title: The evolution of X chromosome inactivation in mammals: the demise of Ohno's hypothesis?
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.
authors.affiliation: Laboratoire 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.
authors.forename: Eugénie
authors.initials: E
authors.lastname: Pessia
authors.forename: Jan
authors.initials: J
authors.lastname: Engelstädter
authors.forename: Gabriel A B
authors.initials: GA
authors.lastname: Marais
mesh_terms: D000818:Animals; D005075:Biological Evolution; D050172:Genes, X-Linked / Q000235:genetics*; D008322:Mammals / Q000235:genetics*; D008957:Models, Genetic; D010802:Phylogeny; D049951:X Chromosome Inactivation / Q000235:genetics*