The evolution of X chromosome inactivation in mammals: the demise of Ohno's hypothesis?
Eugénie Pessia1, Jan Engelstädter, Gabriel A B Marais
1Laboratoire de Biométrie et Biologie Évolutive, Centre National de la Recherche Scientifique, Université Lyon 1, Bat. Gregor Mendel, 16 rue Raphaël Dubois, 69622, Villeurbanne Cedex, France, eugenie.pessia@univ-lyon1.fr.
Abstract:
Ohno's hypothesis states that dosage compensation in mammals evolved in two steps: a twofold hyperactivation of the X chromosome in both sexes to compensate for gene losses on the Y chromosome, and silencing of one X (X-chromosome inactivation, XCI) in females to restore optimal dosage. Recent tests of this hypothesis have returned contradictory results. In this review, we explain this ongoing controversy and argue that a novel view on dosage compensation evolution in mammals is starting to emerge. Ohno's hypothesis may be true for a few, dosage-sensitive genes only. If so few genes are compensated, then why has XCI evolved as a chromosome-wide mechanism? This and several other questions raised by the new data in mammals are discussed, and future research directions are proposed.
Related Concept Videos
X-Inactivation
X-inactivation
Inheritance of Chromatin Structures
Dosage Compensation
In addition to sexual development, the X chromosome has genes involved in autosomal functions such as brain development and the immune system. Therefore, males and females with distinct numbers of X chromosomes will...
The Y Chromosome Determines Maleness
Evolution
Around 300 million years ago, the two sex chromosomes diverged from two identical autosomal chromosomes. Over time, the Y chromosome has lost most of its genes, shrinking in size....
The Ratio of X Chromosome to Autosomes
Normal male Drosophila has a ratio of one X chromosome to two sets of autosomes. In contrast, normal female...


