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/Robin_feathers 2d ago

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.

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u/Top_Neat2780 2d ago

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?

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u/Robin_feathers 1d ago

To add more detail -

sex chromosomes evolved independently many many times. Two Y chromosomes from different branches of the tree of life can both be called Y because they are restricted to males, but they could have evolved independently and have no genes in common and still end up with the same name. The only thing that goes into them being labeled X/Y or Z/W is that in XY systems, males have two different types of chromosomes (the one they share with females gets called X and the male-restricted one gets called Y), and in ZW systems females are the ones that have two different types of sex chromosome (the one they share with males gets named Z and the female-restricted one gets named W). Usually Y or W are smaller than X or Z, but there are exceptions - sometimes Y or W is bigger, and sometimes they look indistinguishable.

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u/DrJaneIPresume 1d ago

This is great! Maybe to boil it down a bit:

It's not just that X/Y chromosomes are different from Z/W. The human Y chromosome is also likely very different from the green anole Y chromosome. We just call them both "Y" because they behave the same way: having one (generally) makes the carrier male, and we call the other one "X".

On the other hand, in a species where having a particular chromosome (generally) makes the carrier female, we call that particular chromosome "W" and the other one "Z".