Determining the evolutionary origin of X inactivation mechanisms in mammals requires knowledge of features of X inactivation across all three major mammal lineages; monotremes, marsupials and eutherians. In the past, research into X inactivation in marsupials and monotremes lagged far behind the major advances made in understanding the mechanisms of X inactivation in human and mouse. Fragmentary knowledge of the genic content and sequence of marsupial and monotreme X chromosomes has been alleviated by the recent release of genome sequences for two marsupials and one monotreme. This has lead to a number of important findings, among which is the absence of XIST in marsupials and monotremes, and the surprising finding that X-borne genes in platypus are subject to stochastic transcriptional inhibition rather than whole chromosome inactivation. Availability of sequence data, and new techniques for studying expression and chromatin modification, now make rapid advance possible.
Deakin, J., Chaumeil, J., Hore, T., & Marshall Graves, J. (2009). Unravelling the evolutionary origins of X chromosome inactivation in mammals: insights from marsupials and monotremes. Chromosome Research, 17, 671-685. https://doi.org/10.1007/s10577-009-9058-6