The Evolutionary Fate of Haptoglobin in Hemoglobinless Antarctic Icefishes
In contrast to the adaptive fitness gain of antifreeze proteins, the seemingly indispensable oxygen transport protein hemoglobin (Hb) experiences evolutionary loss in the icefish family of Antarctic notothenioids. This makes it an excellent model for investigating the evolutionary impact of gene loss on its network of associated protein partners. One of its partner protein is haptoglobin (Hp), which is particularly suited for such a study because it directly binds to free Hb and directs Hb for clearance from circulation. We investigated the evolutionary status and underlying molecular changes of Hp in icefishes via examining its functional integrity at all principal levels of molecular organization (DNA, RNA, and protein) across the icefish phylogeny. Results suggest that the absence of Hb relaxed selective pressure on the maintenance of Hp, resulting in independent loss of Hp in separate icefish lineages via distinct molecular mechanisms. This research indicates how a functional gene gradually become non-functionalized after its selective force was removed, and it also shows a clear co-evolution case of interacting proteins.
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