CONSERVATION OF THE MYOGLOBIN GENE AMONG ANTARCTIC NOTOTHENIOID FISHES

We determined the myoglobin cDNA sequence for seven Antarctic notothen ioid fish species. These data identify mutations in the myoglobin gene for Champsocephalus gunnari and Pagetopsis macropterus, two icefish s pecies that lack detectable quantities of the polypeptide but express myoglobin mRNA. A third species lacking myoglobin polypeptide, Chaenoc ephalus aceratus, is devoid of myoglobin mRNA and accordingly failed t o produce myoglobin products on polymerase chain reaction (PCR) amplif ication. Myoglobin cDNA sequences were highly conserved among the spec ies that express the protein, particularly in the coding region. Seque nce variation among the myoglobin-expressing channichthyid species was 2.0% to 2.9% in the coding region and 2.6% to 3.3% over the entire cD NA. The same extent of variation, 1.6% to 3.2% in the coding sequence and 2.8% to 3.7% overall, was observed between the icefishes and more distantly related, red-blooded nototheniid species. The two species ex pressing mutant myoglobin mRNA, C. gunnari and P. macropterus, exhibit ed the highest degree of sequence variation among the fish myoglobins examined. Drift of the myoglobin sequence in these two species, and co nservation of myoglobin cDNA among fishes from two distinct families, suggest that a selective pressure operates to maintain myoglobin in th e species that express the protein.