title: Activity map of the tammar X chromosome shows that marsupial X inactivation is incomplete and escape is stochastic.
abstract: BACKGROUND
  X chromosome inactivation is a spectacular example of epigenetic silencing. In order to deduce how this complex system evolved, we examined X inactivation in a model marsupial, the tammar wallaby (Macropus eugenii). In marsupials, X inactivation is known to be paternal, incomplete and tissue-specific, and occurs in the absence of an XIST orthologue.
  
  
  RESULTS
  We examined expression of X-borne genes using quantitative PCR, revealing a range of dosage compensation for different loci. To assess the frequency of 1X- or 2X-active fibroblasts, we investigated expression of 32 X-borne genes at the cellular level using RNA-FISH. In female fibroblasts, two-color RNA-FISH showed that genes were coordinately expressed from the same X (active X) in nuclei in which both loci were inactivated. However, loci on the other X escape inactivation independently, with each locus showing a characteristic frequency of 1X-active and 2X-active nuclei, equivalent to stochastic escape. We constructed an activity map of the tammar wallaby inactive X chromosome, which identified no relationship between gene location and extent of inactivation, nor any correlation with the presence or absence of a Y-borne paralog.
  
  
  CONCLUSIONS
  In the tammar wallaby, one X (presumed to be maternal) is expressed in all cells, but genes on the other (paternal) X escape inactivation independently and at characteristic frequencies. The paternal and incomplete X chromosome inactivation in marsupials, with stochastic escape, appears to be quite distinct from the X chromosome inactivation process in eutherians. We find no evidence for a polar spread of inactivation from an X inactivation center.
authors.affiliation: Research School of Biology, The Australian National University, Biology Place, Canberra 0200, Australia. shafagh.alnadaf@anu.edu.au
authors.forename: Shafagh
authors.initials: S
authors.lastname: Al Nadaf
authors.forename: Paul D
authors.initials: PD
authors.lastname: Waters
authors.forename: Edda
authors.initials: E
authors.lastname: Koina
authors.forename: Janine E
authors.initials: JE
authors.lastname: Deakin
authors.forename: Kristen S
authors.initials: KS
authors.lastname: Jordan
authors.forename: Jennifer Am
authors.initials: JA
authors.lastname: Graves
mesh_terms: D000818:Animals; D002460:Cell Line; D002874:Chromosome Mapping; D022202:Chromosomes, Artificial, Bacterial / Q000378:metabolism; D004303:Dosage Compensation, Genetic; D019143:Evolution, Molecular; D005260:Female; D017404:In Situ Hybridization, Fluorescence; D007614:Macropodidae / Q000235:genetics*; D008297:Male; D016133:Polymerase Chain Reaction; D014960:X Chromosome; D049951:X Chromosome Inactivation