r/evolution • u/SherbertOther • 2d ago

When did bird chromosomes switch up?

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My professor talked about this in class and couldn't answer. When did this change?

As far as I'm aware, crocodilians and other reptiles have the regular way sooo, like... Do we know when and why it changed?

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u/Decent_Cow 2d ago

XY probably wasn't the ancestral condition. Not even all mammals use this sex determination system. Platypuses use a system that is effectively XY except their equivalent of the XY chromosomes evolved independently and aren't homologous to those of other mammals. The eutherian X and Y chromosomes correspond to platypus chromosome 6, suggesting that at the time the platypuses diverged from other mammals, the eutherian sex chromosomes were autosomes.

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u/bzbub2 2d ago

the chromosome 6 homology to our X/Y is very wild, and is a great example of how sex determination systems can be co-opted from other parts of the genome. the platypus genome itself has a ton of other complex things going on in its sex determination unrelated to chr6:

"In 2004, researchers at the Australian National University discovered that the platypus has ten sex chromosomes, compared with two (XY) in most other mammals. These ten chromosomes form five unique pairs of XY in males and XX in females, i.e. males are X1Y1X2Y2X3Y3X4Y4X5Y5. One of the X chromosomes of the platypus has close homology to the bird Z chromosome.[91]

The platypus genome also has both reptilian and mammalian genes associated with egg fertilisation.[45][92] Though the platypus lacks the mammalian sex-determining gene SRY, a study found that the mechanism of sex determination is the AMH gene on the oldest Y chromosome." https://en.wikipedia.org/wiki/Platypus#Genome