Science Saturday

Sometimes, Genes That Are Supposed to Be There, Aren't. Meet "Dark DNA"

You may have heard of "dark matter" — the mysterious stuff that apparently makes up 27 percent of the universe but can't be detected by conventional means. But we don't have to look deep into outer space to find an intractable mystery. "Dark DNA" is the biological cousin of dark matter, in the sense that we can't see it but we know it's there. And hiding with it just might be some of evolution's deepest secrets.

The adorable, unfairly named sand rat

Uncharted Genetic Territory

The more scientists map the genome, the more they come to understand exactly how gene sequences are reflected in the fully-formed animal. We've come to understand why giraffes are so tall, why snakes are so long, and soon, we'll probably find the gene that explains why some people don't like "Game of Thrones". But every once in awhile, armed with the knowledge of what genes do what, we'll go searching for something we know is there, and come up empty-handed.

In a piece published on The Conversation, evolutionary biologist Adam Hargreaves describes hitting such an obstacle while researching the sand rat, a species of gerbil that's much cuter than its name implies. These little furballs are particularly prone to type-2 diabetes, so Hargreaves and his colleagues sought to explain the phenomenon by examining a gene called Pdx1, which governs the production of insulin. But it wasn't there. And neither were the 87 other genes that surround it.

The thing is, we know for a fact that the sand rat has the gene Pdx1, since it's absolutely necessary for survival. What's more, plenty of the other 87 missing genes are just as necessary. Just to make sure, Hargreaves and co. looked for certain chemical compounds in the sand rat's muscle tissues that would have been produced by the missing genes — and sure enough, there they were. But why weren't the genes where they were supposed to be? And where were they really?

By the way, sand rats aren't the only animals to have misplaced genes. Actually, well over 200 genes have yet to be discovered in birds, but we know for a fact that they must be in there somewhere. Pdx1 and its surrounding genes, as well the missing bird genes, are all rich in G and C molecules, which have historically been difficult for gene-sequencing technology to detect. So since they aren't turning up where we expect them, they've managed to slip our attention like a needle in a haystack.

The Evolution Will Not Be Gene Sequenced

There was another little surprise that turned up in the gene-sequencing of the sand rat. One section of their genome seemed to be much more likely to mutate than any other part — a section that was rife with G and C molecules. So it's probably not a huge stretch of the imagination to guess that the missing genes are somewhere in that section, even if our modern gene-sequencing technology isn't especially great at identifying them yet.

So the thing is, those "mutation hotspots" could be one of the key ingredients in evolution itself. Evolution, obviously, depends on mutation in order to progress. Once a trait emerges, natural selection determines if it is a good idea or not, which just a fancy way of saying that if the mutation hurts the animal, it's less likely to live long enough to pass it on. Conversely, if the mutation helps the animal, it will have all the time in the world to have tons of babies with very similar genes.

In the case of the sand rat, the mutation hotspot that likely governs insulin production and other traits may have provided the key to its survival in the harsh desert landscape. But even as those mutations help the animal in some ways, they could hurt it in others — it's just that, for now, the good parts outweigh the bad. If the environment changes to the point that desert adaptation isn't so important anymore, then it's likely that the sand rat's poor insulin regulation will start to weigh heavier on its success story.

What You Didn't Know About Your DNA

Written by Reuben Westmaas September 16, 2017
