Concentrations of myoglobin and myoglobin mRNA in heart ventricles from Antarctic fishes.

We used a combined immunochemical and molecular approach to ascertain the presence and concentrations of both the intracellular oxygen-binding hemoprotein myoglobin (Mb) and its messenger RNA (mRNA) in 13 of 15 known species of Antarctic channichthyid icefishes. Mb protein is present in the hearts of eight species of icefishes: Chionodraco rastrospinosus, Chionodraco hamatus, Chionodraco myersi, Chaenodraco wilsoni, Pseudochaenichthys georgianus, Cryodraco antarcticus, Chionobathyscus dewitti and Neopagetopsis ionah. Five icefish species lack detectable Mb protein: Chaenocephalus aceratus, Pagetopsis macropterus, Pagetopsis maculatus, Champsocephalus gunnari and Dacodraco hunteri. Mb concentrations range from 0.44+/-0.02 to 0.71+/-0.08 mg Mb g(-)(1 )wet mass in heart ventricle of species expressing the protein. A Mb-mRNA-specific cDNA probe was used to quantify mRNA in five Mb-expressing icefishes. Mb mRNA was found in low but detectable amounts in Champsocephalus gunnari, one of the species lacking detectable Mb. Mb mRNA concentrations in heart ventricle from Mb-expressing species ranged from 0.78+/-0.02 to 16.22+/-2.17 pg Mb mRNA microg(-)(1 )total RNA). Mb protein and Mb mRNA are absent from the oxidative skeletal muscle of all icefishes. Steady-state concentrations of Mb protein do not parallel steady-state concentrations of Mb mRNA within and among icefishes, indicating that the concentration of Mb protein is not determined by the size of its mRNA pool.

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 hemoglobinless Antarctic icefishes, Channichthyidae. Within this family of fishes, which is endemic to the Southern Ocean surrounding Antarctica, there exist 15 known species and 11 genera. To date, we have examined eight species of icefish (representing seven genera) using immunoblot analyses. Results indicate that Mb is present in heart ventricles from five of these species of icefish. Mb is absent from heart auricle 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 probe from the closely related red-blooded Antarctic nototheniid fish, Notothenia coriiceps. In confirmation that the 0.9-kb mRNA encodes Mb, we report the full-length Mb cDNA sequence of the ocellated icefish, Chionodraco rastrospinosus. Of the eight icefish species examined, three lack Mb polypeptide in heart ventricle, although one of these expresses the Mb mRNA. All species of icefish retain the Mb gene in their genomic DNA. Based on phylogeny of the icefishes, loss of Mb expression has occurred independently at least three times and by at least two distinct molecular mechanisms during speciation of the family.