Scientists have sequenced the genome of the Antarctic midge (Belgica antarctica), which contains only 99 million base pairs of nucleotides, making it smaller than other tiny reported genomes, such as that of the body louse (105 million base pairs) or the winged parasite Strepsiptera (108 million base pairs). It is now the smallest insect genome described to date.
The research is published in the journal Nature Communications, and the authors suspect that the tiny size of the genome can be explained by the midge’s adaptation to its extreme living environment. 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, according to senior author David Denlinger.
“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 — 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.
“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 — the only insect endemic to Antarctica — 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 ten days mating and laying eggs, and then they die. In the Antarctic ecosystem, these midges eat bacteria and algae, as well as nitrogen-rich waste produced by penguins. No other species preys on them, and Denlinger’s lab has not identified any pathogens that might endanger their lives.
Although it is small in architecture, the Antarctic midge genome still contains about 13,500 functional genes, which is similar to the number found in the genomes of other flies.
Denlinger has studied the Antarctic midge for many years, zeroing in on the insect’s unusual stress responses, including the activation of heat-shock proteins. Most animals turn on these proteins only when they’re under acute stress — particularly when they’re exposed to extremely high or low temperatures — and quickly turn them off when the stress has passed. But heat-shock proteins are activated constantly during the Antarctic midge’s larval stage, a trait scientists believe is linked to its survival in harsh conditions.
Denlinger’s lab has cloned and studied several genes connected to these proteins.
“Sequencing the genome gives us access to a broader suite of many other closely related genes that we didn’t have access to before,” he said.
The research also reveals a host of genes called aquaporins, which are involved in water transport into and out of cells. These genes and the proteins they make are also players in the midge’s survival in Antarctica. Most insects can survive losing about 20 percent of the water in their bodies’ cells, but these midges tolerate a loss of up to 70 percent of their water.
“They look like dried up little raisins, and when we pour water on them, they plump up and go on their merry way,” Denlinger said. “Being able to survive that extreme level of dehydration is one of the keys to surviving low temperatures. This midge has some mechanism that enables it to both be dehydrated and stay alive, with its cells functioning normally.”
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