VARIABLE EXPRESSION OF MYOGLOBIN AMONG THE HEMOGLOBINLESS ANTARCTIC ICEFISHES

The important intracellular oxygen-binding protein, myoglobin (Mb), is thought to be absent from oxidative muscle tissues of the family of h emoglobinless Antarctic icefishes, Channichthyidac. Within this family of fishes, which is endemic to the Southern Ocean surrounding Antarct ica, there exist 15 known species and II genera, To date, we have exam ined eight species of icefish (representing seven genera) using immuno blot analyses. Results indicate that Mb is present in heart ventricles from five of these species of icefish, Mb is absent from heart auricl e and oxidative skeletal muscle of all species, We have identified a 0 .9-kb mRNA in Mb-expressing species that hybridizes with a Mb cDNA pro be from the closely related red-blooded Antarctic nototheniid fish, No tothenia coriiceps. in confirmation that the 0.9-kb mRNA encodes Mb, w e report the full-length Mb cDNA sequence of the ocellated icefish, Ch ionodraco rastrospinosus. Of the eight icefish species examined, three lack Mb polypeptide in heart ventricle, although one of these express es the Mb mRNA, All species of icefish retain the Mb gene in their gen omic DNA, Based on phylogeny of the icefishes, loss of Mb expression h as occurred independently at least three times and by at least two dis tinct molecular mechanisms during speciation of the family.