In the human genome, only about 2% of our DNA are genes involved in coding the proteins essential to our existence. The other 98% is noncoding DNA, often called junk DNA because there's no clear purpose for it. That name might seem a bit pejorative, but a new study of the bladderwort genome suggests it's oddly accurate.
The bladderwort is one of the world's most successful carnivorous plants, and it does so with a genome that is 97% genes and 3% junk DNA — that's almost the exact opposite percentage of what we find in humans, not to mention other plant species like corn and tomato. An international team of researchers in Mexico and the United States recently sequenced the genome of Utricularia gibba, and they argue that these results cast major doubt on previous suggestions that noncoding DNA could be crucial in the transformation of DNA into its sibling genetic material RNA. Junk DNA might well serve that or other key functions in complex species, but this study suggests there's no requirement that junk DNA do so. As University of Buffalo biology professor Victor Albert explains:
"The big story is that only 3 percent of the bladderwort's genetic material is so-called 'junk' DNA. Somehow, this plant has purged most of what makes up plant genomes. What that says is that you can have a perfectly good multicellular plant with lots of different cells, organs, tissue types and flowers, and you can do it without the junk. Junk is not needed.
So then, why do so many species have a ridiculous amount of junk DNA while bladderwort has nearly none? In the specific case of U. gibba, it appears to have doubled the size of its genome at three distinct points in its evolutionary history, and each time this occurred it undertook purges of its nonessential DNA. It ended up with 28,500 genes, comparable to plants that have 5 to 10 times as many DNA base pairs in their genome. In a statement, the researchers lay out their arguments as to why some species keep their junk and others get rid of it:
Some species may simply have an inherent, mechanistic bias toward deleting a great deal of noncoding DNA while others have a built-in bias in the opposite direction — toward DNA insertion and duplication. These biases are not due to the fact that one way of behaving is more helpful than the other, but because there are two innate ways to behave and all organisms adhere to them to one degree or the other. The place that organisms occupy on this sliding scale of forces depends in part on the extent to which Darwin's natural selection pressure is able to counter or enhance these intrinsic biases.
The new U. gibba genome shows that having a bunch of noncoding DNA is not crucial for complex life. The bladderwort is an eccentric and complicated plant. It lives in aquatic habitats like freshwater wetlands, and has developed corresponding, highly specialized hunting methods. To capture prey, the plant pumps water from tiny chambers called bladders, turning each into a vacuum that can suck in and trap unsuspecting critters.
Image by Alex Popovkin, Bahia, Brazil on Flickr.