Antarctic midge trades junk DNA for life in the extreme
This fly has the smallest genome in the insect kingdom. Dyani Lewis reports.
The Antarctic midge has just qualified for a second entry in the Guinness Book of Records. Not only is it Antarctica’s only native insect, it turns out it has the tiniest genome of any insect in the world.
Researchers are intrigued by the remarkable adaptations of Belgica antarctica. The half-centimetre long, purplish-black wingless fly spends its first two years as a larva mostly encased in ice, thawing each summer for brief bursts to feed on algae and penguin droppings. Once it reaches adulthood it has only seven to 10 days to find a mate and lay eggs before dying.
During its life on the rocky outcrops of the Antarctic Peninsula it has to withstand extreme cold, UV exposure, high salinity and extreme dehydration. It can lose 70% of its body fluids, taking on the appearance of a “little raisin”, says David Denlinger from Ohio State University.
Denlinger was curious as to what secrets might lie in this extremophile’s DNA. One thing he didn’t expect was to find that there was so little of it. As published last week in Nature Communications the midge’s DNA code stretches a mere 99 million letters. By contrast, a honeybee has over double that at 236 million letters. And the DNA of the mosquito that transmits dengue fever, Aedes aegypti, runs 14 times longer.
Several genes have already been identified that appear to make the midge better adapted to life in the extreme. “Heat shock” proteins, which in other animals are switched on briefly during periods of extreme stress, are continuously active in the midge. Genes for antioxidant enzymes, which protect against UV, are also in constant “on” mode.
Further genes important for Antarctic life are likely to be revealed, but with its tiny genome the midge has unwittingly entered the long-standing debate over how much of the DNA code is meaningless “junk”. For some creatures, such as mosquitoes or us, the vast majority of DNA does not appear to have a clear function, such as coding for proteins. In humans 98.5% of our DNA fits that category.
Some scientists argue vehemently that even if DNA does not code for protein it may still carry other vital information (see "What is our junk DNA for?"). The counter-argument is based on the existence of organisms that survive perfectly well without the junk, such as the fugu fish or the bladderwort plant. Now the Antarctic midge joins their ranks. Like these other species it carries a respectable number of protein-coding genes – around 13,500, which is average for an insect – but little junk DNA. Only 10% of its genome is junk, Denlinger found.
In particular, it only carries a tiny proportion of a virus-like sub-type of junk DNA known as “transposons”. "Most genomes are full of transposons,” says Denlinger. The dengue mosquito devotes half of its genome to them, but the midge sets aside just 0.12% for the cause.
One possible explanation could be the midge’s isolation. Transposons often use viruses or bacteria to jump between species. In the barren environs of Antarctica, where no other insects live, opportunities to acquire transposons to bulk up the genome are probably rare.
In wondering why some organisms end up with more junk DNA than others researchers had thought temperature would play a role… but in the opposite direction. In warmer temperatures faster metabolism and growth are favoured so that an organism can complete its lifecycle in the shortest possible time. This might spur organisms to dump their junk to allow faster growth, because the bigger the genome the longer it takes to replicate.
"Clearly this midge did not read those rules and decided to go the other way,” says Denlinger.
Biologist Dag Hessen from the University of Oslo is also fascinated that the midge has bucked the trend. “This points to the fact that there is probably not one unifying cause for small genomes," he says. Precisely what mechanisms have caused the midge to downsize its genome remain, for now, a mystery.