Al Nadaf, ShafaghWaters, Paul D.Koina, EddaDeakin, Janine E.Jordan, Kristen S.Graves, Jennifer A.2016-01-032016-01-03Genome Biology. 2010 Dec 23;11(12):R1221465-6906http://dx.doi.org/10.1186/gb-2010-11-12-r122http://hdl.handle.net/1885/95201BACKGROUND: 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.This project was funded by grants to JAMG and PDW from the Australian Research Council.© 2010 Nadaf et al.; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Keywords: animal cell; article; cell nucleus; chromosome analysis; chromosome map; controlled study; female; fibroblast; gene expression; gene location; gene locus; Macropus eugenii; male; marsupial; nonhuman; orthology; paralogy; stochastic model; X chromosome inaenNadaf et al..2010-12-2310.1186/gb-2010-11-12-r1222016-02-24