Not all of our cells have the same genome. A few cell types, mostly immune cells, undergo a programmed deletion of small sections of DNA, and so don't end up with the genome that started in the fertilized egg that ultimately produced them. But most cells in the body end up with the same genome, which is one of the reasons it's possible to convert many of them back to stem cells.

That said, most specialized adult tissues don't need all the genes they inherit. In fact, activating the unneeded genes—turning on liver enzymes in a muscle cell, for example—would generally be harmful. So, it's easy to envision that there might be an evolutionary advantage to the controlled elimination of some DNA.

Although it's rare in mammals, we haven't looked at the genomes of most other animals carefully enough to really know whether these sorts of controlled genomic deletions might be common elsewhere. About two years ago, there was a hint that they took place in at least one group of animals: the jawless fish that branched off near the base of the vertebrate family tree. Researchers working on them reported something a bit odd: a stretch of DNA they were looking at was present in the germ line (reproductive cells that give rise to sperm and eggs), but not in many of the other cells of the animal's body.

But the follow-up work is where things got rather surprising. As the researchers checked more areas of the genome, they found more and more places with the same pattern: present in the germ cells, missing from everywhere else in the body. By the time they were done totaling it up, the number was staggering: the cells that make up most of the lamprey were missing about 20 percent of the lamprey genome. Individual deletions could take out hundreds of thousands of bases, and included all sorts of material: genes, repetitive "junk" DNA, you name it.

In their latest study, the authors identify the genes that the lamprey gets rid of, and find the total numbers in the hundreds. And, when they looked at them in detail, they found a clear bias: many of the genes were involved in controlling a cell's ability to divide or establish its identity.

The authors suggest that, if these genes were activated in a regular cell, they might lead to problems with its normal function, or even produce a condition like cancer. So, rather than relying on the usual mechanisms of shutting something down, the lamprey simply gets rid of the gene. This isn't absolutely necessary, as mammals can shut genes down through a combination of proteins that attach to DNA and the structure of the chromosomes themselves. Of course, that process can be error prone, while the lamprey's would leave little room for the genes to be reactivated later.

It's easy to think of this as a bit of an aberration. After all, the jawless vertebrates branched off very early in our lineage's history, and there are only about 100 species total left in the lineage. And it's entirely possible that this is an oddity that only evolved after the lineage had split off.

But there's an alternate possibility, namely that this was a feature of our entire lineage, but has been lost somewhere back in the history of mammals. If that's the case, then these sorts of controlled genome reductions might be far more widespread within the animal kingdom. It's certainly a possibility worth examining in more detail.

Current Biology, 2012. DOI: 10.1016/j.cub.2012.06.028 (About DOIs).