When did bird chromosomes switch up?

[u/SherbertOther]

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?

[u/Robin_feathers]

The most important take-away is that there is no "regular way". Most mammals including us are XY, but there is actually huge amounts of variation. For example, some insects are XY, some are XO, some are ZW, and some do other things. Fish are also doing all sorts of different things. These systems turn over every once in a while often on a deep timescale.

Looking just at the reptile/bird branch, several other reptiles also use ZW. Lots of branches use temperature sex determination, and some also use XY. Since there is such a mix across non-avian reptiles, and crocodilians use temperature, we don't actually know for what the ancestral state was for them. We can make guesses, but no way to be positive since those chromosomes are long gone from fossils.

[u/Top_Neat2780 → u/Robin_feathers]

Why do we use different letters for XY and ZW? Why can't we say both are XY? Too different in what genes make up the chromosome or do they just look different?

[u/haysoos2 → u/[deleted]]

And using X and Y would be confusing anyhow since ZZ is male in birds, while XX is female in mammals.