Activity map of the tammar X chromosome shows that marsupial X inactivation is incomplete and escape is stochastic

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.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.In therian mammals (eutherians and marsupials), the sex of an embryo is determined by the presence or absence of a Y chromosome, whereby males have a Y and a single X, and females have two X chromosomes. The eutherian X and Y chromosomes show homology within a pseudoautosomal region that pairs at meiosis, and most Y genes have a homologue on the X chromosome, from which they clearly evolved. This supports the hypothesis that the X and Y evolved from an ordinary autosome pair via degradation of the Y, after it acquired a testis-determining factor, SRY (reviewed in [1]).The sex chromosomes of eutherian and marsupial mammals share extensive homology, although the marsupial sex chromosomes lack the aut