ANTARCTIC FISH HEMOGLOBINS - EVIDENCE FOR ADAPTIVE EVOLUTION AT SUBZERO TEMPERATURE

Notothenioids represent a large group of marine teleosts that are most ly endemic to the Antarctic Ocean. In this environment, the low metabo lic demand and the high oxygen concentration reduce the need for hemog lobin(s) [Hb(s)]. The extreme condition is represented by the icefish (Channichthyidae, Notothenioidei), the only vertebrates that lack Hb. We obtained the nucleotide sequence coding for the beta-globin chain o f the single major Hb form in six red-blooded notothenioids. These inc luded Gymnodraco acuticeps, one of the closest species to the Hb-less icefish, which is also the only known fish having a single Hb without Bohr effect. This species shows a higher rate of nonsynonymous substit utions (K-A), in contrast with the homogeneity of synonymous substitut ion (K-S) rates, and K-A/K-S ratios significantly greater than one in the majority of comparisons. These results are suggestive of positive selection, diversifying the single major Hb toward specialized functio ns. A single Hb that is free to diversify means that its role in routi ne oxygen transport can be reduced in the presence of a combination of physiological, ecological, and environmental factors. Although a redu ced ''routine'' function for Hb, as is apparent in G. acuticeps, might , indeed, evoke the lack of Hb in icefish, evidence of diversifying se lection reported here is at variance with the hypothesis of a simple t rend from a single Hb toward the Hb-less condition.