Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes

@article{Wallis2007SexDI,
  title={Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes},
  author={M. C. Wallis and Paul D. Waters and Margaret L. Delbridge and Patrick J. Kirby and Andrew J. Pask and Frank Gr{\"u}tzner and Willem Rens and Malcolm Andrew Ferguson-Smith and Jennifer A. Marshall Graves},
  journal={Chromosome Research},
  year={2007},
  volume={15},
  pages={949-959}
}
In eutherian (‘placental’ mammals, sex is determined by the presence or absence of the Y chromosome-borne gene SRY, which triggers testis determination. Marsupials also have a Y-borne SRY gene, implying that this mechanism is ancestral to therians, the SRY gene having diverged from its X-borne homologue SOX3 at least 180 million years ago. The rare exceptions have clearly lost and replaced the SRY mechanism recently. Other vertebrate classes have a variety of sex-determining mechanisms, but… 

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TLDR
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...

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