Metabolic Adaptation To High Altitude Of Two Phrynocephaius Lizards On Tibetan Plateau

The Tibetan plateau is the highest plateau in the world, averaging more than4000m above sea level in altitude. High altitude is a major challenge to life, but native animals can survive vigorously in the cold and hypoxic conditions associated with these environments. Compared with a mass of studies on Tibetan plateau mammals and birds, limit information was gathered on the adaptation characteristics of reptilian species at high altitude. Some lizards in the genus of Phrynocephalus are widely distribute on the Tibetan plateau and are thought to be the exceptional choice for investigations of reptile metabolic adaptation to high altitude. Phrynocephalus erythrurus is dwelling at altitudes higher than any other living lizards in the world and Phrynocephalus vlangalii distributes with a widely range of altitudes on the Tibetan plateau. Phrynocephalus przewalskii, which inhabits desert and semi-desert areas in north China (altitude from1000to1500m), was selected as a reference species for the present study. Here, we conducted our research to three different aspects (intraspecific, interspecific and different oxygen concentration acclimatization), and these three Phrynocephalus lizards were used to analyse some metabolism-related characteristics, including morphological traits, body temperature selection, mitochondrial respiratory rates, some metabolic enzymes and metabolism-related genes. These studies could accumulate some useful information on lizards’metabolic characteristics and provide new insights into the adaptation mechanisms of reptiles at high altitude.In the first part of my dissertation, the metabolic characteristics of P. erythrurus, which inhabits at high altitudes (4500m) and P. przewalskii, which inhabits low altitudes, were analysed to explore the adaptation strategies of lizards to high-altitude environments. The results indicated that the mitochondrial respiratory rates of P. erythrurus were significantly lower than those of P. przewalskii, and that proton leak accounts for74~79%of state4and7～8%of state3in P. erythrurus vs.43～48%of state4and24～26%of state3in P. przewalskii. Lactate dehydrogenase (LDH) activity in P. erythrurus was lower than in P. przewalskii, indicating that at high altitude the former does not, relatively, have a greater reliance on anaerobic metabolism. A higher activity related to β-hydroxyacyl coenzyme A dehydrogenase (HOAD) and the HOAD/citrate synthase (CS) ratio suggested there was a possible higher utilization of fat in P. erythrurus. The lower expression of PGC-la, PPAR-y and UCP2in P. erythrurus suggested their expression was not influenced by cold and low PO2at high altitude. Analysis of mitochondrial H2O2production indicated that it was largely related to the mitochondrial respiratory rate, rather than the effect of UCP2. These distinct characteristics of P. erythrurus are considered to be necessary strategies in metabolic adaptation to high altitude and may effectively compensate for the negative influence of cold and low PO2.In the second part of my dissertation, a high altitude population (from Maduo, Qinghai Province, with an altitude of4300m) and a low altitude population (from Golmud, Qinghai Province, with an altitude of2800m) of P. vlangalii were used to analyze the possible intraspecific differences on the morphological traits, body temperature selection, resting metabolic rate, mitochondrial respiratory rate, proton leak and some metabolic enzymes. The snout-vent length, arm length and leg length of Maduo population (MD) was not significantly shorter than those of Golmud population (GM), these results suggested the morphological traits were conversed to the Bergmann’s rule for this lizard species. The body temperature of MD was significantly lower than that of GM, and the heating rate of MD may faster than that of GM. The mitochondrial respiratory rates of MD were significantly lower than those of GM, this result was consistent with our previous study on the mitochondrial respiratory rates of P. erythrurus and P. przewalskii. While, there were some differences between P. erythrurus and P. vlangalii of MD on the proton leak regulation; the proton leak of liver mitochondria in MD account for21.4～27.2%of State4, and this proportion of P.erythrurus reached to74～79%. We speculate that the MD did not elevate their body temperature by proton leak thermogenesis, but by means of reducing mitochondrial respiratory rate to improve the efficiency of mitochondrial respiration and ATP production, thereby the energy metabolism could maintain the basic ATP needs of organism for a longer time. In addition, LDH activity and LDH/CS ratio of MD liver was significantly lower than those of GM, which suggested that the liver metabolism at high altitude may less depend on anaerobic metabolism. Moreover, the comparisons of CS and HOAD activity between the two populations indicated nutrients utilization ability may varied with organs, especially, the fat metabolism may make a major contribution to skeletal muscle metabolism.Many studies about reptile adaptations at high altitude were mainly focused on effect of low temperature, while relative less information about adaptation to hypoxia were gathered in these reptile species. In order to examine the impact of oxygen concentration on metabolic adaptation, the P. vlangalii were divide into two groups to acclimated a high (21%, HO group) or a low (10%, LO group) oxygen concentration by artificial simulation in our lab for45days, respectively. The results showed that P. vlangalii in the LO group was tended to select lower body temperatures and sustained lower resting metabolic rates, this was consistent with our previous result in the comparison between MD and GM of P. vlangalii. Compared to HO group, the LDH activities in skeletal muscle was significantly lower in LO group, but there was no difference in liver. This result was not consistent with our previous studies on liver LDH activity in P. erythrurus and P. vlangalii. We conjecture that the lower liver activity in these lizards may be as the result of living in high altitude, which have a synergy effect of both hypoxia and low temperature. In addition, the HOAD activities in liver and skeletal muscle did not differ between LO and HO group, which was also not consistent with P. erythrurus and P. vlangalii. These results indicated that activation of fat metabolism pathway in lizards at high altitude maybe greatly affected by low ambient temperature, while depend on glucose metabolism during hypoxia with warm temperature may be more helpful to raise the utilization rate of oxygen. The production rate of H2O2has no difference between the two groups, which differed with our prediction that the regulation of free radical mainly rely on the mitochondrial respiratory rate previously, the detail mechanism needs further study.In conclusion, we found that the P. erythrurus and P. vlangalii have some special characteristics coping with the hypoxia and low temperature at high altitude, these results fill the gap of the high altitude adaptation in genus of Phrynocephalus. Furthermore, we conclude some possible mechanisms of reptile adaptation facing to hypoxia and low temperature, which helps to further enrich the theoretical system of animals adaptation at high altitude. These work were useful to better protect the biodiversity of the plateau, and may also provide innovative references for the research and development of plateau medicine. However, the research on the reptile adaptation to plateau is in its infancy, our research on the genus of Phrynocephalus adaptation to high altitude still exist some uncertain questions, we need to carry on more intensive research of the related field in the future.