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Y chromosome

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Y Chromosome Is More Than a Sex Switch

 

Here to stay. The Y chromosome is small compared with the X, but is required to keep levels of some genes high enough for mammals to survive.

http://www.sciencemag.org/sites/default/files/styles/article_main_small/public/images/sn-genes.jpg?itok=7mnkSPKy&timestamp=1398272995

Here to stay. The Y chromosome is small compared with the X, but is required to keep levels of some genes high enough for mammals to survive.

Andrew Syred/Science Source

The small, stumpy Y chromosome—possessed by male mammals but not females, and often shrugged off as doing little more than determining the sex of a developing fetus—may impact human biology in a big way. Two independent studies have concluded that the sex chromosome, which shrank millions of years ago, retains the handful of genes that it does not by chance, but because they are key to our survival. The findings may also explain differences in disease susceptibility between men and women.

“The old textbook description says that once maleness is determined by a few Y chromosome genes and you have gonads, all other sex differences stem from there,” says geneticist Andrew Clark of Cornell University, who was not involved in either study. “These papers open up the door to a much richer and more complex way to think about the Y chromosome.”

The sex chromosomes of mammals have evolved over millions of years, originating from two identical chromosomes. Now, males possess one X and one Y chromosome and females have two Xs. The presence or absence of the Y chromosome is what determines sex—the Y chromosome contains several genes key to testes formation. But while the X chromosome has remained large throughout evolution, with about 2000 genes, the Y chromosome lost most of its genetic material early in its evolution; it now retains less than 100 of those original genes. That’s led some scientists to hypothesize that the chromosome is largely indispensable and could shrink away entirely.

To determine which Y chromosome genes are shared across species, Daniel Winston Bellott, a biologist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, and colleagues compared the Y chromosomes of eight mammals, including humans, chimpanzees, monkeys, mice, rats, bulls, and opossums. The overlap, they found, wasn’t just in those genes known to determine the sex of an embryo. Eighteen diverse genes stood out as being highly similar between the species. The genes had broad functions including controlling the expression of genes in many other areas of the genome. The fact that all the species have retained these genes, despite massive changes to the overall Y chromosome, hints that they’re vital to mammalian survival.

“The thing that really came home to us was that these ancestral Y chromosome genes—these real survivors of millions of years of evolution—are regulators of lots of different processes,” Bellott says.

Bellott and his colleagues looked closer at the properties of the ancestral Y chromosome genes and found that the majority of them were dosage-dependent—that is, they required two copies of the gene to function. (For many genes on the sex chromosomes, only one copy is needed; in females, the copy on the second X chromosome is turned off and in males, the gene is missing altogether.) But with these genes, the female has one on each X chromosome and the male has a copy on both the X and Y chromosomes. Thus, despite the disappearance of nearby genes, these genes have persisted on the Y chromosome, the team reports online today in Nature.

“The Y chromosome doesn’t just say you’re a male; it doesn’t just say you’re a male and you’re fertile. It says that you’re a male, you’re fertile, and you’re going to survive,” Bellott explains. His group next plans to look in more detail at what the ancestral Y chromosome genes do, where they’re expressed in the body, and which are required for an organism’s survival.

In a second Nature paper, also published online today, another group of researchers used a different genetic sequencing approach, and a different set of mammals, to ask similar questions about the evolution of the Y chromosome. Like Bellott’s paper, the second study concluded that one reason that the Y chromosome has remained stable over recent history is the dosage dependence of the remaining genes.

“Knowing now that the Y chromosome can have effects all over the genome, I think it becomes even more important to look at its implications on diseases,” Clark says. “The chromosome is clearly much more than a single trigger that determines maleness.” Because genes on the Y chromosome often vary slightly in sequence—and even function—from the corresponding genes on the X, males could have slightly different patterns of gene expression throughout the body compared with females, due to not only their hormone levels, but also their entire Y chromosome. These gene expression variances could explain the differences in disease risks, or disease symptoms, between males and females, Clark says.

 
(1) http://www.sciencemag.org/news/2014/04/y-chromosome-more-sex-switch
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Spermatogenic Defects in Sex Reversed Mice

Reporter and Curator: Dr. Sudipta Saha, Ph.D.

 

“Sex reversed” (Sxr) is an inherited form of sex reversal that causes XX and XO mice to develop as phenotypically normal males. Adult XYSxra mice exhibit varying degrees of spermatogenic deficiency but are usually fertile, while XOSxra mice have severe spermatogenic failure and are always sterile. The present quantitative spermatogenic analysis reports when these anomalies first appear during puberty. The results demonstrate that in XYSxra mice there was increased degeneration of pachytene spermatocytes and, to a lesser extent, meiotic metaphase stages. On average, there were only one-half the number of spermatids compared with the XY controls. The defect in XOSxra mice appeared a little later, with an almost complete arrest and degeneration during the meiotic metaphases.

 

A minority of XYSxra mice are sterile, and these may have testes as small as those from XOSxra mice. Adult XOSxra mice have consistently small testes and are invariably sterile. The reported results document the testicular defects in XYSxra and XOSxra testes as they first arise during puberty. The only other quantitative data on XYSxra and XOSxra spermatogenesis are for adult mice. A previous report described XYSxra testes as being a “mosaic” of normal and defective spermatogenesis. Recently a more extensive analysis was carried out of adult XYSxra and XOSxra testes. Once again there is good agreement with the present results in that the spermatogenic failure in XYSxra testes was predominantly between pachytene and diplotene, while in XOSxra testes the block was predominantly during the meiotic metaphases. To explain the spermatogenic anomalies in XYSxra and XOSxra testes, Burgoyne and Baker (1984) invoked the “meiotic pairing site” hypothesis of Miklos (1974). The other notable feature of the present study was the demonstration that the testicular deficiency is manifested earlier (with respect to age and spermatogenic stage) in XYSxra testes than in XOSxra testes. Krzanowska (1989) recently reported increased levels of X-Y univalence in pubertal XY males. So, it is suggested that this reduced efficiency of X-Y pairing at puberty that leads to the increased incidence of diploid spermatids in pubertal XYSxra males and to the presence of diploid spermatids in pubertal XY males. The other feature of pubertal XYSxra testes that deserves mention is the increase in the number of differentiating spermatogonia.

 

The conclusion is that most of the spermatogenic deficiencies in XYSxra and XOSxra testes can be explained in terms of the “meiotic pairing site” hypothesis, which links spermatogenic failure with sex chromosome univalence during meiosis. In XYSxra testes a variable proportion of pachytene spermatocytes have the X and Y unpaired, and the elimination of these cells explains the variable reduction in testis size and fertility. In XOSxra testes all spermatocytes have a univalent sex chromosome, accounting for the almost total spermatogenic block in these mice. It is suggested that the affected spermatocytes are eliminated earlier in XYSxra testes than in XOSxra testes, because two univalent sex chromosomes have more unpaired sites than the univalent X alone.

 

References:

 

Sutcliffe M. J., Darling S. M., Burgoyne P. S. (1991) Spermatogenesis in XY, XYSxra and XOSxra Mice: A quantitative analysis of spermatogenesis throughout puberty. Molecular Reprod. Dev. 30(2), 81–89.

 

Burgoyne P. S., Baker T. G. (1984) Meiotic pairing and gametogenic failure. In CW Evans and HG Dickinson (eds): “Controlling Events in Meiosis (38th Symp SOC Exp Biol).” Cambridge Company of Biologists, pp 349-362.

 

Miklos G. L. G. (1974) Sex-chromosome pairing and male fertility. Cytogen. Cell Genet. 13, 558-577.

 

Krzanowska H (1989) X-Y chromosome dissociation in mouse strains differing in efficiency of spermatogenesis: Elevated frequency of univalents in pubertal males. Gamete. Res. 23, 357-365.

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