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Mammalian X Chromosome Dosage Compensation: Perspectives From the Germ Line

Sangrithi, Mahesh N., Turner, James M. A.
BioEssays 2018 v.40 no.6 pp. e1800024
X chromosome, aneuploidy, epigenetics, evolution, females, gene dosage, gene expression regulation, genes, germ cells, males, mammals, sex determination
Sex chromosomes are advantageous to mammals, allowing them to adopt a genetic rather than environmental sex determination system. However, sex chromosome evolution also carries a burden, because it results in an imbalance in gene dosage between females (XX) and males (XY). This imbalance is resolved by X dosage compensation, which comprises both X chromosome inactivation and X chromosome upregulation. X dosage compensation has been well characterized in the soma, but not in the germ line. Germ cells face a special challenge, because genome wide reprogramming erases epigenetic marks responsible for maintaining the X dosage compensated state. Here we explain how evolution has influenced the gene content and germ line specialization of the mammalian sex chromosomes. We discuss new research uncovering unusual X dosage compensation states in germ cells, which we postulate influence sexual dimorphisms in germ line development and cause infertility in individuals with sex chromosome aneuploidy.