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View Diary: Birds do it, paramecia do it (51 comments)

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  •  There's no satisfying short answer to this (8+ / 0-)

    so the long one is below.  The short answer is that XY did not evolve from ZW and in fact both systems have evolved multiple times.

    I assume you are referring to the bird ZW system in which males are ZZ and females are ZW.  It is clear from even a brief examination of the data on sex determination systems that the ZW system is not ancestral to the XY system in vertebrates.  In fact both of them seemed to have evolved independently more than once.  Here's some info.

    All birds have a ZW system as far as I know.  The closest living relatives of birds, crocodilians, completely lack sex chromosomes and sex is determined by incubation temperature.

    Some turtles have temperature dependent sex determination and others have sex chromosomes.

    The squamate reptiles (snakes and lizards) have examples of ZW, XY, and temperature dependent sex determination.  The location of the sex determining genes indicates that these systems evolved independently in this group from birds and mammals.  The tuatara, a lizard-like reptile from New Zealand, is the closest living relative to the squamates has temperature dependent sex determination.

    Mammals almost all have the XY system.  The duck-billed platypus has a bizarre 5X5Y male and 10X female system but the X and Y chromosomes are not the same genetically as in other mammals.

    Both fish and amphibians have both XY and ZW systems - in fact there are examples of species in both groups in which different populations of the same species have different mechanisms (well that's what people claim - I would argue that having different sex determination systems automatically make them different species and would call them very closely related species).  There are also fish that change sex over their lifetimes and some fish that are both male and female at the same time.

    So sex determination in vertebrates has been the subject of a lot of evolution.  I'm not that familiar with the literature on why there has been so much evolution.  If anyone's interested this review might be a good place to start (free for anyone).

    "To see both sides of a quarrel, is to judge without hate or alarm" - Richard Thompson

    by matching mole on Sat Jun 07, 2014 at 01:10:15 PM PDT

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    •  Thanks for taking the time to give the long answer (2+ / 0-)
      Recommended by:
      RiveroftheWest, matching mole

      Yes, I was thinking of ZW but couldn't call up the letters off the top of my head. I knew it was the system for birds, had read that it was for certain lizards, and over-extrapolated. (I also knew of the temperature dependent sex determiniation in crocodilians, which should have set my cognitive dissonance sirens blaring, but didn't.)

      I'll make time for that review over the next week.

      •  The review is on insect sex determination (0+ / 0-)

        (I didn't realize that initially) so it won't necessarily give insight into vertebrates in particular but it should help with more general questions.

        "To see both sides of a quarrel, is to judge without hate or alarm" - Richard Thompson

        by matching mole on Sun Jun 08, 2014 at 04:14:17 AM PDT

        [ Parent ]

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