Key Features of the X Inactivation Process Are Conserved between Marsupials and Eutherians

In female marsupials, X chromosome inactivation (XCI) is imprinted, affecting the paternal X chromosome. One model, supported by recent studies [1, 2], proposes that XCI in marsupials is achieved through inheritance of an already silent X chromosome from the father [3-6], with XCI initiated by meiotic sex chromosome inactivation (MSCI) [7, 8]. This model is appealing because marsupials have no Xist gene [9-12] and the marsupial inactive X chromosome is epigenetically dissimilar to that of mice, apparently lacking repressive histone marks such as H3K27 trimethylation [13]. A central prediction of the meiotic inactivation model of XCI is that silencing of genes on the X chromosome, initiated during male meiosis, is stably maintained during subsequent spermiogenesis. Here we characterize XCI in the male germline and female soma of the marsupial Monodelphis domestica. Contrary to the meiotic inactivation model, we find that X genes silenced by MSCI are reactivated after meiosis and are subsequently inactivated in the female. A reexamination of the female somatic inactive marsupial X chromosome reveals that it does share common properties with that of eutherians, including H3K27 trimethylation and targeting to the perinucleolar compartment. We conclude that aspects of the XCI process are more highly conserved in therian mammals than previously thought.