Rosser, Z., Balaresque, P., & Jobling, M. (2009). Gene Conversion between the X Chromosome and the Male-Specific Region of the Y Chromosome at a Translocation Hotspot The American Journal of Human Genetics, 85 (1), 130-134 DOI: 10.1016/j.ajhg.2009.06.009
There is new paper is out in American Journal of Human Genetics about how the X and Y chromosome might not be as separate as we think, and in fact might undergo regular recombination in certain regions (you can read a press release for the paper here).
Specifically, the paper is a resequencing study of the X and Y chromosome homologues PRKX and PRKY in around 60 individuals, looking for signatures of recombination. In summary; it is an interesting and well supported paper in as far as it goes, but it raises more questions about Y chromosome evolution than it answers
The Study
The PRKX and PRKY genes stopped undergoing regular recombination about 30 million years ago, but are known to still occasionally recombine, generally causing sex abnormalities such as XX Males or Swyer Syndrome; because people with these syndromes are infertile, the recombined chromosomes are never passed on into the gene pool as a whole.
The authors took 12 men from many diverse populations, and amplified up about 2kb of the PRKX region from their X chromosomes and 2kb of the PRKY region from the Y chromosomes. 11 of them were identical, but one differed in two locations 4bp apart; differences that matched up with the X chromosome, but not the other Y chromosomes. Unless two very unlikely mutations occurred in just the right way, we can conclude that a small part of DNA (at most 138bp) has been copied from the X to the Y chromosome (some sort of gene conversion).
They then went back to resampled 45 more males, and found a few more instances of the same gene conversion, as well as another, entirely different gene conversion. These two gene conversions are almost certainly a case where a chromosomal abnormality recombination started to happen, but didn’t resolve in a damaging way (perhaps it was repaired by a natural mechanism, or something similar). Based on this, they calculate that X-Y gene conversion is probably as common as Y-Y gene conversion in the PRKX/Y regions.
The Significance
This is all interesting, and well worth knowing. The authors say that this mechanism may play a part in the evolution of the other intersex abnormality genes (both known and current undiscovered), which probably undergo similar recombination events. They even suggest that this mechanism may play a part throughout the X-degenerate and X-transposed regions of the Y chromosome; this may be the case, but further studies would have to done (perhaps a job for the 1000 Genomes Project there).
So, a nice paper, well supported data, a cool result, all is good. But it is worth noting that this doesn’t generalise well to the entire Y chromosome, and it does not come close to solving the problem of the degeneration of Y chromosome genes. The majority of the Y chromosome genes have no homology with the X chromosome; as the Y chromosome sequence paper reported, the bulk of them are found in the ampliconic regions; stretches of DNA that are palindromic (and thus carry two copies of each section of sequence), which undergo frequent gene conversion.
Whats more, the amplicionic regions are also enriched for non-degenerated genes; the genes in the regions that would undergo XY gene conversion are exactly the genes that do tend to decay, and it is the genes that would not undergo such recombination that do not decay. The paper gives a reason that the genes that decay the most might decay slower than expected, but that just raises the question of why the ampliconic genes decay even less given that they do not have access to this mechanism. To me, the answer how the why the Y chromosome genes avoid decay lies in the Y-Y gene conversion, and this paper does not chance that conclusion.
Tags: evolution, genetics, sequencing
I see you’ve started research blogging too
I remember having a discussion about this in first year and I seem to remember coming away with the impression that X and Y had been homologous at one point, and the Y had picked up the ‘maleness’ gene, shrunk as bits got lost (not an evolutionary dissadvantage because there was a copy on the X chromosome). That was the last time I ever thought seriously about the issue, so I’m probably well overdue for some catch-up reading.
I did some research blogging stuff a few months ago, but this is the first one I’ve done for a while. I see you’ve started up doing some recently as well; woop for science blogging!
By the way, I loved your post about cell-wall-less bacteria.
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