How does the bald notothen, a small fish that lives in the freezing, icy waters of Antarctica, manage to survive? Clues are to be had from the genes it expresses, which are revealed in a BMC Genomics study published this week.
Kevin T Bilyk and C-H Christina Cheng have decoded the fish's entire transcriptome -- a record of all expressed genes -- in different tissue types and various environments. By comparing the notothen transcriptome against that of the tropical zebrafish (Danio rerio), they were able to highlight 58 elements thought to play essential roles in icy water survival. These include components related to protein binding and transcription -- the first step of gene expression.
The bald notothen (Pagothenia borchgrevinki) lives under the surface ice of the McMurdo Sound, the southern limit of marine life and arguably the coldest, iciest extreme of all Antarctic fish habitats. As a result, it has evolved various adaptations to life in this extreme environment. These include newly-minted genes that encode antifreeze glycoproteins, and modifications of pre-existing genes that help it to function at low temperatures.
Researchers would like to pinpoint exactly which genes endow the bald notothen with its hardy nature, but in the absence of a DNA sequence, the transcriptome -- the output of gene activity -- is a useful proxy.
Transcriptome sequences are already available for a handful of notothenoid species, but they don't include any of the high-latitude coastal species, such as the bald notothen. This reference transcriptome serves as a useful model of gene expression in extreme cold, and will likely fuel further molecular studies of other polar fish and cold-blooded creatures.
The bald notothen is also intriguing because, despite living in sub-zero temperatures, it can handle warmer waters -- given a few days to acclimatise to water at 4 oC, it can then tolerate small incremental increases in temperature. With this in mind, Bilyk and Cheng also studied the transcriptomes of fish kept in warmer waters. They hope to unravel the molecular mechanisms that enable the fish to survive both extreme cold and warmer temperatures -- a feature that may serve the fish well as climate change threatens the temperature of polar waters.
- Kevin T Bilyk and C-H Christina Cheng. Model of gene expression in extreme cold - reference transcriptome for the high-Antarctic cryopelagic notothenioid fish Pagothenia borchgrevinki. BMC Genomics, 2013
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