Enter the platypus. This egg-laying mammal from Australia specializes in being contrary, and its sex chromosomes are no different.
Lucy Cooke, Bitch
While discussing the infinite complexity and variation of what sex is and how it’s determined, Lucy Cooke touches on the research of Jennifer Graves. While most known for her work with the SRY gene, she was also part of the team which discovered the platypus uses five pairs of chromosomes to determine sex.
This was to be my first project. How does this work? why don’t we get more combinations with that number of chromosomes? Are alternate combinations less viable?
In my quest for answers I found the paper A platypus’ eye view of the mammalian genome (top tier title BTW) by Frank Grutzner and Jennifer Graves. This paper mentions that the sex chromosomes do not segregate independently during male meiosis, with the 5 x chromosomes going to one pole and the 5 y chromosomes to the other. The author proposes in a later paper that this is facilitated by a linked chain of alternating sex chromosomes at meiosis in the male, with no other chains of this size having been observed in mammals (Grutzner et al. 2006).
Both papers were fascinating and I highly recommend them
Gruetzner, Frank, et al. “How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals.” Chromosoma, vol. 115, no. 2, 13 Dec. 2005, pp. 75–88, https://doi.org/10.1007/s00412-005-0034-4.
Grützner, F., & Graves, J. A. (2004). A platypus’ eye view of the mammalian genome. Current Opinion in Genetics & Development, 14(6), 642–649. https://doi.org/10.1016/j.gde.2004.09.006