Sex-dependent RNA expression in opossum brain and liver.

To compensate for the increased dosage of X chromosomes in females one of the two X chromosomes is inactivated in eutherian mammals in an Xist-dependent manner. Xist is, however, not found in metatherians. Here we describe long non-coding RNA Rsx (RNA-on-the-silent X) that exhibits properties consistent with a role in X-inactivation in opossum. Rsx is abundantly expressed in females but not males and is transcribed from, and coats, the inactive X chromosome copy. Furthermore, when both X chromosomes are active, Rsx is silenced. We used RNA-Seq to determine exon-intron structure and expression level of Rsx in males and females. Overall design: RNA sequencing was used to detect and characterize transcripts with strongly biased female expression.