Sex determination and sex chromosome evolution is a lot less well-understood than most people would probably assume. One of the most puzzling cases is that of the platypus and echidna, the only members left of a group of mammals that diverged from theria about 210 million years ago.
Way more than just two
The platypus has 10 sex chromosomes, while the echidna has nine. Sex determination occurs by the sex chromosomes aligning during male meiosis as X1Y1X2Y2X3Y3X4Y4X5Y5, so five X chromosomes go into one sperm cell, and five Y chromosomes go into another. What happens if the chromosomes align “incorrectly” is entirely unknown, and considering how understudied monotremes are, unlikely that we’d get to see any examples (but I’d imagine one could learn a lot about their sex-determination from that).
No SRY or DMRT1
In placental mammals and marsupials, the SRY gene on the Y chromosome is what determines sex, as it controls the formation of testes.
Birds however, have a ZZ/ZW system where the females are actually the heterogamateic sex having a Z and a W sex chromosome. Sex determination in birds is controlled in a dosage-dependent manner where the females only have one copy of the DMRT1 gene, while the males, being ZZ have two copies.
What’s really interesting though, is that the SRY gene is absent in monotremes and very few genes have actually been identified on the Y. One gene suspected to play a role in sex determination is Y chromosome gene, anti-Müllerian hormone, Amhy, though the molecular bases actually determining sex in monotremes is still not known.
Platypus X1Y1X2Y2X3 are homologous to echidna X1Y1X2Y2X3, however the platypus X5 is homologous to echidna X4, platypus Y5 is homologous to echidna Y3, and platypus X4 and Y3 homologs are in echidna autosome 27 whereas echidna X5 and Y4 are in autosomal platypus chromosomes.
Genes found on the therian X chromosome are orthologous to platypus chromosome 6, the homolog of echidna autosome 16. SOX3, thought to be the sort of “X-version of SRY” is autosomal in monotremes, as well as other non-therian vertebrates. SRY is probably a derived trait that only evolved after the divergence of monotremes from other mammals.
That being said, no homology has been shown so far between any platypus X chromosomes and therian X. Monotremes XY chromosomes are unique but show some homology with bird chromosomes, which is especially interesting considering in birds sex-determination works very differently from mammals and females are the heterogametic sex.
Whether sex-determination works in monotremes as a dosage-dependent set up (like in birds), or in a Y-gene set up as seen in humans, is still unknown.
Monotremes are relatively ignored because there’s just not very many of them, but for an evolutionary biologist they can be very useful in trying to answer questions related to amniotic lineages and divergence. While still mammals, they are genetically very distant from other mammals, even though people tend to maybe think of marsupials and monotremes as being more similar (both Australian, both not placentals), monotremes are a real anomaly.
- Diego Cortez, Ray Marin, Deborah Toledo-Flores, Laure Froidevaux, Angélica Liechti, Paul D. Waters, Frank Grützner, Henrik Kaessmann. Origins and functional evolution of Y chromosomes across mammals. Nature, 2014
- Grützner F., et al. Nature, doi:10.1038/nature03021 (2004).
Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes. M. Wallis-P. Waters-M. Delbridge-P. Kirby-A. Pask-F. Grützner-W. Rens-M. Ferguson-Smith-J. Graves – Chromosome Research – 2007