Although virtually all vertebrates make red blood cells containing hemoglobin to transport oxygen to their tissues throughout their life histories, a group of Antarctic fish known as the Channichthyidae (icefish) has lost both hemoglobin and red blood cells in their evolutionary history. The lack of red blood cells give these fish colorless blood, a characteristic unique to them among all vertebrates. New research by a team of scientists including MSC Professor William Detrich sheds light on how this highly unusual condition evolved.

Recently published in the journal Marine Genomics, the study examines microRNA genes known to participate in formation of red blood cells (erythromiRs). Four species of Antarctic fish, including two white-blooded icefish and two red-blooded relatives, were examined to determine whether mutation of one or more of these regulatory genes contributed to disappearance of red blood cells and loss of hemoglobin in icefish.

Results demonstrated that the Blackfin icefish genome possessed, and its red-cell-forming kidney marrow expressed, all of the erythromiR genes. Therefore, loss of expression of known erythromiRs cannot explain the colorless blood of icefish. The Hooknose icefish, in contrast, failed to express one erythromiR gene in its kidney marrow, which the authors proposed occurred after the loss functional globin genes by the most recent common ancestor of the icefish. Conservation of erythromiR genes in icefish genomes in the absence of red blood cell formation indicates that these microRNA genes may have other important cellular functions.

This study provides important insight that will guide researchers as they seek to understand the evolutionary history of the loss of red blood cells and hemoglobin in icefish.