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Antarctic fish hemoglobins: evidence for adaptive evolution at subzero temperature
L Bargelloni1, S Marcato, T Patarnello
1Dipartimento di Biologia, Università di Padova, Via Ugo Bassi, 58/B I-35131 Padova, Italy.
Summary
Antarctic fish like Gymnodraco acuticeps show positive selection on their single hemoglobin form. This suggests hemoglobin diversification, not a simple trend towards losing hemoglobin entirely.
Area of Science:
- Marine biology
- Evolutionary genetics
- Fish physiology
Background:
- Notothenioids are Antarctic fish with reduced need for hemoglobin due to cold, high-oxygen waters.
- Icefish are unique vertebrates lacking hemoglobin entirely.
- Gymnodraco acuticeps possesses a single hemoglobin form without the Bohr effect, closely related to icefish.
Purpose of the Study:
- To investigate the evolutionary pressures on the single hemoglobin form in red-blooded notothenioids.
- To understand the functional diversification of hemoglobin in Antarctic fish.
- To test the hypothesis of a trend towards hemoglobin loss in Antarctic fish.
Main Methods:
- Obtained nucleotide sequences for the beta-globin chain of hemoglobin in six red-blooded notothenioids.
- Analyzed rates of nonsynonymous (KA) and synonymous (KS) substitutions.
- Calculated KA/KS ratios to infer positive selection.
Main Results:
- Gymnodraco acuticeps exhibited a higher rate of nonsynonymous substitutions (KA) compared to synonymous substitutions (KS).
- KA/KS ratios significantly greater than one were observed in most comparisons for G. acuticeps.
- These findings suggest positive selection is diversifying the single major hemoglobin.
Conclusions:
- The single hemoglobin in G. acuticeps is diversifying under positive selection for specialized functions.
- Reduced routine oxygen transport by hemoglobin is possible due to environmental and physiological factors.
- Diversifying selection on hemoglobin contradicts a simple evolutionary trend towards hemoglobin loss in Antarctic fish.