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James Turner group project:

Maintenance of MSCI / MSUC during sperm differentiation

According to the traditional view, MSCI is transient, with the X and Y chromosomes reactivating at the onset of spermiogenesis. Recently, we and others found that the X and Y chromosomes appear heterochromatic in spermatids and that many X-linked genes stay inactive in the post-meiotic period. This maintenance of MSCI into late spermatogenesis is a general phenomenon - unsynapsed autosomes also retain a transcriptionally inactive state in spermatids. The reason for maintaining the X chromosome in a repressed state is unclear:- it has been suggested that maintained X inactivation may serve to provide daughters with an inactive X chromosome and thereby serve a role in dosage compensation in the pre-implantation period, but this is controversial.

We are currently studying the extent of X chromosome inactivation in spermatids more directly using RNA FISH approaches. In addition, we are investigating whether unsynapsed autosomes, which are repressed during both meiosis and spermiogenesis, retain an inactive state in offspring. Finally, we are identifying genes involved specifically in the maintenance of X chromosome repression in spermatids.

Figure 1

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Continued repression of the X and Y chromosomes in spermatids. The sex chromosomes appear as DAPI-bright structures (arrows) located next to the centromeres. They are transcriptionally repressed, as shown by Cot-1 RNA FISH, which highlights areas of ongoing transcription within the nucleus. Chromosome painting unambiguously identifies the sex chromosomes.