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Planet Earth


The LoomBy Carl ZimmerNovember 5, 2003 8:39 AM


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My hotel here in Wisconsin has a great high-speed connection and I have some downtime, and so I'll post on a really interesting paper that just came out that may tell us a lot about how we got so complex. When I say "we," I'm speaking very broadly. Humans, other mammals, reptiles, birds, amphibian, and fish are all very complex, particularly compared to our closest invertebrate relatives. The picture I've attached here is of Ciona, one of these closest relatives. Little more than a small sleeve-shaped filter feeder, it's not too impressive. In particular, its body is not too complicated. It doesn't have ears, eyes, noses, stomachs, livers, and the many other organs that vertebrates have--organs that have to be constructed from many kinds of cell types. Scientists have been studying the genes of Ciona and our other invertebrate cousins to find some clues to what happened to give rise to that complexity. One possibility is that in our lineage, a big portion of the genome was duplicated--or perhaps the whole genome was duplicated. Perhaps it even doubled more than once. In the latest issue of Genome Biology, Spanish researchers take a new look at Ciona's genome. They looked at so-called "mobile elements" in these animals' DNA. Mobile elements are not actually genes, but viral-like stretches of DNA that make copy of themselves that can be inserted into other places in the genome. We humans have a staggering amount of mobile elements or their defunct relicts--perhaps close to half of our genome is composed of them. Most vertebrates studied have hefty amounts of mobile elements too. Among other branches of life, some have lots of this jumping DNA, while others have little. The Spanish researchers have found that Ciona has very little jumping DNA. On top of that, it doesn't defend against jumping DNA the way vertebrates do--by clamping down on these stretches of the genome in a process called methylation to keep the jumping DNA from jumping. These mobile elements seem to be an innovation in vertebrates, not seen in our closest invertebrate relatives. So what may have happened some 550 million years ago is that our genomes duplicated. This opened up the possibility for jumping DNA to move around the genome without causing too much harm, since each gene came in pairs. If one gene got knocked out by an intruding mobile element, the other gene could still operate. Vertebrates evolved methylation as a way to keep these mobile elements from getting too far out of control. But mobile elements may also have played a positive role in vertebrate evolution--they became the main source of mutations, carrying surrounding parts of genes with them to new places and creating new genes. These mutations were the raw material for evolution. New genes led to new cell types, and a new way of living.

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