title: The X factor: X chromosome dosage compensation in the evolutionarily divergent monotremes and marsupials.
abstract: Marsupials and monotremes represent evolutionarily divergent lineages from the majority of extant mammals which are eutherian, or placental, mammals. Monotremes possess multiple X and Y chromosomes that appear to have arisen independently of eutherian and marsupial sex chromosomes. Dosage compensation of X-linked genes occurs in monotremes on a gene-by-gene basis, rather than through chromosome-wide silencing, as is the case in eutherians and marsupials. Specifically, studies in the platypus have shown that for any given X-linked gene, a specific proportion of nuclei within a cell population will silence one locus, with the percentage of cells undergoing inactivation at that locus being highly gene-specific. Hence, it is perhaps not surprising that the expression level of X-linked genes in female platypus is almost double that in males. This is in contrast to the situation in marsupials where one of the two X chromosomes is inactivated in females by the long non-coding RNA RSX, a functional analogue of the eutherian XIST. However, marsupial X chromosome inactivation differs from that seen in eutherians in that it is exclusively the paternal X chromosome that is silenced. In addition, marsupials appear to have globally upregulated X-linked gene expression in both sexes, thus balancing their expression levels with those of the autosomes, a process initially proposed by Ohno in 1967 as being a fundamental component of the X chromosome dosage compensation mechanism but which may not have evolved in eutherians.
authors.affiliation: School of Veterinary Science, University of Queensland, Gatton, Queensland 4343, Australia. Electronic address: d.whitworth@uq.edu.au.
authors.forename: Deanne J
authors.initials: DJ
authors.lastname: Whitworth
authors.affiliation: School of BioSciences, The University of Melbourne, Parkville, Victoria 3010, Australia.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Pask
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004303:Dosage Compensation, Genetic; D006801:Humans; D008394:Marsupialia / Q000235:genetics*; D014960:X Chromosome / Q000235:genetics*
keywords: Marsupial; Monotreme; X chromosome inactivation