Sex chromosomes come in different shapes and sizes. In species with differentiated sex chromosomes, it is thought that having some form of sex chromosome dosage compensation system is critical. In an ideal word, such systems should restore parity of gene expression from the X chromosomes with the autosomes, and from the X chromosome in males (with one X) and females (with two Xs). A critical part of sex chromosome dosage compensation is X chromosome inactivation (XCI).
The marsupial specific RSX long non-coding RNA (lncRNA) is as a functional counterpart to the eutherian specific XIST, both of which play pivotal roles in mediating XCI. We investigated the RSX interactome in the opossum (Monodelphis domestica), identifying 135 interacting proteins. Remarkably, 54 of these proteins have orthologues in the XIST interactome. Both RSX and XIST interactomes are enriched in biological pathways related to RNA processing, translation regulation, and epigenetic transcriptional silencing. This highlights the functional coherence of independently evolved lncRNAs across diverse mammalian lineages, and wider comparison of sex chromosome dosage compensation in other vertebrates showcases extreme plasticity in the system.