Transposable elements TEs can jump around in the genome.
Received Jul 13; Accepted Oct This article has been cited by other articles in PMC. Abstract Background During meiosis, the X and Y chromosomes are transcriptionally silenced. The persistence of repressive chromatin marks on the sex chromatin after meiosis initially led to the assumption that XY gene silencing persists to some extent in spermatids.
Considering the many reports of XY-linked genes expressed and needed in the post-meiotic phase of mouse spermatogenesis, it is still unclear whether or not the mouse sex chromatin is a repressive or permissive environment, after meiosis.
Results To determine the transcriptional and chromatin state of the sex chromosomes after meiosis, we re-analyzed ten ChIP-Seq datasets performed on mouse round spermatids and four RNA-seq datasets from male germ cells purified at different stages of spermatogenesis.
Our study shows that coverage of active epigenetic marks H3K4me3 and Kcr is similar on the sex chromosomes and on autosomes. The post-meiotic sex chromatin nevertheless differs from autosomal chromatin in its enrichment in H3K9me3 and its depletion in H3K27me3 and H4 acetylation.
We also identified a posttranslational modification, H3K27ac, which specifically accumulates on the Y chromosome. In parallel, we found that the X and Y chromosomes are enriched in genes expressed post-meiotically and display a higher proportion of spermatid-specific genes compared to autosomes.
Finally, we observed that portions of chromosome 14 and of the sex chromosomes share specific features, such as enrichment in H3K9me3 and the presence of multicopy genes that are specifically expressed in round spermatids, suggesting that parts of chromosome 14 are under the same evolutionary constraints than the sex chromosomes.
Conclusions Based on our expression and epigenomic studies, we conclude that, after meiosis, the mouse sex chromosomes are no longer silenced but are nevertheless regulated differently than autosomes and accumulate different chromatin marks. We propose that post-meiotic selective constraints are at the basis of the enrichment of spermatid-specific genes and of the peculiar chromatin composition of the sex chromosomes and of parts of chromosome Electronic supplementary material The online version of this article doi: At the basis of those differences is the fact that they are under different evolutionary constraints due to loss of recombination and sexual antagonism i.
In mammalian males, sex chromosomes are highly heteromorphic and only recombine via a small region usually located at the tip of the chromosome the PAR, pseudoautosomal regionwhile in females the X chromosome can recombine.
Meaning that, compared to a classic pair of autosomes, the X only recombines two-thirds of the time and most of the Y chromosome does not recombine at all, and is hence called MSY for male-specific region on the Y [ 78 ]. One particular phenomenon which affects mammalian sex chromosome gene content and expression takes place in male germ cells: During meiosis, the X and Y chromosomes are transcriptionally silenced by a series of chromatin-based events.
The precise role of meiotic sex chromosome inactivation MSCI is unknown, but it is conserved and essential in all mammals studied so far and in more distant species such as C. While XY chromosomes are enriched in genes expressed in spermatogonia, they are devoid of genes expressed during meiosis, as a consequence of MSCI [ 1314 ].
Whether or not XY chromosomes remain to some extent silent after meiosis is still controversial see below. Based on many studies performed using mostly mouse as a model, MSCI is known to start in spermatocytes at the pachytene stage of meiotic prophase I with phosphorylation of histone H2A variant X by ATR- and MDC1-mediated spreading of this signal over the sex chromosomes [ 1516 ].
This is followed by changes in histone posttranslational modifications, such as di- and trimethylation of the lysine 9 of histone H3 H3K9me2 and me3 and ubiquitination of histone H2A uH2Aand recruitment of heterochromatin proteins CBX1 and CBX3 [ 17 — 21 ].
Some other changes in the sex chromatin appear later, such as deacetylation of histones H3 and H4 [ 19 ], replacement of the canonical histones H3 H3. All these changes in the composition of the sex chromatin are accompanied by its compaction and re-localization at the periphery of the spermatocyte nucleus in a structure called the sex body [ 23 ].
After meiosis, in spermatids, the sex chromatin either from the X or the Y chromosome since spermatids are haploid can still be easily distinguished from autosomal chromatin, as a more DAPI-dense structure immediately adjacent to the constitutive heterochromatin regrouped into one or two chromocenters Fig.
Some of the repressive chromatin marks and chromatin-associated proteins observed on the sex body during meiosis H3K9me2 and H3K9me3, CBX1 and CBX3 are still visibly enriched on the post-meiotic sex chromatin [ 24 — 26 ].
There are nevertheless many reports of sex chromosome-encoded genes expressed and needed in the post-meiotic phase of spermatogenesis [ 29 — 36 ] and, inMueller et al.Two X chromosomes, one Y chromosome, and two sets ofautosomes The fly is female because the ratio of X chromosomes to sets ofautosomes is Assuming that such a fly would be viable, what would be the sex of a fruit fly with the following chromosomal composition?
Gene order is not random with regard to gene expression in mammals: coexpressed genes, and in particular housekeeping genes, are clustered along chromosomes more often than expected by chance.
To understand the origin of these clusters and to quantify the impact of this phenomenon on genome. The Y chromosome is present in males, who have one X and one Y chromosome, while females have two X chromosomes. Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies.
Idiograms show . Men have XY or (YX) chromosomes and women have XXchromosomes. X-linked recessive genetic diseases (such as juvenileretinoschisis) occur when there is a defective X chromosomes thatoccurs without a paired X chromosome that is good. By definition, the first two numbers in the Fibonacci sequence are either 1 and 1, or 0 and 1, depending on the chosen starting point of the sequence, and each subsequent number is the sum .
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