Antarctic midge has smallest insect genome

Belgica antarctica adults mating - larger female on left and male on right

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A flightless fly from Antarctica has been found to have the smallest insect genome sequenced to date.

The Antarctic midge can survive high levels of salt, freezing and intense ultraviolet radiation, which are all common to the Antarctic climate.

But the fly's genome has just 99 million base pairs (the building blocks of DNA), compared with 3.2 billion in the human genome.

It is thought the small size may be an adaptation to its extreme environment.

Results from the study are reported in Nature Communications.

The wingless midge, Belgica antarctica, lives on the rocky outcrops of the Antarctic peninsula and is the only insect and fully terrestrial animal endemic to the continent.

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Its larvae develop over two Antarctic winters, losing nearly half their body mass through dehydration each time. The adult midge's lifespan is 7-10 days, during which mating and egg laying takes place.

Unusual adaptations including winglessness and the ability to tolerate extreme cold and dryness allow the fly to endure high winds and extreme temperatures.

The scientists, led by Dr Joanna Kelley, from Washington State University, US, wanted to sequence the genome to understand how the midge adapted to the extreme environment it lives in.

"It's tiny," said Dr Kelley. "That was a huge surprise. I was very impressed."

At only 99 million base pairs of nucleotides, the genome of the midge is smaller than the genomes reported for the body louse (105 million base pairs) and the winged parasite Strepsiptera (108 million base pairs).

"This is the first insect extremophile to be sequenced, which means it is a fantastic foundation for future comparative studies," Dr Kelley added.

"In regards to genome evolution, previous work has suggested that small effective population sizes should lead to larger genomes, which is the opposite of what we see in the midge."

But while the genome is small in size, the midge has about 13,500 genes, similar to the number in other flies.

Larvae of the midge Belgica antarctica The midge's larvae lose nearly half their body weight as they develop over two Antarctic winters

The researchers compared the genes with their known functions in other animals and found an abundance of genes geared towards regulation and development.

They also found that the midge's genome had few genes involved in odour reception, which could be due to the fly having to crawl around to look for food and mates rather than rely on odour for such tasks.

The midge's genome was also said to be "extremely economic" compared with other related insects, with very few repeated genetic sequences and shorter stretches of DNA, called introns, separating coding regions of the genome.

"We think the small genome may be an adaptation to the extreme environment the fly encounters," she said.

"Other small insect genomes appear to be related to developmental time. Since the midge spends two Antarctic winters as a desiccated larva and then has the short Antarctic summer for growth, this is an open question for Belgica antarctica."

Dr Kelley now hopes to compare the other Antarctic and sub-Antarctic organisms to see if they have similar genomes.

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