Tibetan antelopes (Panthelops hodgsonii) live at high altitude on the Tibetan plateau. They have outstanding physical capability at altitudes (up to 5,500m) that, for most mammals, is extreme and very strenuous, if not impossible, to survive.
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One way animals resolve problems relating to oxygen supply is by using different forms of haemoglobin, the oxygen transport protein in red blood cells. In mammals it takes the form of an assembly of four protein molecules called globins, and there are different globins that can be included in that four-component structure. For example, most mammals have foetal and adult specific globins, which differ in how strongly they attract and bind oxygen (i.e. their affinity for oxygen). Haemoglobin in adults partly comprises adult specific globin (there are others as well but, for clarity, this is detail we can ignore without compromising the point we’re describing). But in foetal blood adult globin is wholly or partially replaced with a foetal one, forming haemoglobin molecules that, having higher oxygen affinity, contributes to the difficult and critical process of transferring sufficient oxygen across the placenta, to the foetus.
”Sheep and goats also have another use for this third globin, in a plastic response to hypoxia.”
Sheep and goats have a third form of haemoglobin, one partly composed of a globin that is present in lambs and kids. This juvenile globin also has a comparatively high oxygen affinity and additional blood oxygen may help infant animals at a stage when they remain very delicate. Sheep and goats also have another use for this third globin, in a plastic response to hypoxia. When adults are subjected to low oxygen concentrations they change their haemoglobin constituents, reducing the proportion of adult globin and replacing it with the juvenile one. The physiological benefit of this is obvious. Molecular biological studies have shown that this ability to switch from adult to juvenile globin use when oxygen is decreased, have been inherited from a common ancestor of sheep and goats.
Tibetan antelopes share that ancestor, and they have adapted to extremely high altitude by further refinement of these globins1. First, a deletion of their DNA has resulted in the adult globin being lost; these antelopes do not have the gene for it. Second, that lost globin has been functionally replaced by the formerly juvenile one, which is no longer plastic, but expressed constitutively in adults. A critical aspect of the evolution of these highly specialised animals, then, has been the redeployment and assimilation of a formerly plastic response to altitude possessed by an ancestor (and still present in sheep and goats), resulting in the default use of a high oxygen affinity form of haemoglobin much more suited to the difficult environment these animals now endure.
”A critical aspect of the evolution of these highly specialised animals has been the redeployment of a formerly plastic response to altitude possessed by an ancestor.”
Their being athletic at altitude is central to the ecological strategy of Tibetan antelopes; it is how they exploit their extremely high altitude home. Athletic capability in a hypoxic environment almost certainly requires greater physiological capacity than sheep and goats have, and constant rather than inducible expression and deployment of the juvenile haemoglobin is likely to contribute to that. That the constitutive condition probably evolved from an inducible one in an ancestor of the antelopes is logical because of the plasticity’s clear role in evolution of altitude tolerance as shown by its importance to the sheep and goats (the research paper describes, with evidence, the evolutionary sequence of this animal group). Furthermore the plasticity is likely to have contributed to the assimilation events in which genetic change (in this case a deletion) led to permanence of a position in the former range of juvenile haemoglobin expression levels the plasticity involved. This is because natural selection is essential for such genetic changes to spread and eventually establish new, adapted populations. In this case, the selecting environmental factor would partly have been the high altitudes the ancestors were already encountering enabled by their plasticity. There would therefore have been a mechanistic link between the plasticity in juvenile haemoglobin expression and its own genetic assimilation in the form of permanent expression.
Clearly, there is far more to the identity of Tibetan antelopes than this single evolutionary adaptation. However, it is distinctive and critical to this species; without it, these animals would not have secured the advantage of being able to survive in an environment with few or no resource competitors and one that is difficult for predators. Their evolution has involved many and diverse adaptive changes, but that redeployment of a plastic response to altitude change was a formative event and beautifully illustrates plasticity as an innate resource for adaptation and how this can instigate the formation of new species.
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References
- Signore, Anthony V., and Jay F. Storz. ‘Biochemical Pedomorphosis and Genetic Assimilation in the Hypoxia Adaptation of Tibetan Antelope’. Science Advances 6, no. 25 (June 2020): eabb5447. https://doi.org/10.1126/sciadv.abb5447. [↩]