the journal Genome Biology sheds light on this long-standing mystery. Our research team has discovered compelling evidence suggesting that the sex determination process in monotremes hinges on a single gene, which shares similarities with genetic mechanisms found in certain fish and amphibians, rather than the conventional mammalian approach.

Understanding the Secrets of Monotreme Sex

Monotremes, which include the platypus and echidna, represent the most ancient group of living mammals. These fascinating animals are noted for being the only mammals that lay eggs, and they exhibit various reptilian characteristics. In contrast, most mammals, including humans, determine sex through a pair of chromosomes known as X and Y. An embryo that inherits an XX combination develops into a female, while an XY combination results in a male.

In many mammalian species, the male sex is primarily determined by the SRY gene located on the Y chromosome. This gene is responsible for triggering male development. However, attempts to identify the SRY gene in monotremes have been unsuccessful, raising questions about how these animals determine their sex.

Two decades ago, researchers made an important discovery regarding monotremes: they follow a different sex-determining system that involves multiple X and Y chromosomes. Initially, scientists hypothesized that the Y chromosomes must contain a gene responsible for sex determination, but limited information was available to support this idea.

In 2008, a significant advancement was made when scientists published a full genome sequence of the platypus. However, the genome studied was derived from a female, meaning it lacked vital information about the Y chromosomes. It wasn't until 2021 that researchers managed to sequence a more comprehensive platypus genome along with the first echidna genome, revealing sequences for multiple Y chromosomes.

From these new findings, a candidate gene known as the anti-Müllerian hormone (AMH) emerged as a potential key player in sex determination for monotremes. This hormone is known to be involved in sexual development across numerous animal species.

A 100-Million-Year-Old Evolutionary Shift

The latest research marks a pivotal moment in understanding monotreme reproduction, presenting the first clear evidence that an adapted version of the AMH gene, found on one of the monotreme Y chromosomes and referred to as AMHY, serves as the sex determination gene in these unusual mammals.

The study reveals that significant changes in the AMH gene occurred early in the evolution of monotremes, which could explain how the AMHY variant emerged and assumed a critical role in male sexual development. This evolutionary milestone likely set the foundation for the unique sex chromosome system observed in the ancestors of today’s platypus and echidna around 100 million years ago, marking the point at which the AMH gene on the XY chromosomes diverged in function.

Our research demonstrates that despite considerable alterations to the AMHY gene compared to its predecessor, AMHX, it has preserved its core characteristics. Crucially, for the first time, we established that the AMHY gene activates in the appropriate tissues and at the right developmental stages to facilitate the formation of testes during male development, providing a crucial piece of the biological puzzle.

A Milestone for Mammalian Genetics

What sets the AMHY gene apart from other known mammalian sex determination genes is its hormonal nature. Rather than directly interacting with DNA to activate or deactivate other genes responsible for male development, AMHY operates at the cellular surface to influence gene expression.

Evidence is mounting that AMHY also plays a role in sex determination in various fish and amphibian species. However, the identification of AMHY as a sex-determining hormone in monotremes represents the first documented instance of such a mechanism occurring within the mammalian lineage.

Looking Ahead: Future Research Directions

As our studies continue, we aim to delve deeper into the molecular functions of AMHX and AMHY, exploring the distinct ways in which they operate in monotremes compared to other mammals. This groundbreaking research was conducted by a collaborative team of scientists from esteemed institutions, including the University of Adelaide, the University of Melbourne, the University of Queensland, Monash University, and Currumbin Wildlife Sanctuary.