Scientists who sequenced the genome of the Antarctic midge（蚊，蠓） suspect the genome's small size -- the smallest in insects described to date -- can probably be explained by the midge's adaptation to its extreme living environment. The midge is a small, wingless fly that spends most of its two-year larval stage frozen in the Antarctic ice. Upon adulthood, the insects spend seven to 10 days mating and laying eggs, and then they die.

Its genome contains only 99 million base pairs of nucleotides（核苷酸）, making it smaller than other tiny reported genomes for the body louse (105 million base pairs) and the winged parasite Strepsiptera (108 million base pairs), as well as the genomes of three other members of the midge family.

The midge genome lacks many of the segments of DNA and other repeat elements that don't make proteins, which are found in most animal genomes. The lack of such "baggage" in the genome could be an evolutionary answer to surviving the cold, dry conditions of Antarctica, said senior author David Denlinger, Distinguished Professor of entomology（昆虫学） and of evolution, ecology and organismal biology at The Ohio State University.

"It has really taken the genome down to the bare bones and stripped it to a smaller size than was previously thought possible," Denlinger said. "It will be interesting to know if other extremophiles（极端微生物） -- 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."

Once called "junk DNA," these DNA segments and repeat elements in genomes are now known to have important functions related to gene regulation. They also are implicated in many disease processes. So could a bare-bones genome be the secret to midge survival?

"We don't yet understand what the implications are of not having all that extra baggage. It seems like a good thing in many ways, but organisms do get some beneficial things from this baggage, too," Denlinger said.

The midge genome is small in architecture but not in the number of genes, the researchers noted: The Antarctic midge genome, like genomes of other flies, contains about 13,500 functional genes.

The research is published in the journal Nature Communications.