The Antarctic midge, Belgica antarctica, is the continent’s only native insect... and it’s tough. The extremophile survives in the most intense landscape: freezing temperatures, desiccation, high salt concentrations, strong winds, and severe UV exposure. And it does all that with a surprisingly compact genome -- likely an adaptation to life in extreme environments, according to work published in Nature Communications this week.
This small fly is the only fully terrestrial animal native to Antarctica, spending its life on frozen rocky outcrops, eating bacteria, algae, and nitrogen-rich penguin waste. The larvae develop over two winters, losing half their body mass each time. When they mature into the wingless adults, they spend just a week or so mating and laying eggs before the cycle begins again.
To see how they evolved to cope with these conditions, a team led by Joanna Kelley from Washington State University and David Denlinger from Ohio State University assembled the B. antarctica genome and discovered the smallest insect genome ever sequenced: just 99 million base pairs of nucleotides (a basic unit of DNA).
“It’s tiny,” Kelley says in an university statement. “That was a huge surprise. I was very impressed.” The previous insect record holder and runner-up was the body louse (105 megabase pairs) followed by twisted-wing parasites, Strepsiptera (108 megabase pairs); three other midges were also near the top. Just for comparison, the human genome has 3.2 billion base pairs, while the bacterium Carsonella ruddii has 159,662 base pairs.
Compared to other insects, this “extremely economic” genome contains fewer repetitive genetic sequences that don’t code for proteins (known as “junk DNA”), as well as shorter stretches of DNA called introns, which separate the genome’s coding regions. The lack of such “extra baggage” could be an evolutionary answer to surviving those cold, dry conditions, Denlinger explains in a news release. “It has really taken the genome down to the bare bones and stripped it to a smaller size than was previously thought possible.”
Like other flies, the midge has 13,500 functional genes. Among these, the team found an abundance of genes associated with development, regulation of metabolism, and responses to external stimuli.
As larvae, their heat-shock proteins are constantly activated; most animals turn these proteins on only when they’re very stressed out. They also have a host of genes called aquaporins, which are involved with transporting water in and out of cells. Other bugs can lose about 20 percent of the water in their cells, but these guys tolerate a loss of up to 70 percent -- surviving an extreme level of dehydration. “They look like dried up little raisins," Denlinger says, "and when we pour water on them they plump up and go on their merry way."
On the other hand, their genome contains few odorant receptors. Being flightless, the midge probably doesn’t need to detect things that are farther than they can walk, and they don’t venture far.
“It’s a pretty exciting fly,” Kelley adds.
Images: mating pair (top), wingless adult (middle), larvae (bottom) by Richard E. Lee, Jr