Do you have the X-Factor? Seems like marsupial and monotreme mammals don’t!

               Inactivation of X chromosome in females is nature’s way of playing fair.  Since females have two X chromosomes and male only one, one of the X chromosomes is inactivated in females to prevent over expression of the same chromosome. This is known as dosage compensation and is a common feature in mammals and is one of the best examples of an epigenetic system. The gene responsible for this is the XIST (X-inactive specific transcript) which is present in the X-chromosome inactivation centre. This gene sequence conserved in most of the mammals is missing in non- eutherian (Non-placental) mammals like marsupials and monotreme (egg-laying). Infact on mapping this region scientists not only failed to find this gene but also the region homologus to it.
        This now leaves us with many questions. Since X inactivation is also seen in non-eutherian mammals if not XIST then what controls it?  Eutherian(placental) and non-eutherian mammals diverged 130 million years ago with monotreme mammals diverged 30 million years before marsupials. So does this mean that X inactivation centre reconstituted in eutherian mammals was independently fused in marsupials and monotreme? This finding though leaves us pondering  about the mechanism and evolution of the epigenetic system confirms one thing ,nature not only likes playing fair but also mysterious...
Lynnmaria Nazareth
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