Evolutionary biologists have uncovered a surprising secret in the genome of a common intestinal parasite. According to a new study, a primitive protozoan called Giardia lamblia has at least one intron, a piece of "junk DNA" that exists in the middle of genes but doesn't help code for a protein. The discovery suggests that introns--considered a mark of genetic complexity--evolved 1 billion years earlier than researchers had thought.
For years, evolutionary biologists have argued over when introns evolved. Although the purpose of the segments is poorly understood, they make life more difficult for cells because they because they have to be snipped out of RNA copies of a gene before it can be turned into a protein. Prokaryotes--those organisms whose cells lack nuclei--don't have introns. Nearly all eukaryotes, in contrast, do. But until now, experts thought the most primitive eukaryotes were the exception to this rule. This group includes Giardia, a bug infamous for causing diarrhea in unfortunate hikers who drink from contaminated streams.
Mitchell Sogin of the Marine Biological Laboratory in Woods Hole, Massachusetts, and his colleagues discovered an intron in Giardia after working out the order of the bases in its genome, recognizing that its sequence was characteristic of introns in other organisms. Team members Julie Nixon and John Samuelson of Harvard University found the 35-base intron in the middle of the gene for a protein called ferredoxin. The team also discovered genes for several of the proteins that make up the molecular scissors, or spliceosomes, that remove introns as the gene is translated into a protein. The existence of these genes bolsters the idea that Giardia's intron is processed like those in other eukaryotes, says Sogin.
The new findings, published in the 19 February early online issue of the Proceedings of the National Academy of Sciences, suggest that introns evolved about 2 billion years ago--before different eukaryotic groups split off from one another. In recent years, more and more researchers have supported the notion that introns evolved relatively late in eukaryote evolution, says W. Ford Doolittle, an evolutionary biologist at Dalhousie University in Halifax, Nova Scotia. However, he says, this discovery "opens the question" again.