Genetics of Adaptation Graduate Seminar
Author: Sunita Yadav
One normally does not think of llamas and beluga whales in the same sentence; they look very different and one lives in the ocean whereas the other in the Andes Mountains. Nonetheless, they have adapted to low-oxygen environments in much the same fashion. Llamas belong to the camel family and are divided into 4 species, llama, alpaca, guanaco, and vicuña, all in the genusLama. They live at altitudes above 5000 feet where atmospheric pressure is lower and therefore less available oxygen. The Beluga Whale, Delphinapterus leucas, is ghostly white with a circumpolar distribution in frigid waters around the Arctic Sea. It is one of two members in the toothed order Monodontidae, the other species being the Narwhal. Beluga whales have been known to dive to depths of 500 m, also an oxygen deficient (hypoxic) habitat, and for periods that can exceed 15 minutes at a time in a dive. A third type of hypoxic habitat would be underground environments inhabited by mole rats. The work to date reflects that there are multiple mechanisms that underlie various physiological responses to hypoxic environments.
Mammals employ different physiological adaptations to survive in low-oxygen environments such as: high oxygen affinity to hemoglobin (the respiratory pigment), a reduced heart rate, a lower metabolic rate, a higher blood volume percentage, higher myoglobin content (a protein that stores oxygen in muscle tissue and allows muscle tissue to be active for prolonged periods), and altered distribution of blood flow to tissues.
A first question to ask is why there is less oxygen in deep oceans and high mountains. With increasing altitude, atmospheric pressure is lower resulting in what is generally called ‘thin air’. Basically this means there are less oxygen molecules available per unit of air. In contrast, oxygen in oceans is incorporated by surficial mixing; deep ocean water simply does not mix at the surface very often and therefore is hypoxic. When mammals are exposed to such habitats, their normal oxygen transport does not work well (the reason why we hyperventilate when mountain climbing).
So what happens when a beluga whale dives? If it were not adapted to its deep sea lifestyle, at some point during the dive, the amount of carbon dioxide (CO2) in it’s blood would increase leading to lower pH, reduced oxygen binding in blood (due to reduced oxygen binding to hemoglobin) and probable death because of oxygen starvation. Since beluga whales are not extinct, they have found a way to survive during their deep diving. The question is how?
Under normal pressures, most mammals in a resting state have excess oxygen releasing only about 25% oxygen to tissues. Under active conditions when cells need more oxygen, hemoglobin quickly unloads oxygen to the active cells. Typically, an oxygen disassociation curve is used to describe percent saturation of hemoglobin with oxygen at different atmospheric pressures. Since one hemoglobin molecule can attach 4 oxygen molecules, a 50% saturation (referred to as p50) value is commonly used for comparison. Temperature, pH and other organic molecules affect how quickly an individual reaches the p50 value. |  |
Since belugas and llamas have been living in hypoxic environments for thousands of years, their bodies have adapted to efficiently use lower oxygen amounts. One way they accomplish this is to modulate blood chemistry to make oxygen more attracted to the hemoglobin molecule (this would be a left shift on the oxygen dissociation curve depicted above).
The balance of CO2 and oxygen maintains blood pH. At low pH, hemoglobin has lower affinity for oxygen(curve shifts to the right) whereas at higher pH, hemoglobin has a higher affinity for oxygen (curve shifts to the left). An increase in CO2 reduces pH and restricts binding of oxygen to hemoglobin. Reduced pH occurs because CO2 is converted to carbonic acid by a group of enzymes known as carbonic anhydrases (CA). They form carbonic acid from CO2 at different rates. One form of the CA enzymes is CA II which is 10x to 100x faster than CA I or CA III in forming carbonic acid. The hypothesis is that mammals (like the beluga) living in hypoxic environments would show reduced levels of CA II to maintain pH balance in blood thereby maintaining high oxygen attraction for hemoglobin.
By comparing the expression levels (quantities) of CAs in different species, researchers have begun to explain the underlying physiological adaptations that beluga whales and other mammals employ. One research group (H. Yang and colleagues) used blood samples from the dromedary camel, domestic llamas, and beluga whales to separate and quantify CAs. Remember that CA II was the fastest in converting CO2 to carbonic acid; not good if you are a beluga whale or llama. Well, what did they find? CA II was not detected in red cells of beluga whales and reduced levels were found in llamas. Camels though closely related to llamas displayed a typical mammalian ratio with greater CA I levels in relation to CA II. In studies of mole rats (underground lifestyle), CA II was also deficient in red blood cells.
Storz and colleagues used deer mice (Peromyscus maniculatus) that live at different elevations to study adaptation to altitude. Genetic studies comparing mice populations at various elevation zones revealed differences in allelic forms (version of a gene) of α and β globin genes (type of protein) that code for hemoglobin structure. Mice that live at higher elevations show a higher prevalence of the genetic code for the version of the gene that ultimately leads to a destabilized hemoglobin molecule. What does this mean? An unstable hemoglobin molecule shows a higher attraction for oxygen. Mice in lowlands had a different β –globin (more stable hemoglobin) than mice in highlands (unstable hemoglobin, i.e. higher affinity to oxygen). This implies some sort of divergent selection based on habitat locations of deer mice.
More recent work on humans living on the Tibetan plateau by Xan Yi and colleagues shows a strong positive selection on a different pathway. Exomes (coding regions in the DNA) of 50 Tibetan individuals were sequenced. Differences were found at a single location (single nucleotide polymorphism or SNP) on the genome for a gene called EPAS1 between highland Tibetan and lowland Han populations. EPAS1 is a transcription factor gene involved in hemoglobin concentration in red blood cells. Most humans react to low oxygen levels by producing more hemoglobin which can make blood more viscous (and also lead to altitude sickness), Tibetans, however, have relatively low hemoglobin concentrations. Therefore, Tibetans avoid the symptoms of altitude sickness. How they actually get enough oxygen is yet to be decoded. If you have ever wondered why Ethiopian and Kenyan athletes frequently win marathons, you will not be surprised to learn that the runners all come from highland areas in those countries. Their adaptation to high altitude allows them to use oxygen more efficiently.
Researchers are also actively studying different species such as, plateau pika, mountain sheep, and orca whales to study variation in adaptation to hypoxic habitats across species. The earlier studies in llamas and beluga whales have not been repeated to verify DNA sequence level differences in gene expression (amount of a gene that is produced to make a certain number of protein molecules) or regulation (whether a gene is activated or not). Thus far, the research in llamas, belugas, deer mice, and humans suggests that there are multiple pathways for adaptation to low oxygen habitats.
References
Storz, J.F., Runck, A.M., Sabatino, S.J., Kelly, J.K., Ferrand, N., Moriyama, H., Weber, R.E. and A. Fago, 2009, Evolutionary and functional insights into the mechanism underlying high-altitude adaptation of deer mouse hemoglobin, PNAS, Vol. 106(34): 14450-14455.
Yang, H., Hewett-Emmett,D. and Richard E. Tashian, 2000, Absence or Reduction of Carbonic Anhydrase II in the Red Cells of the Beluga Whale and Llama: Implications for Adaptation to Hypoxia, Biochemical Genetics, Vol. 38(7/8):241-252.
Yi, Xan et al., 2010, Science, Vol. 329(5987): 75-78.
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