Recent research reveals that our version of the actions of the Y chromosome may need revision. Studies indicate the Y chromosome may be changing more rapidly than any other part of the human genome. We teach our students that the sex of most mammals is determined by the X and Y sex chromosomes. Females have two Xs, males have one X and one Y, so the Y chromosome makes the offspring a male. Geneticists have shown that the Y was originally identical to the X, but over 300 million years it has shrunk and is now the smallest human chromosome. The Y chromosome occurs singularly, and has no counterpart that allows for swapping or exchanging genetic material. The integrity and potential survival of the Y chromosome is being questioned. Scientists suggested that the Y chromosome will eventually just waste away entirely in 125,000 years or so. The worst case scenario would lead to the end of males in a species.
However, recent work carried out by Dr David Page of the Massachusetts Institute of Technology and published in Nature indicates that the male chromosome is changing more quickly than any other part of humanity's genetic code. Dr Page first sequenced the Y chromosome in 2003. He and colleagues have now sequenced the Y chromosome of the chimp, humanity's closest relative, and compared this with the human version. Human and chimp DNA generally differ by less than 2 percent, yet more than 30 percent of the Y chromosome differed between the two species. Page and colleagues suggest that mutations arise quite readily in the Y chromosome. The Y chromosome uses gene conversion to repair its genetic code; a repair mechanism less efficient than other repair strategies used in the human genome.
Consequently, more mutations can arise in the Y chromosome. The Y mutations are subjected to fairly rapid and consequential selective pressures. If beneficial they are passed on with the Y chromosome. If detrimental, they are likely eliminated. The Y chromosome is of course critical to sperm production. Advantageous mutations would be expected to be preserved as they would increase fertility or sperm integrity. Damaging mutations would be thus eliminated since the Y chromosomes possessing these harmful changes would not fertilize an egg, survive to fertilize an egg, or perhaps damage sperm in some way. Interestingly, this idea was supported when the parts of the Y chromosome involved in sperm production were compared between humans and chimpanzees. This part of the chromosome showed the most difference between the species.
Wes Warren of Washington University in St Louis, Missouri, who contributed to the study, said: "This work clearly shows that the Y is pretty ingenious at using different tools than the rest of the genome to maintain diversity of genes. These findings demonstrate that our knowledge of the Y chromosome is still advancing."
Source: Henderson, M. January 14, 2010. Why the Y chromosome is a hotbed of evolution. TimesOnLine. Available athttp://www.timesonline.co.uk/tol/news/science/genetics/article6987092.ece. Accessed on January 18, 2010.
Submitted by: Robin Pattillo, PhD, RN, News Editor at[email protected].