Two species of antarctic icefishes (genus Champsocephalus) share a common genetic lesion leading to the loss of myoglobin expression

Abstract

Species of the suborder Notothenioidei dominate the fish fauna of coastal Antarctic waters. Members of one notothenioid family, Channichthyidae (Antarctic icefishes), are unique among all vertebrates in lacking the circulating oxygen-binding protein hemoglobin. Icefish species also do not uniformly express the intracellular oxygen-binding protein myoglobin (Mb) in their oxidative muscles. Our laboratory previously characterized the pattern of cardiac Mb expression in 13 of the 16 known icefish species. In this paper, we complete the survey of cardiac Mb expression among all 16 known species of icefishes. Using PAGE and immunoblot analyses, we demonstrate that both Channichthys rhinoceratus and Cryodraco atkinsoni express Mb in heart ventricle, while Champsocephalus esox does not express the protein. We report Mb gene sequences from Channichthys rhinoceratus and Champsocephalus esox genomic DNA. The Mb gene of C. esox contains the identical 5-bp duplication/insertion to that observed in congeneric Champsocephalus gunnari, a species that also does not produce Mb. This duplication in exon 2 of the Champsocephalus spp. gene causes a shift in reading frame at a position normally encoding for amino acid 91 and also results in a premature stop codon, thus disrupting translation of the normal protein. Thus, 6 of the 16 known icefish species do not express cardiac Mb. These results confirm earlier conclusions that losses of Mb expression have occurred via at least four independent events during the evolution of the icefish family. Extreme similarity of Mb genes in Champsocephalus congeners further suggests recent speciation despite early divergence of this group from the lineage leading to more derived icefishes.