Evolutionary analysis of the transferrin gene in Antarctic Notothenioidei: A history of adaptive evolution and functional divergence

Organisms living in the Southern Ocean are exposed to strong environmental constraints, especially temperature. The Perciform suborder Notothenioidei, today largely endemic to the Antarctic, has developed numerous cold-adapted characters. The most striking peculiarity of this group of fish is the drastic reduction of hemoglobin content in their blood. This condition gives rise to a low metabolic demand of iron. Recently, we have undertaken a study to add new insights on iron metabolism in hemoglobinless fish. By taking advantage to our previous studies on transferrins from Antarctic Notothenioids, in this article we compared the evolutionary properties of these sequences to those from other fish groups and mammals. Our findings based on analysis of dN/dS ratio and on the results of the relative rate ratio test suggest that the transferrin gene has undergone positive selection in fish but not in mammals. The results of type I functional divergence provide statistical evidence for shifted evolutionary rate after speciation. Finally, when superimposed onto the three-dimensional structure of transferrin, the sites identified as responsible of the divergence in Notothenioids appear to cluster preferentially at non-iron binding locations, occupying surface locations that might affect the rate of iron binding and/or release in a cold environment.