For years, scientists have been puzzled by the sex determination process in platypuses and echidnas, the egg-laying mammals of Australia. Unlike most mammals, they do not follow the typical genetic pattern for male and female development.

A recent study published in Genome Biology sheds light on this mystery. Researchers have identified a single gene that dictates sex in monotremes, resembling the mechanism seen in some fish and amphibians.

The Quest for the Monotreme Sex Gene

Monotremes, such as the platypus and echidna, represent the oldest group of mammals. These unique creatures lay eggs and possess reptile-like characteristics. While many mammals use X and Y chromosomes to determine sex, monotremes have a different system.

In most mammals, the SRY gene on the Y chromosome triggers male development. However, this gene was missing in monotremes. Previous studies revealed that monotremes have multiple X and Y chromosomes, but the sex-determining gene remained elusive.

Recent advancements in genome sequencing provided crucial insights. The discovery of the anti-Muellerian hormone (AMH) on a monotreme Y chromosome, known as AMHY, emerged as a strong candidate for sex determination.

A Genetic Evolution 100 Million Years Ago

The research unveiled that changes in the AMH gene led to the emergence of AMHY as the sex determination gene in monotremes. This evolutionary event occurred approximately 100 million years ago, shaping the unique sex chromosome system of platypuses and echidnas.

Unlike conventional mammal sex genes, AMHY functions as a hormone that regulates gene expression on cell surfaces. This mechanism differs from the direct DNA interaction seen in other species.

Future Directions

Further investigations aim to explore the distinct roles of AMHX and AMHY in monotreme sexual development. This groundbreaking discovery opens new avenues for understanding sex determination in mammals.

Collaborating institutions include the University of Adelaide, the University of Melbourne, the University of Queensland, Monash University, and Currumbin Wildlife Sanctuary.