Tuesday, 20 March 2012

Auf Wiedersehen Men- The Disappearing Y Chromosome

By Oliver Creagh, 42644794

Hey bloggers; O-dizzle here. On a recent Internet perambulation, my search results came up with an article about ‘human sex chromosomes’ (the latter being a freebie given my original Google intentions). This article, from the magazine Nature, was about the sex-determining chromosome for human men (also known as the Y chromosome) and its dramatic loss of genetic content since its inception. Previously, it has been thought that the Y chromosome will disappear in 5 million years (a 'yikes!' from us men, but perhaps a 'hurrah' from women?). However, a recent paper by Jennifer Hughes and colleagues at the Whitehead Institute in Cambridge, Massachusetts, has put to bed the notion that the Y chromosome will eventually become degenerate altogether.

As a refresher; the Y chromosome contains a gene that, when activated, engenders testes development and the secretion of male hormones. Sex chromosome combination for men is X,Y, for women, X,X. According to Nature, the Y chromosome emerged in a mammalian ancestor roughly 200-300 million years ago, evolving from an ordinary pair of autosomal chromosomes- a gene named SRY appeared on one of these, and exhibited the function that, in short, made males ‘male’. The chromosome with this SRY gene became the first Y chromosome, and chromosome with the gene that SRY developed from, named SOX3, became the first X chromosome- hence it is likely that both human chromosomes shared a common ancestor. However, since then, the Y chromosome has considerably deteriorated in the amount of its genetic content, some suggesting by upwards of 720 genes- this being particularly noticeable when compared with the X chromosome (see below).

A human X (left) and Y (right) chromosome- there is an obvious disparity in content

Now, while the mechanism by which the degeneration of the Y chromosome  is beyond the scope of the blogosphere, sources such as Hughes, Charlesworth and Charlesworth and Bowdler suggest that this is due to the Y chromosomes lack of participation in sexual recombination, a process associated with the “crossing over” stage of Meiosis. During Prophase 1 of Meiosis, there is a genetic rearrangement between non-sister chromatids, called “crossing-over”. This involves the exchange of corresponding sections of DNA molecules between non-sister chromatids, thus furthering the diversity of genetic content produced via Meiosis with these new ‘recombinant chromosomes’. 

A diagrammatic version of "Crossing Over", with the new recombinant chromosomes on the far right- while the X chromosome participates in  this process, the Y does not as willingly

According to Hughes, female X chromosomes participate wholeheartedly in the crossing-over process with eachother- however, there is little to no cross-over of significance between X and Y chromosomes, with the Y chromosome unwilling to engage. This results in the Y chromosome being reasonably undiversified, maintaining any mutations or deletions in the base sequence that leave the chromosome open to forces of erosion over time. The result of these scientific and anecdotal observations has lead many to claim that, alarmingly, the Y chromosome will eventually deteriorate altogether, in approximately 5 million years.

This is where Hughes and her team come in. In order to investigate whether this “impending demise” theory had any validity, they sequenced the Y chromosome of the Rhesus Macaque, an Old Monkey that is separate from humans by about 25 millions years of evolution. What they found was that the human Y chromosome had only lost one gene compared to that of the Rhesus Macaque in the last 25 million years, and no genes in the last 6 million years. Such a minimal amount of deterioration seemingly blows the hypothesis of a 5 million year time-bomb on the Y chromosome out of the water.

The "Rhesus Macaque", an Old Monkey separate from humans by about 25 million years of evolution- it was this species'  Y-chromosome that Hughes and her team sequenced the genetic content of.

As men everyone now rest slightly more easily, knowing that their manliness (at least in a genetic sense) isn't going anywhere, there still remains an element of debate on the subject. However, as Hughes has argued, the Y chromosome is going no where in the near future, and then some.

Also bloggers, as for some supplementary material-

To read a previous precursor article to the one discussed, also by Hughes

What BBC Science Says on the issue: http://www.bbc.co.uk/news/science-environment-17127617
Nature's analysis of the paper: 

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