The neo-X does not form a Barr body but shows a slightly condensed structure in the Okinawa spiny rat (Tokudaia muenninki).

Abstract

X chromosome inactivation (XCI) is an essential mechanism for gene dosage compensation between male and female cells in mammals. The Okinawa spiny rat (Tokudaia muenninki) is a native rodent in Japan with XX/XY sex chromosomes, like most mammals; however, the X chromosome has acquired a neo-X region (Xp) by fusion with an autosome. We previously reported that dosage compensation has not yet evolved in the neo-X region; however, X-inactive-specific transcript (Xist) RNA (long non-coding RNA required for the initiation of XCI) is partially localized in the region. Here, we show that the neo-X region represents an early chromosomal state in the acquisition of XCI by analyses of heterochromatin and Barr body formation. We found no evidence for heterochromatin formation in the neo-X region by RBA (R-banding by acridine orange) assays and immunostaining of H3K27me3. Double-immunostaining of H3K27me3 and HP1, a component of the Barr body, revealed that the entire ancestral-X chromosome region (Xq) showed a bipartite folded structure. By contrast, HP1 was not localized to the neo-X region. However, BAC FISH revealed that the signals of genes on the neo-X region of the inactive X chromosome were concentrated in a narrow region. These findings indicated that although the neo-X region of the inactive X chromosome does not form a complete Barr body structure (e.g., it lacks HP1), it forms a slightly condensed structure. These findings combined with the previously reported partial binding of Xist RNA suggest that the neo-X region exhibits incomplete inactivation. This may represent an early chromosomal state in the acquisition of the XCI mechanism.