The evolution of mammalian X chromosomes and X chromosome inactivation

Claudia L.Rodríguez Delgado*, Janine E. Deakin

*Corresponding author for this work

    Research output: Chapter in Book/Report/Conference proceedingChapterpeer-review


    For many years it was thought that the X chromosome in all three major lineages of mammals (monotremes, marsupials and eutherians) had a shared evolutionary history. However, monotreme and marsupial genome projects and the employment of molecular cytogenetic techniques have enabled the evolutionary history of mammalian X chromosomes to be more accurately traced. This research revealed that monotremes have a complex sex chromosome system consisting of multiple X chromosomes that share no homology to the X chromosome of therian (marsupial and eutherian) mammals. Therian sex chromosomes arose from a pair of autosomes with a subsequent addition to the X chromosome having occurred in the eutherian lineage. The evolution of mammalian sex chromosomes has led to large, gene-rich X chromosomes and smaller, gene-poor Y chromosomes, which is thought to create an imbalance in transcriptional output between the X chromosome and autosomes in males that requires dosage compensation. This hypothesis has only been thoroughly interrogated since the advent of microarray and RNA-sequencing technologies but the findings of these studies remain controversial. However, it is more generally accepted that the dosage imbalance for X-borne genes between the sexes requires some form of compensation; otherwise females would be expressing twice as much of an X-linked gene as their male counterparts. Compensation for this imbalance is deemed essential, as an extra copy of autosomal genes is usually deleterious. In eutherian mammals, this is achieved by transcriptionally silencing one X chromosome in female somatic cells. Fifty years of research has uncovered many of the features of this remarkable epigenetic phenomenon. However, research into X chromosome inactivation in marsupials and monotremes has lagged far behind that of their eutherian counterparts, largely due to the limited knowledge of genic content of their X chromosomes. Fortunately, genome sequencing of the platypus and several marsupial genomes has alleviated this problem and enabled such research to forge ahead at a great pace. This has led to a number of seminal findings and new hypotheses regarding the evolution of X chromosome inactivation.

    Original languageEnglish
    Title of host publicationSex Chromosomes
    Subtitle of host publicationNew Research
    PublisherNova Science Publishers, Inc.
    Number of pages52
    ISBN (Print)9781624171437
    Publication statusPublished - Jan 2013


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