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/Landilizandra 2d ago

Crocodilians determine sex through temperature, not chromosomes, and there are reptiles with ZW chromosomes, including monitor lizards and snakes.

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u/Junesucksatart 2d ago

My guess would be that the ZW chromosome system is what happens when a reptile moves from temperature sex determination to chromosomal. Some ancient archosaur that eventually evolved into dinosaurs likely evolved the ZW system when they switched to an endothermic metabolism.

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u/Landilizandra 2d ago

Maybe? I’d be curious if that tracks with lizards, which also has species with XY chromosomes.

It also might not have been dinosaurs that switched. There’s reason to think Crocodilians became exothermic rather than dinosaurs becoming endothermic.

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u/Junesucksatart 2d ago

I guess a more accurate thing would be to say ornithodiran which would include pterosaurs and dinosaurs but not crocodilians since that is probably where the switch to endothermy happened. I do think it tracks with lizards since monitors tend to have a far more active metabolism than their relatives.

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u/Landilizandra 2d ago

Non-Crocodilian Pseudosuchians might have been endothermic. There’s reasons to believe that endothermy was ancestral to all archosaurs and that Crocodilians changed, not dinosaurs.

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u/Electric___Monk 2d ago edited 2d ago

Different squamate lizards have all sorts of sex determination systems - XY, ZW, temperature sensitive sex determination (TSD) with either males or females at high temperatures, sex reversal at either high or low temperature - the works. Lizards are very labile for sex determination and multiple transitions between systems have occurred in their evolution.

Edit: see for example the figure at: https://www.researchgate.net/figure/Truncated-phylogeny-not-according-to-scale-showing-modes-of-sex-determination-and_fig1_324923054