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Re: Ratite evolution



   To add further fuel to ratite evolution fire, in PNAS (Proceedings of the
National Academy of Sciences of the United States of America), 1998, vol.
95, Is 14, pp. 8147-8152 there is an article titled _Evolution of the avian
sex chromosomes from an ancestral pair of autosomes_ , by AK Fridolfsson,
et.al.
   In this, the authors compared the chromosomal ancestry of the sex
determining genes in mice, humans, chickens, and ostriches.   In mammals, as
most of us know, sex determination of the offspring is chromosomally
determined, with females being XX and males being XY.   The term for this
pairing of different chromosomes is heteromorphism.   Birds and Reptiles
also have a heteromorphic pair of chromosomes that _can_ also determine the
sex of the offspring, but these chromosomes are not the same as the ones in
mammals and so are called the W and Z chromosomes.   In birds, sex is
determined by female heterogamety- the females are ZW and the males are ZZ.
   Using a chromo-helicase-DNA-binding protein gene (CHD1) for a comparative
mapping analysis, the authors concluded that they could trace the evolution
of the avian sex chromosomes from a pair of autosomes, with a divergence
date of between 60-100 mya.   They also concluded that the sex chromosomes
of ratites are different from those of the other extant avians, and that
ratite sex chromosomes could have 1) originated from a different pair of
autosomes or 2) ratites branched off before the chromosomes of all other
modern birds began to differentiate.
   This would put the separation of ratites as a group somewhere in the mid
Cretaceous, which is the time period that a lot of fantastic new avian and
dinosaurian fossils are coming from.   Does anyone (Ron Orenstein?) know if
there is fossil evidence to support this?    Ratites are commonly accepted
by one and all to be closely related to neornithines; but just how closely
are they related to enantiornithines?  Do they posses any characters that
would be found in a highly derived enantiornthine? 
   Is there a connection between homeothermy and chromosomal sex
determination (CSD)?   In many fish, some amphibs, some reptiles, sex of the
offspring is determined by the incubation temperature of the eggs (TSD).
Yet birds are CSD, apparently.   Are there any birds which are TSD?   (the
Megapodes of Australia spring to mind as possibilities, anyone know for
sure?)   What implications would this have on dinosaur sex determination?
Could hadrosaurs, or others, control the sex of their offspring ala mama
alligator, moving rotten junk on and off the nest to ensure a litter of
mostly girls (or boys)?
   Just some random thoughts after a long day....

-Bruce Mortensen
back to the lab, again!