When It Comes to Genomes, Smaller Is Sometimes Better

The Antarctic midge travels very light, genetically speaking. A new study shows that this tiny creature, which spends most of its short life frozen in ice (save for a weeklong bender of mating and egg-laying), packs the smallest genome found in insects to date. The length of that genome is only 99 million base pairs (the building blocks of DNA, either a guanine-cytosine couple or an adenine-thymine couple). For comparison, the silk moth’s genome is an estimated 436 million base pairs long; our own Homo sapiens genome spans 3.2 billion base pairs.

But does size really matter when it comes to your genetic code? The human genome is big, but it pales in comparison to the marbled lungfish (130 billion base pairs) and is utterly dwarfed by the Japanese flower Paris japonica (149 billion base pairs). In the 1960s, scientists declared that the amoeba Polychaos dubium had an even more gargantuan genome, clocking in at 670 billion base pairs, but this was before modern genetic sequencing techniques, so that figure is disputed.

On the smaller end of the scale, there are creatures that get by with even smaller genomes than the Antarctic midge: The carnivorous plant Genlisea margaretae, clocking in at 63 million base pairs; or the E. coli bacterium, which trucks along with 4.6 million base pairs. The bacterium Nasuia deltocephalinicola, which lives in the digestive tracts of leafhopper insects, has just over 112,000 base pairs in its genome.

How can these animals get by with such little DNA? It sounds a little less implausible when you consider that just around 2 percent of the human genome, corresponding to some 20,000 genes, contains instructions for making the proteins our body needs. The rest, called non-coding DNA, is a genetic wilderness that scientists are still trying to map. Sometimes referred to as “junk DNA,” the non-coding regions of genomes are more valuable than you might think. They contain bits of genetic code inserted into the genome by viruses (some of which may have a function, some of which may just be hitching a ride), and stretches of DNA that do not code for proteins but still play an important role in our daily life. Some of these regions regulate DNA replication and gene expression; others produce useful RNA molecules that can act as chemical sensors. In 2012, a group of scientists working on a genetic mapping effort called the ENCODE project conservatively estimated that 8 to 9 percent of the human genome was involved in regulation alone.

But a smaller genome may confer advantages to an organism. Researchers have found a gradual shrinking of genomes occurring in those dinosaurs that eventually became birds. Less DNA to pack has been linked to faster nutrient transport and faster cellular signaling, possibly freeing up the energy needed for a metabolically demanding process such as flight.

The Antarctic midge may be a doorway into investigating the relationship between genome sizes and surviving in extreme environments (such as staying frozen for most of your life). “[The insect] has really taken the genome down to the bare bones and stripped it to a smaller size than was previously thought possible,” Ohio State University researcher David Denlinger, one of the scientists who described the midge genome in Nature Communications this week, told Phys.org. “It will be interesting to know if other extremophiles—ticks, mites, and other organisms that live in Antarctica—also have really small genomes, or if this is unique to the midge. We don’t know that yet.”