Antarctica conjures up a deep frozen place, bitterly cold and inhospitable to life. The frigidity is indeed severe, but the Southern Ocean (SO) surrounding Antarctica is teaming with marine life. A group of >100 related species of fishes called Antarctic notothenioids abundantly populate this iciest and coldest body of water on Earth, critically sustaining the rich Antarctic food web that includes the iconic penguins and seals.  By physical principles, ectothermic (cold-blooded) fishes could not have existed in the SO. Freezing water temperatures, replete with ice crystals, are a deadly combination as it will quickly freeze the fish body fluids into solid ice resulting in death.  Under such exigent life-or-death selection pressure, the Antarctic notothenioid ancestor evolved life-saving antifreeze proteins (AFPs) and conquered this forbidding marine realm. With freezing death from freezing, fish could continue to evolve and adapt to become proficient in performing physiological functions in extreme cold.  This project was carried out to gain an integrated understanding of the mechanisms and evolution of these two sets of crucial and intertwined adaptive traits through studies that span the environment, organism and genome.

Our findings reveal surprising twists to long held paradigms, bringing new insights into evolutionary complexities and trade-offs.  AFPs are known to protect fish from freezing by binding to invading ice crystals and arresting ice growth. We found this tight binding also prevents water molecules from leaving the ice crystal, inhibiting melting. Thus antifreeze proteins are also anti-melt proteins, a double-edged sword.  Accumulation of AFP-stabilized internal ice could eventually become lethal.  We tested whether ice can melt during summer warming episodes. We collected long-term temperature record of a high-latitude habitat and found that summer warming does not overcome AFP inhibition of melting to reliably eliminate ice. Thus evolution of the life-saving AFPs exacts a costly trade-off - the risk of lifelong accumulation of damaging internal ice crystals. Another long held paradigm is that AFP protection is obligatory for notothenioid freeze avoidance. One species, nicknamed Antarctic grey cod, lacks detectable AFPs but survives in many SO sites. We found the grey cod has not lost its AFP genes, but transcription defect resulted in no messenger RNA and thus no protein. How then does it survive in the wild? We found its native habitat is the non-freezing Antarctic bottom water layer, and in a test thermal gradient it seeks to occupy the non-freezing region. Thus the grey cod has forgone its AFPs for behavioral freeze avoidance. Evolutionary ingenuity can be unpredictable and odd.

Evolutionary adaptation gaining functioning abilities in constant cold inexorably progresses to cold specialization, with loss of functions previously needed in thermally variable environments of the ancestor. This project addressed this progression, of the notothenioid genome from a non-polar to polar character. To this end, we sequenced the transcriptomes (the expressed gene set of the genome) of a basal non-Antarctic species as ancestral proxy, and two cold-adapted Antarctic species, one red-blood and one white-blooded, aptly comprising an evolutionary progression from ancestral non-Antarctic to highly-derived polar character. We selected a functional system – cellular heat shock response (HSR) - to assess this progression. The basal species executed an intense classical HSR, establishing HSR is an ancestral trait in the precursor of the Antarctic notothenioids. Prior studies have documented Antarctic notothenioids are unable to mount a HSR, a functional loss during their evolution in unchanging cold. We found, contrary to this homogeneous loss paradigm, it is species dependent. While the red-blooded species did not exhibit a HSR, the white-blooded notothenioid was able to mount a minimal HSR, and additionally (and surprisingly) displayed robust broader cellular stress responses outside of the HSR.  Understanding notothenioid ability to mitigate heat challenge is relevant as it correlates with thermal plasticity of these fish now facing a warming SO.

The research outcomes of this project have advanced our understanding of the interplay of environmental, organismal, molecular and evolutionary processes of notothenioid freeze avoidance and physiological functioning in the cold, relevant to the broader picture of the limits of environmental adaptation, climactically driven genomic changes, and adaptive potential of this ecologically important fishes to a warmer world. The project lead to 17 published papers (4 in PNAS) and one in review. Genomic libraries and transcriptome data resources generated from the project are available for use by the broader community. The project provided the training of three post-doctoral researchers, one PhD and two MS students (all graduated) in field and lab research, and five undergraduate students conducted supervised research. Public outreach included “Science at the Market” for the local city residents visiting the weekly farmers’ market, and the PI and coPI contributed to the production of a French documentary film about Antarctic sciences and scientists, aired on the acclaimed European cultural TV channel “Arte”.

Last Modified: 11/30/2017
Modified by: Chi-Hing C Cheng