There are eight families of notothenioid fish, and five of them inhabit the Southern Ocean, the frigid sea that encircles the Antarctic continent.
These fish can withstand temperatures that would turn most fish to ice. Their ability to live in the cold - and oxygen-rich - extremes is so extraordinary that they make up more than 90 percent of the fish biomass of the Southern Ocean.
Dissostichus mawsoni, the Antarctic toothfish, is the largest of the notothenioid fish to inhabit the Southern Ocean.
University of Illinois (U. of I.) animal biology professor Arthur DeVries discovered in the late 1960s that some notothenioids manufacture their own "antifreeze proteins." These proteins bind to ice crystals in the blood to prevent the fish from freezing.
In the new study, U. of I. animal biology professor C.H. Christina Cheng and her colleagues at the Chinese Academy of Sciences sought comprehensive genetic clues that would help explain how the Antarctic notothenioids survive.
Cheng and her colleagues wanted to know which genes were being expressed at high levels in one representative species of Antarctic notothenioid, Dissostichus mawsoni.
They analyzed gene expression in four tissues: the brain, liver, head kidney (the primary blood-forming organ in fish) and ovary of D. mawsoni.
"We saw this very peculiar profile where in each of these tissues the proteins that are highly expressed are from a small set of genes," Cheng said.
"Each tissue makes all kinds of transcripts - the genetic messages that are made into proteins - but we found that a small group of genes dominates the transcriptional process," she added.
The researchers reasoned that any proteins that gave the fish an advantage in a cold, oxygen-rich environment would be expressed at high levels in the Antarctic fish.
To get a better idea of whether the genes that were "upregulated" in D. mawsoni enhanced its survival in the Antarctic, the researchers compared gene expression in D. mawsoni and in the same tissues of several unrelated, warm-water fish.
They found that most of the genes that were highly expressed in the Antarctic fish were not elevated in the warm-water fish.
When they analyzed the upregulated genes, the researchers found that many of them coded for proteins that respond to environmental stress.
There were many chaperone proteins, including "heat shock proteins," for example, which protect other proteins from being damaged by stresses such as extreme cold or heat.