title: The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z.
abstract: BACKGROUND
  Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.
  
  
  RESULTS
  Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X1, X2, X3, X5, and in chromosome Y1.
  
  
  CONCLUSIONS
  Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.
authors.affiliation: Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 OES, UK. wr203@mole.bio.cam.ac.uk
authors.forename: Willem
authors.initials: W
authors.lastname: Rens
authors.forename: Patricia C M
authors.initials: PC
authors.lastname: O'Brien
authors.forename: Frank
authors.initials: F
authors.lastname: Grützner
authors.forename: Oliver
authors.initials: O
authors.lastname: Clarke
authors.forename: Daria
authors.initials: D
authors.lastname: Graphodatskaya
authors.forename: Enkhjargal
authors.initials: E
authors.lastname: Tsend-Ayush
authors.forename: Vladimir A
authors.initials: VA
authors.lastname: Trifonov
authors.forename: Helen
authors.initials: H
authors.lastname: Skelton
authors.forename: Mary C
authors.initials: MC
authors.lastname: Wallis
authors.forename: Steve
authors.initials: S
authors.lastname: Johnston
authors.forename: Frederic
authors.initials: F
authors.lastname: Veyrunes
authors.forename: Jennifer A M
authors.initials: JA
authors.lastname: Graves
authors.forename: Malcolm A
authors.initials: MA
authors.lastname: Ferguson-Smith
mesh_terms: D000818:Animals; D001717:Birds / Q000235:genetics*; D020223:Chromosome Painting; D022202:Chromosomes, Artificial, Bacterial; D005260:Female; D006801:Humans; D007621:Karyotyping; D008297:Male; D008856:Microscopy, Fluorescence; D010987:Platypus / Q000235:genetics*; D016133:Polymerase Chain Reaction; D012730:Sex Chromosomes; D004442:Tachyglossidae / Q000235:genetics*