Platypus and echidnas break mammal rules with weird sex system

Tiny, spiny echidnas and the duck‑billed platypus already look like misfits, but the bigger oddity hides in their DNA. For most mammals, a gene on the Y chromosome called SRY flips the switch for testis formation and sets sex development in motion.

Researchers have now shown that these monotremes wield a completely different master key: a Y‑linked copy of anti-Müllerian hormone (AMHY).

Dr. Linda Shearwin of the University of Adelaide’s School of Biological Sciences led the international team that cracked the case.

Different sex gene in monotremes

Platypuses carry five X and five Y chromosomes that line up in an alternating chain during male meiosis, a structure unlike anything in other mammals.

Females hold five paired X chromosomes, yet the species thrives, proof that complicated does not always mean fragile.

Genomic clocks place the monotreme split from other mammals at roughly 187 million years ago, long before SRY arose in placental and marsupial lineages.

Their last common ancestor with us never carried SRY, so a different mechanism had to evolve.

Monotremes have 10 sex chromosomes

Monotreme males don’t just have an X and a Y. They juggle a system with up to 10 sex chromosomes, five Xs and five Ys in platypuses, and a slightly smaller set in echidnas.

These chromosomes form a chain during sperm production, a configuration that looks more like a daisy chain than the standard X–Y pair mammals typically carry.

This odd setup baffled scientists for decades. Unlike therian mammals, monotreme sex chromosomes share homology with bird Z chromosomes, not with the mammalian Y.

That raised suspicions early on that monotremes must use a completely different trigger to determine sex, but proving it required access to rare developmental stages.

Anti‑Müllerian hormone normally turns off female tract development in male embryos and later guides ovarian follicle growth. In people, doctors measure AMH to gauge fertility or predict menopause timing.

Most mammals keep AMH on an ordinary chromosome, far downstream of SRY and SOX9. In echidnas and platypuses, evolution duplicated the gene, parked the copy on a Y chromosome, and rewired its control panel to switch on early in male gonads.

Sex starts outside the cell’s nucleus

“This new understanding fills a significant knowledge gap and ultimately can be used to support conservation of these iconic Australian mammals. AMHY would be the first known example of a growth factor playing a sex‑determining role in mammals,” said Dr. Shearwin.

Her team tracked AMHY activity in developing echidna testes well before visible sex differences appeared. 

The protein works outside the nucleus, binding a surface receptor and relaying signals rather than directly steering DNA.

That makes monotremes the only known mammals where a secreted factor, not a transcription factor, calls the first shot in sex development.

What makes AMHY different from AMHX

Although a X‑linked copy of anti-Müllerian hormone (AMHX) and AMHY both belong to the TGF-β protein family and share key structural features, they diverged significantly after an ancient gene inversion.

The two proteins are only 52% identical, with differences concentrated in regions that affect hormone cleavage and signaling.

Their promoter regions also diverge. AMHY lacks the usual SOX9, GATA4, and SF-1 binding sites seen in AMHX and other mammalian AMH genes.

Instead, AMHY uses unique transcription factor sites, like DMRT1 and GATA1, which are active only in male gonads, suggesting a custom regulatory setup designed for sex determination.

Wider view of sex genes

AMH duplications have taken over sex determination more than once. A male‑specific AMHY copy steers testis formation in Patagonian pejerrey fish and old‑world silversides.

Some amphibians use the same trick, showing that hormones can win the evolutionary race against classic DNA‑binding proteins.

The discovery also underlines how fragile Y chromosomes can be. SRY itself did not appear until about 160 million years ago and was identified in humans only in 1990.

If SRY could pop up late, there is no reason other genes cannot step in when needed, as monotremes demonstrate.

Why monotreme sex matters

Platypus populations face habitat loss and climate change, and captive breeding programs struggle because eggs are delicate and reproductive cycles poorly understood.

Knowing that AMHY, not SRY, dictates sex gives biologists a tool for early‑stage sexing of embryos in conservation labs.

The work also helps physicians dissect AMH’s many roles in humans. Because monotreme AMHY triggers an entire sex pathway on its own, comparing its sequence to human AMH may spotlight sites essential for receptor binding or signal strength. That insight could refine AMH‑based fertility tests or therapies.

To test AMHY’s function, researchers tried introducing platypus AMHX and AMHY into developing chicken embryos, a model known to respond clearly to sex-determining signals.

Overexpressing chicken AMH in females causes visible masculinization, so scientists hoped platypus versions might do the same.

But nothing changed. The monotreme proteins didn’t cause male traits in chickens, even when highly expressed.

Structural analysis showed that key binding sites between mammalian AMH and the bird receptor don’t line up well.

This mismatch confirms that early mammalian changes in AMH–AMHR2 interaction likely limit cross-species functionality.

The full study is published in Genome Biology.

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