Saturday, October 23, 2004

Humans, mice... pandas thumb.

I'm getting very frustrated trying to put together a science post. Most of what I find is behind subscription walls, and I have been REALLY disappointed in the Public Access journals this time around.
What I have been trying to follow up on is the presence of repetitive DNA, which is made, up among other things, of copies of transposable elements. This DNA makes up almost 1/3 of the mouse and human genomes, with one particular family of sequences, the MaLR repetitive elements, occuring more than 380,000 times in the mouse genome! Such massive representation must confer some sort of selective advantage to outweigh the cost of keeping it. However, mammalian cells appear to be doing their darndest to suppress and control these regions of DNA by a variety of strategies.

A bit of further reading has given me two more discussion points.
The first , by Pearson et al. at Jackson Labs, shows that although these transposable elements are supressed through most of the mouse's life cycle, during the oocyte to pre-implantation embryo stages, they become very active, representing almost 3% of the total RNA output of the mature oocyte! Furthermore, the authors identify many instances of transposable elements spliced onto host mRNA transcripts, displacing elements present in the "normal" transcript made at other stages. In effect, the lunatics have taken over the asylum, at least during these developmental stages.

The second reading appears to underscore the parasitic nature of these repetitive elements. Nobrega et al. report mutant mice missing big chunks of their "gene desert" DNA, of which repetitive DNA is a part. Mice homozygous for these big genomic deletions are... perfectly normal, as far as laboratory tests can show, supporting the idea that at least a lab mouse could do pretty well without these regions. It should be said that the authors mentioned above, Pearson et al., believe that only a few transposable elements are positioned correctly to "break out" during earliest embryogenesis in the phenomena they study.

What appeals to me about this pretty confusing segment of the biological world is that it appears to be another instance of the Panda's Thumb. This notion, made famous by Stephen Jay Gould's book of the same title, states that natural selection is a master improvisor, and acts on all heritable differences without regard to human notions of elegance. As highly refined as the beings living all around us are, they followed a windy, twisty road to be here. In the case of repetitive DNA, the hosts genomes' relationship with these "parasites" makes a coevolutionary spiral in which, seemingly despite themselves, both players have benefited in their evolutionary struggles.

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