Meiotic Sex Chromosome Inactivation


The differentiation of sex chromosomes through evolution has resulted in significant loss of homology and gene content which has profound consequences on gene dosage between the sexes and impacts on their behaviour through meiosis. During spermatogenesis the unpaired DNA, of heteromorphic sex chromosomes becomes transcriptionally silenced and sequestered into a ‘sex body’, a process termed ‘meiotic sex chromosome inactivation’. This essential meiotic event involves an ordered hierarchy of epigenetic changes to the sex chromosome DNA initiated by conserved DNA damage, repair and checkpoint machinery. Although the reason for this repression remains unclear, it is thought to prevent recombination between nonhomologous sex chromosome DNA and to avoid stringent meiotic checkpoints. Recent work has extended our understanding of the evolutionary conservation of meiotic silencing to include the avian ZW sex chromosome system providing new insights into the evolutionary dynamics of meiotic sex chromosome organization in diverse species.

Key concepts:

  • The accumulation of mutation and rearrangements on sex chromosomes lead to blocks in recombination, accumulation of sexually antagonistic genes and the differential degradation of sex chromosomes.

  • Heteromorphic sex chromosomes result in gene dosage imbalance between the sexes and unsynapsed DNA in meiosis I.

  • Meiotic sex chromosome inactivation protects unpaired sex chromosome DNA from aberrant recombination and checkpoint surveillance machinery.

  • Sex chromosomes undergo transcriptional silencing during late prophase due to the accumulation of conserved epigenetic changes.

  • Therian mammals have active X‐borne retrogenes on autosomes to compensate for the loss of transcription of sex chromosome genes important for spermatogenesis.

Keywords: spermatogenesis; meiotic silencing; spigenetics; sex chromosomes

Figure 1.

Evolutionary perspective of sex chromosome systems. Figure indicates sex chromosome complements, current knowledge on MSCI, and identification of retrotransposon movements from the X chromosome. Question mark indicates current lack of knowledge on monotreme MSCI. Ma; million years ago.

Figure 2.

The synaptonemal complex mediates chromosomal synapsis at pachytene. Mouse pachytene spermatocyte immunostained using antibodies for synaptonemal complex protein 3 (SCP3) to visualize lateral elements of the condensed chromosomal axes (left panel) and synaptonemal complex protein 1 (SCP1) to visualize transverse filaments of the synapsed homologous regions (middle panel). The right panel shows the merged image with DAPI (4′,6′‐diamidino‐2‐phenylindole) in blue. Sex chromosomes appear green in the merged image (circled with broken line). Images produced by TJD.

Figure 3.

The sex body accumulates signature epigenetic modifications to effect transcriptional silencing of the sex chromosomes at pachytene. Image shows phosphorylation of H2AX (γH2AX) is concentrated on the sex chromatin at pachytene in a mouse spermatocyte. Text lists common modifications to sex chromatin during MSCI. Image produced by TJD.

Figure 4.

Comparative organizational behaviour of sex chromosomes in pachytene spermatocytes.



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Further Reading

Burgoyne PS, Mahadevaiah SK and Turner JMA (2009) The consequences of asynapsis for mammalian meiosis. Nature Reviews. Genetics 10(3): 207–216.

Daish T and Grützner F (2009) Location, location, location! Monotremes provide unique insights into the evolution of sex chromosome silencing in mammals. DNA and Cell Biology 28(2): 91–100. Review.

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How to Cite close
Daish, Tasman J, and Grützner, Frank(Feb 2010) Meiotic Sex Chromosome Inactivation. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0022109]