Few organisms can survive in harsh environments, such as extreme cold or dry conditions, but some species equipped with special adaptations can thrive. The Antarctic midge, Belgica Antarctica, the only wingless insect native to Antarctica, has the smallest insect genome among those sequenced, a likely adaptation to the extreme conditions it is exposed to, according to a new study. Led by Professor Joanna Kelley at Washington State University, US, the study is the first to sequence the genome of an insect found in the poles.
The size of the genome was completely unexpected. Prof Kelley said: “We were very surprised to find that it is so small.”
Living in the rocky outcrops of the Antarctic Peninsula, the midge has unusual adaptations, such as losing its wings and continuously producing heat shock proteins, to endure not only freezing temperatures and high levels of ultraviolet radiation, but also some of the driest and windiest conditions on the planet.
Its larvae take two years to develop before emerging as adults, and spend most of their time burrowed under the ice. Adults, only about 3 millimeters in length, similar in size to a small black worker ant, are the largest land residents native to the continent. Although most people might think penguins and seals are largest animals in Antarctica, they spend most of their time in the water, and some of them are mere visitors. Adult midges live for only 7 to 10 days, during which they mate and lay eggs.
By sequencing the genome, an organism’s entire set of genes, the researchers aimed to shed light on which genes allow the midge to cope in such an inhospitable environment.
To their surprise, the genome turned out to have only 99 million base pairs, the building blocks that make up our DNA sequence, making it the smallest insect genome sequenced to date. This is smaller than the genome of a body louse, which at around 105 million base pairs, was thought to be tiny. In comparison, the genome of the fruit fly, Drosophila Melanogaster, also from Diptera, the same order as the midge, is around 170 million base pairs. We have over 3 billion base pairs.
Most animal genomes, including ours, are filled with large portions of repeating sequences, non-coding regions of DNA called introns that are interspersed in a gene, and mobile DNA elements—which used to be referred to as “junk” DNA. However, the midge’s genome composition is different: it has few repeats, short stretches of introns, and a small number of mobile DNA elements. These reductions allow it to compact its genome, which the authors believe is an adaptation to its harsh environment.
Despite its small size, it has around 13,500 genes that code for proteins, similar to those of other insects belonging to Diptera, such as mosquitoes and fruit flies. Many of its genes are associated with their roles in development, regulating metabolism, and response to external stimuli. When compared with the genes from the fruit fly and three mosquito species, it has around 5,000 unique genes.
Having shed its repeated sequences, it boasts a more streamlined genome that can pack a higher proportion of protein coding genes compared with the genomes of other insects. Approximately 14% of the fruit fly’s genome codes for proteins, and for Aedes aegypti, the mosquito that can spread yellow fever, the figure is only 1.6%.
The larvae persist despite severe dehydration, losing 70% of the water in its cells. But, the midge lacks genes coding for late embryogenesis abundant proteins, known for their crucial role in coping with dehydration in the African sleeping midge, a close relative. How then can the larvae survive the intense desiccation? It is likely that other genes, and constant production of heat shock proteins, among others, might help them survive, the researchers suggest.
Interestingly, the team noticed fewer genes related to odor perception, reflecting a decreased reliance on this sense. The researchers believe this could be because they have less choice of food available to detect when foraging and shorter distances to find mates as they cannot fly.
However, Prof Kelley stresses the need to “study additional genomes to understand whether the conditions are placing a constraint on genome size or whether there are other forces at play. The Antarctic environment may place energetic constraints on the organism or there may be developmental limitations.”
“There are other native Subantarctic midge species and now other species that are on the Antarctic continent due to human introductions. I am very interested in comparing the genomes of introduced species and other native Subantarctic species,” she said.
Now that the midge genome has been sequenced, the team believes it will lay the foundation for comparisons of genomes of other species living in extreme environments.
Kelley, J. L. et al. Compact genome of the Antarctic midge is likely an adaptation to an extreme environment. Nat. Commun. 5:4611 doi: 10.1038/ncomms5611 (2014).