Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Dissecting genomes of multiple yak populations: unveiling ancestry and high-altitude adaptation through whole-genome resequencing analysis

The present study was undertaken to elucidate the population structure and differentiation of Indian yak from Chinese and wild cohorts on genome-wide scale by identifying the selection sweeps and genomic basis of their adaptation across different comparisons while analyzing whole genome sequencing (WGS) data using latest bioinformatics tools. The study included 105 individuals from three distinct yak populations i.e., Indian yak (n = 29); Chinese yak (n = 61) and wild yak (n = 15), hypothesized to be related along the evolutionary timescale. Efficient variant calling and quality control in GATK and PLINK programs resulted in around 1 million (1,002,970) high-quality (LD-independent) SNPs with an average genotyping rate of 96.55%. The PCA, ADMIXTURE and TREEMIX analysis revealed stratification of the yak groups into three distinct clusters. The empirical distribution pattern of minor allele frequency (MAF) of SNPs on genome-wide scale was also elucidated for three yak cohorts revealing unique distribution across five different bins. The selection signature analysis revealed candidate genes that are important for the adaptation of Indian yak against harsh environmental conditions in their habitats. Under iHS analysis, several genes were identified to be under selection pressure in Indian yak including ABCA12, EXOC1, JUNB, KLF1, PRDX2, NANOS3, RFX1, RFX2, and CACNG7. On the other hand, across population analysis revealed the genes like NR2F2, OSBPL10, CIDEC, WFIKKN2, ADCY, THSD7A, ADGRB3, TRPC1, VASH2, and ABHD5 to be part of selective sweeps under these comparisons. A total of 53 genes were found common between intra- and inter-population selection signature analysis of Indian yak. Notably, the genes harbouring the SNPs under selection pressure were significant for adaptation traits including lipidogenesis, energy metabolism, thermogenesis, hair follicle formation, oxidation-reduction reactions, hypoxia and reproduction. These genes may be evaluated as candidate genes for livestock adaptation to harsh environmental conditions and to further the research and application in the present era of climate change.

The catecholamine response to graded high-altitude flight in yellow-rumped warblers (Setophaga coronata)

Chronic exposure to low oxygen (hypoxia) leads to amplification of the hypoxic chemoreflex, increasing breathing and sympathetic nervous system (SNS) activation. Prolonged SNS activation redistributes blood to hypoxia-sensitive tissues away from muscles. Recent tracking studies have shown that migratory songbirds can fly 5,000 m or higher above sea level, leading us to hypothesize that migratory birds may have a blunted hypoxic chemoreflex to maintain blood flow to muscles during migratory flight at high altitudes. To test this hypothesis, we used a hypobaric wind tunnel and measured circulating plasma catecholamines after maximal altitude flight, flight at 75% of maximal altitude, flight at ground level (∼250 m), and after rest at 75% of maximal altitude and ground level in migratory myrtle yellow-rumped warblers (Setophaga coronata). Yellow-rumped warblers were capable of flying above 4,000 m simulated altitude above sea level (average maximum altitude of ∼3,600 m) and would maintain flights at 75% of individual maximum altitudes (∼2,700 m). Circulating dopamine and noradrenaline were similar between resting and flight conditions at ground level and with exposure to 75% of maximal altitude, whereas adrenaline significantly increased with flight, but did not change further with flight at 75% of maximal altitude. In contrast, both adrenaline and noradrenaline concentrations increased after maximum altitude flights compared with 75% and ground-level flights. Our findings show that exercise increases plasma adrenaline in migratory songbirds and suggest that warblers flying at high altitudes below their maximum altitude may be minimally hypoxic, allowing them to maintain oxygen transport to flight muscles.NEW & NOTEWORTHY Yellow-rumped warblers, a small songbird that conducts migratory flights, were found to fly to altitudes above 4,000 m above sea level in simulated flights using a hypobaric wind tunnel. Circulating adrenaline suggests that warblers flying at 75% of individual maximum altitudes are not experiencing arterial hypoxia, allowing them to maintain aerobically demanding migratory flight at high altitudes.

Gas exchange, oxygen transport and metabolism in high-altitude waterfowl

High-altitude life poses physiological challenges to all animals due to decreased environmental oxygen (O2) availability (hypoxia) and cold. Supporting high metabolic rates and body temperatures with limited O2 is challenging. Many birds, however, thrive at high altitudes. The O2-transport cascade describes the pathway involved in moving O2 from the environment to the tissues encompassing: (i) ventilation, (ii) pulmonary O2 diffusion, (iii) circulation, (iv) tissue O2 diffusion, and (v) mitochondrial O2 use for ATP production. Shared avian traits such as rigid lungs with cross-current gas exchange and unidirectional airflow aid in O2 acquisition and transport in all birds. Many high-altitude birds, however, have evolved enhancements to some or all steps in the cascade. In this review, we summarize the current literature on gas exchange and O2 transport in high-altitude birds, providing an overview of the O2-transport cascade that principally draws on the literature from high-altitude waterfowl, the most well-studied group of high-altitude birds. We close by discussing two important avenues for future research: distinguishing between the influences of plasticity and evolution and investigating whether the morphological and physiological differences discussed contribute to enhanced locomotor or thermogenic performance, a potential critical link to fitness.This article is part of the theme issue 'The biology of the avian respiratory system'.

Born high, born fast: Does highland birth confer a pulmonary advantage for sea level endurance?

Less than 7% of the world's population live at an altitude above 1500 m. Yet, as many as 67% of medalists in the 2020 men's and women's Olympic marathon, and 100% of medalists in the 2020 men's and women's Olympic 5000 m track race may have been born or raised above this otherwise rare threshold. As a possible explanation, research spanning nearly a quarter of a century demonstrates that indigenous highlanders exhibit pulmonary adaptations distinct from their lowland counterparts. These adaptations may then promote endurance performance. Indeed, healthy indigenous highlanders often exhibit a larger aerobic exercise capacity compared to sea-level residents who travel to high altitude. However, questions remain on whether high-altitude birth is advantageous for sea-level competitions. In this review, we ask whether being born at a high altitude generates an ergogenic advantage for endurance performance in the Summer Olympics-a venue that is generally held at sea level. In so doing, we distinguish between three groups of high-altitude residents: (i) the indigenous highlander, (ii) the highland newcomer, and (iii) the highland sojourner. Concentrating specifically on altitude-induced alterations to pulmonary physiology beginning in the perinatal period, we propose that if altitude-related maladaptations are avoided, genomic and developmental alterations accompanying highland birth may present benefits for endurance competitions at sea level.

Recovery of ventilatory and metabolic responses to hypoxia in neonatal rats after chronic hypoxia

Chronic hypoxia (CH) during postnatal development attenuates the hypoxic ventilatory response (HVR) in mammals, but there are conflicting reports on whether this plasticity is permanent or reversible. This study tested the hypothesis that CH-induced respiratory plasticity is reversible in neonatal rats and investigated whether the initial plasticity or recovery differs between sexes. Rat pups were exposed to 3 d of normobaric CH (12 % O2) beginning shortly after birth. Ventilation and metabolic CO2 production were then measured in normoxia and during an acute hypoxic challenge (12 % O2) immediately following CH and after 1, 4-5, and 7 d in room air. CH pups hyperventilated when returned to normoxia immediately following CH, but normoxic ventilation was similar to age-matched control rats within 7 d after return to room air. The early phase of the HVR (minute 1) was only blunted immediately following the CH exposure, while the late phase of the HVR (minute 15) remained blunted after 1 and 4-5 d in room air; recovery appeared complete by 7 d. However, when normalized to CO2 production, the late phase of the hypoxic response recovered within only 1 d. The initial blunting of the HVR and subsequent recovery were similar in female and male rats. Carotid body responses to hypoxia (in vitro) were also normal in CH pups after approximately one week in room air. Collectively, these data indicate that ventilatory and metabolic responses to hypoxia recover rapidly in both female and male neonatal rats once normoxia is restored following CH.

High altitude polycythemia and its maladaptive mechanisms: an updated review

High altitude polycythemia is a maladaptation of highlanders exposed to hypoxic environment, leading to high blood viscosity and severe cardiorespiratory dysfunction. Prolonged hypoxia causes respiratory depression and severe hypoxemia, and further mediates changes in genetic and molecular mechanisms that regulate erythropoiesis and apoptosis, ultimately resulting in excessive erythrocytosis (EE). This updated review investigated the maladaptive mechanisms of EE, including respiratory chemoreceptor passivation, sleep-related breathing disorders, sex hormones, iron metabolism, and hypoxia-related factors and pathways.

Potential role of the gut microbiota of bumblebee Bombus pyrosoma in adaptation to high-altitude habitats

The gut microbiota affects the health and overall fitness of bumblebees. It can enhance the host's ecological range by leveraging their metabolic capacities. However, the diversity of the gut microbiota and adaptive functional evolution in high-altitude regions remain unclear. To explore how the gut microbiota helps the host adapt to high-altitude environments, we analyzed the differences in diversity and function of the gut microbiota between high- and low-altitude regions through full-length 16S rRNA sequencing. Our results show that high-altitude regions have a lower abundance of Fructobacillus and Saccharibacter compared to low-altitude regions. Additionally, some individuals in low-altitude regions were invaded by opportunistic pathogens. The gut microbiota in high-altitude regions has a greater number of pathways involved in "Protein digestion and absorption" and "Biosynthesis of amino acids," while fewer carbohydrate pathways are involved in "digestion and absorption" and "Salmonella infection." Our finding suggests that plateau hosts typically reduce energy metabolism and enhance immunity in response to adverse environments. Correspondingly, the gut microbiota also makes changes, such as reducing carbohydrate degradation and increasing protein utilization in response to the host. Additionally, the gut microbiota regulates their abundance and function to help the host adapt to adverse high-altitude environments.

Multiscale Impact of Environmental and Socio-Economic Factors on Low Physical Fitness among Chinese Adolescents and Regionalized Coping Strategies

As low physical fitness in adolescents increases their risk of all-cause mortality in future adulthood as well as regional public health budgets, many scholars have studied the factors influencing physical fitness in adolescents. However, the spatial non-stationarity and scale between physical fitness and influencing factors in adolescents are often neglected. To rectify this situation, this study constructed a multi-scale geographically weighted regression model based on data from the China National Student Fitness Survey and the China Statistical Yearbook in 2018 to investigate the spatial patterns of factors influencing low physical fitness among adolescents. The results showed that the influencing factors for measuring the physical fitness of Chinese adolescents had significant spatial heterogeneity and multi-scale effects. The local R² values were relatively low in the western region of China. Consideration should be given to increasing the lifestyle and ethnic and cultural characteristics of local residents when selecting influencing factors in the future. The physical fitness of men was mainly influenced by socio-economic factors, while that of women was influenced by natural environmental factors. According to the different spatial distribution patterns of MGWR, this study suggests that each region should develop regionalized strategies to cope with the low physical fitness of adolescents, including taking advantage of the natural environment to develop physical fitness promotion projects, accelerating the upgrading of industrial structures in the north-eastern and western regions, and the need to remain cautious of rapid urbanization in the east.

Genomic Diversity Profiling and Breed-Specific Evolutionary Signatures of Selection in Arunachali Yak

Arunachali yak, the only registered yak breed of India, is crucial for the economic sustainability of pastoralist Monpa community. This study intended to determine the genomic diversity and to identify signatures of selection in the breed. Previously available double digest restriction-site associated DNA (ddRAD) sequencing data of Arunachali yak animals was processed and 99,919 SNPs were considered for further analysis. The genomic diversity profiled based on nucleotide diversity, π (π = 0.041 in 200 bp windows), effective population size, Ne (Ne = 83) and Runs of homozygosity (ROH) (predominance of shorter length ROHs) was found to be optimum. Subsequently, 207 regions were identified to be under selective sweeps through de-correlated composite of multiple signals (DCMS) statistic which combined three individual test statistics viz. π, Tajima's D and |iHS| in non-overlapping 100 kb windows. Mapping of these regions revealed 611 protein-coding genes including KIT, KITLG, CDH12, FGG, FGA, FGB, PDGFRA, PEAR1, STXBP3, olfactory receptor genes (OR5K3, OR5H6 and OR1E1) and taste receptor genes (TAS2R1, TAS2R3 and TAS2R4). Functional annotation highlighted that biological processes like platelet aggregation and sensory perception were the most overrepresented and the associated regions could be considered as breed-specific signatures of selection in Arunachali yak. These findings point towards evolutionary role of natural selection in environmental adaptation of Arunachali yak population and provide useful insights for pursuing genome-wide association studies in future.

Commentary: Hierarchical reductionism approach to understanding adaptive variation in animal performance

Aerobic capacity is a complex performance trait with important consequences for fitness, and is determined by the integrated function of the O₂ transport pathway. The components of the O₂ pathway interact and function as an integrated physiological system, which could strongly influence the contribution of each component to variation in aerobic capacity. In this commentary, we highlight the value of hierarchical reductionism - combining studies of how component parts work in isolation with studies of how components interact within integrated systems - for understanding the evolution of aerobic capacity. This is achieved by focussing on the role of haemoglobin in adaptive increases in aerobic capacity in high-altitude deer mice (Peromyscus maniculatus). High-altitude deer mice have evolved increased aerobic capacity in hypoxia, in association with evolved changes in several subordinate traits across the O₂ pathway. This includes an evolved increase in Hb-O₂ affinity - which helps safeguard arterial O₂ saturation in hypoxia - and reductionist approaches have been successful at identifying the genetic, structural, and biochemical underpinnings of variation in this trait. However, theoretical modelling and empirical measurements suggest that increased Hb-O₂ affinity may not augment aerobic capacity on its own. The adaptive benefit of increased Hb-O₂ affinity in high-altitude deer mice appears to have been contingent upon antecedent changes in other traits in the O₂ pathway, particularly an increased capacity for O₂ diffusion and utilization in active tissues. These findings highlight the importance of understanding the interactions between the components of integrated systems for fully appreciating the evolution of complex performance phenotypes.

Heterogeneity in Hematological Parameters of High and Low Altitude Tibetan Populations

Introduction

High altitude hypoxia is believed to be experienced at elevations of more than 2500 meters above sea level. Several studies have shed light on the biochemical aspects of high altitude acclimatization, where participants were sojourners to the high altitude from low altitude areas. However, information regarding the difference between the high altitude adapted Tibetans living at high altitude and their counterparts who reside at low altitude are lacking. To understand this, we have measured various hematological parameters in the Tibetan populations, who are residing in both high and low altitudes in India.

Methods

A total of 168 individuals (79 from high altitude (≥4500 meters) and 89 from low altitude (~850 meters) were recruited for this study. Hematological parameters such as red blood cells (RBC) count, hematocrit (HCT), hemoglobin concentration (Hb), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) were measured from the individuals from high and low altitudes. Serum erythropoietin (EPO) was measured by ELISA. Statistical analyses were performed to compare data from both of the altitudes. Gender-wise comparison of data was reported. Correlation analysis was performed within relevant parameters.

Results

Highly significant differences (p <0.0001) between high and low altitude Tibetans were detected in RBC count, HCT, Hb, MCHC in both males and females and in MCV in females. In the case of MCHC, however, age and BMI were potential confounders. Nominally significant differences (p <0.05) were detected in MCV and MCH within males. No significant difference in serum EPO level was found between altitude groups, in any gender. No significant correlation was found between serum EPO with Hb as well as serum EPO with HCT.

Discussion

Our study explores significantly lower RBC count, HCT, Hb, MCH, MCHC and higher MCV in long-term Tibetan residents living at low altitude compared to their high altitude counterparts, which is likely due to the outcome of hematological adaptation to a relatively hyperoxic environment in low altitude areas.

High-Altitude Adaptation: Mechanistic Insights from Integrated Genomics and Physiology

Population genomic analyses of high-altitude humans and other vertebrates have identified numerous candidate genes for hypoxia adaptation, and the physiological pathways implicated by such analyses suggest testable hypotheses about underlying mechanisms. Studies of highland natives that integrate genomic data with experimental measures of physiological performance capacities and subordinate traits are revealing associations between genotypes (e.g., hypoxia-inducible factor gene variants) and hypoxia-responsive phenotypes. The subsequent search for causal mechanisms is complicated by the fact that observed genotypic associations with hypoxia-induced phenotypes may reflect second-order consequences of selection-mediated changes in other (unmeasured) traits that are coupled with the focal trait via feedback regulation. Manipulative experiments to decipher circuits of feedback control and patterns of phenotypic integration can help identify causal relationships that underlie observed genotype–phenotype associations. Such experiments are critical for correct inferences about phenotypic targets of selection and mechanisms of adaptation.

Tibetan PHD2, an allele with loss-of-function properties

Tibetans have adapted to the chronic hypoxia of high altitude and display a distinctive suite of physiologic adaptations, including augmented hypoxic ventilatory response and resistance to pulmonary hypertension. Genome-wide studies have consistently identified compelling genetic signatures of natural selection in two genes of the Hypoxia Inducible Factor pathway, PHD2 and HIF2A The product of the former induces the degradation of the product of the latter. Key issues regarding Tibetan PHD2 are whether it is a gain-of-function or loss-of-function allele, and how it might contribute to high-altitude adaptation. Tibetan PHD2 possesses two amino acid changes, D4E and C127S. We previously showed that in vitro, Tibetan PHD2 is defective in its interaction with p23, a cochaperone of the HSP90 pathway, and we proposed that Tibetan PHD2 is a loss-of-function allele. Here, we report that additional PHD2 mutations at or near Asp-4 or Cys-127 impair interaction with p23 in vitro. We find that mice with the Tibetan Phd2 allele display augmented hypoxic ventilatory response, supporting this loss-of-function proposal. This is phenocopied by mice with a mutation in p23 that abrogates the PHD2:p23 interaction. Hif2a haploinsufficiency, but not the Tibetan Phd2 allele, ameliorates hypoxia-induced increases in right ventricular systolic pressure. The Tibetan Phd2 allele is not associated with hemoglobin levels in mice. We propose that Tibetans possess genetic alterations that both activate and inhibit selective outputs of the HIF pathway to facilitate successful adaptation to the chronic hypoxia of high altitude.

Selection signatures for high-altitude adaptation in ruminants

High-altitude areas are important socio-economical habitats with ruminants serving as a major source of food and commodities for humans. Living at high altitude, however, is extremely challenging, predominantly due to the exposure to hypoxic conditions, but also because of cold temperatures and limited feed for livestock. To survive in high-altitude environments over the long term, ruminants have evolved adaptation strategies, e.g. physiological and morphological modifications, which allow them to cope with these harsh conditions. Identification of such selection signatures in ruminants may contribute to more informed breeding decisions, and thus improved productivity. Moreover, studying the genetic background of altitude adaptation in ruminants provides insights into a common molecular basis across species and thus a better understanding of the physiological basis of this adaptation. In this paper, we review the major effects of high altitude on the mammalian body and highlight some of the most important short-term (coping) and genetically evolved (adaptation) physiological modifications. We then discuss the genetic architecture of altitude adaptation and target genes that show evidence of being under selection based on recent studies in various species, with a focus on ruminants. The yak is presented as an interesting native species that has adapted to the high-altitude regions of Tibet. Finally, we conclude with implications and challenges of selection signature studies on altitude adaptation in general. We found that the number of studies on genetic mechanisms that enable altitude adaptation in ruminants is growing, with a strong focus on identifying selection signatures, and hypothesise that the investigation of genetic data from multiple species and regions will contribute greatly to the understanding of the genetic basis of altitude adaptation.

Life Ascending: Mechanism and Process in Physiological Adaptation to High-Altitude Hypoxia

To cope with the reduced availability of O₂ at high altitude, air-breathing vertebrates have evolved myriad adjustments in the cardiorespiratory system to match tissue O₂ delivery with metabolic O₂ demand. We explain how changes at interacting steps of the O₂ transport pathway contribute to plastic and evolved changes in whole-animal aerobic performance under hypoxia. In vertebrates native to high altitude, enhancements of aerobic performance under hypoxia are attributable to a combination of environmentally induced and evolved changes in multiple steps of the pathway. Additionally, evidence suggests that many high-altitude natives have evolved mechanisms for attenuating maladaptive acclimatization responses to hypoxia, resulting in counter-gradient patterns of altitudinal variation for key physiological phenotypes. For traits that exhibit counteracting environmental and genetic effects, evolved changes in phenotype may be cryptic under field conditions and can only be revealed by rearing representatives of high-and low-altitude populations under standardized environmental conditions to control for plasticity.

Control of breathing and respiratory gas exchange in high-altitude ducks native to the Andes

We examined the control of breathing and respiratory gas exchange in six species of high-altitude duck that independently colonized the high Andes. We compared ducks from high-altitude populations in Peru (Lake Titicaca at ∼3800 m above sea level; Chancay River at ∼3000-4100 m) with closely related populations or species from low altitude. Hypoxic ventilatory responses were measured shortly after capture at the native altitude. In general, ducks responded to acute hypoxia with robust increases in total ventilation and pulmonary O₂ extraction. O₂ consumption rates were maintained or increased slightly in acute hypoxia, despite ∼1-2°C reductions in body temperature in most species. Two high-altitude taxa - yellow-billed pintail and torrent duck - exhibited higher total ventilation than their low-altitude counterparts, and yellow-billed pintail exhibited greater increases in pulmonary O₂ extraction in severe hypoxia. In contrast, three other high-altitude taxa - Andean ruddy duck, Andean cinnamon teal and speckled teal - had similar or slightly reduced total ventilation and pulmonary O₂ extraction compared with low-altitude relatives. Arterial O₂ saturation (S _aO2 ) was elevated in yellow-billed pintails at moderate levels of hypoxia, but there were no differences in S _aO2  in other high-altitude taxa compared with their close relatives. This finding suggests that improvements in S _aO2  in hypoxia can require increases in both breathing and haemoglobin-O₂ affinity, because the yellow-billed pintail was the only high-altitude duck with concurrent increases in both traits compared with its low-altitude relative. Overall, our results suggest that distinct physiological strategies for coping with hypoxia can exist across different high-altitude lineages, even among those inhabiting very similar high-altitude habitats.

Human Genetic Adaptation to High Altitude: Evidence from the Andes

Whether Andean populations are genetically adapted to high altitudes has long been of interest. Initial studies focused on physiological changes in the O₂ transport system that occur with acclimatization in newcomers and their comparison with those of long-resident Andeans. These as well as more recent studies indicate that Andeans have somewhat larger lung volumes, narrower alveolar to arterial O₂ gradients, slightly less hypoxic pulmonary vasoconstrictor response, greater uterine artery blood flow during pregnancy, and increased cardiac O₂ utilization, which overall suggests greater efficiency of O₂ transfer and utilization. More recent single nucleotide polymorphism and whole-genome sequencing studies indicate that multiple gene regions have undergone recent positive selection in Andeans. These include genes involved in the regulation of vascular control, metabolic hemostasis, and erythropoiesis. However, fundamental questions remain regarding the functional links between these adaptive genomic signals and the unique physiological attributes of highland Andeans. Well-designed physiological and genome association studies are needed to address such questions. It will be especially important to incorporate the role of epigenetic processes (i.e.; non-sequence-based features of the genome) that are vital for transcriptional responses to hypoxia and are potentially heritable across generations. In short, further exploration of the interaction among genetic, epigenetic, and environmental factors in shaping patterns of adaptation to high altitude promises to improve the understanding of the mechanisms underlying human adaptive potential and clarify its implications for human health.

Control of breathing and ventilatory acclimatization to hypoxia in deer mice native to high altitudes

AIM

We compared the control of breathing and heart rate by hypoxia between high- and low-altitude populations of Peromyscus mice, to help elucidate the physiological specializations that help high-altitude natives cope with O₂ limitation.

METHODS

Deer mice (Peromyscus maniculatus) native to high altitude and congeneric mice native to low altitude (Peromyscus leucopus) were bred in captivity at sea level. The F1 progeny of each population were raised to adulthood and then acclimated to normoxia or hypobaric hypoxia (12 kPa, simulating hypoxia at ~4300 m) for 5 months. Responses to acute hypoxia were then measured during stepwise reductions in inspired O₂ fraction.

RESULTS

Lowlanders exhibited ventilatory acclimatization to hypoxia (VAH), in which hypoxia acclimation enhanced the hypoxic ventilatory response, made breathing pattern more effective (higher tidal volumes and lower breathing frequencies at a given total ventilation), increased arterial O₂ saturation and heart rate during acute hypoxia, augmented respiratory water loss and led to significant growth of the carotid body. In contrast, highlanders did not exhibit VAH - exhibiting a fixed increase in breathing that was similar to hypoxia-acclimated lowlanders - and they maintained even higher arterial O₂ saturations in hypoxia. However, the carotid bodies of highlanders were not enlarged by hypoxia acclimation and were similar in size to those of normoxic lowlanders. Highlanders also maintained consistently higher heart rates than lowlanders during acute hypoxia.

CONCLUSIONS

Our results suggest that highland deer mice have evolved high rates of alveolar ventilation and respiratory O₂ uptake without the significant enlargement of the carotid bodies that is typical of VAH in lowlanders, possibly to adjust the hypoxic chemoreflex for life in high-altitude hypoxia.

Measuring high-altitude adaptation

High altitudes (>8,000 ft or 2,500 m) provide an experiment of nature for measuring adaptation and the physiological processes involved. Studies conducted over the past ~25 years in Andeans, Tibetans, and, less often, Ethiopians show varied but distinct O₂ transport traits from those of acclimatized newcomers, providing indirect evidence for genetic adaptation to high altitude. Short-term (acclimatization, developmental) and long-term (genetic) responses to high altitude exhibit a temporal gradient such that, although all influence O₂ content, the latter also improve O₂ delivery and metabolism. Much has been learned concerning the underlying physiological processes, but additional studies are needed on the regulation of blood flow and O₂ utilization. Direct evidence of genetic adaptation comes from single-nucleotide polymorphism (SNP)-based genome scans and whole genome sequencing studies that have identified gene regions acted upon by natural selection. Efforts have begun to understand the connections between the two with Andean studies on the genetic factors raising uterine blood flow, fetal growth, and susceptibility to Chronic Mountain Sickness and Tibetan studies on genes serving to lower hemoglobin and pulmonary arterial pressure. Critical for future studies will be the selection of phenotypes with demonstrable effects on reproductive success, the calculation of actual fitness costs, and greater inclusion of women among the subjects being studied. The well-characterized nature of the O₂ transport system, the presence of multiple long-resident populations, and relevance for understanding hypoxic disorders in all persons underscore the importance of understanding how evolutionary adaptation to high altitude has occurred.NEW & NOTEWORTHY Variation in O₂ transport characteristics among Andean, Tibetan, and, when available, Ethiopian high-altitude residents supports the existence of genetic adaptations that improve the distribution of blood flow to vital organs and the efficiency of O₂ utilization. Genome scans and whole genome sequencing studies implicate a broad range of gene regions. Future studies are needed using phenotypes of clear relevance for reproductive success for determining the mechanisms by which naturally selected genes are acting.

Altitude matters: differences in cardiovascular and respiratory responses to hypoxia in bar-headed geese reared at high and low altitudes

Bar-headed geese (Anser indicus) fly at high altitudes during their migration across the Himalayas and Tibetan plateau. However, we know relatively little about whether rearing at high altitude (i.e. phenotypic plasticity) facilitates this impressive feat because most of what is known about their physiology comes from studies performed at sea level. To provide this information, a comprehensive analysis of metabolic, cardiovascular and ventilatory responses to progressive decreases in the equivalent fractional composition of inspired oxygen (FiO2: 0.21, 0.12, 0.09, 0.07 and 0.05) was made on bar-headed geese reared at either high altitude (3200 m) or low altitude (0 m) and on barnacle geese (Branta leucopsis), a low-altitude migrating species, reared at low altitude (0 m). Bar-headed geese reared at high altitude exhibited lower metabolic rates and a modestly increased hypoxic ventilatory response compared with low-altitude-reared bar-headed geese. Although the in vivo oxygen equilibrium curves and blood-oxygen carrying capacity did not differ between the two bar-headed goose study groups, the blood-oxygen carrying capacity was higher than that of barnacle geese. Resting cardiac output also did not differ between groups and increased at least twofold during progressive hypoxia, initially as a result of increases in stroke volume. However, cardiac output increased at a higher FiO2 threshold in bar-headed geese raised at high altitude. Thus, bar-headed geese reared at high altitude exhibited a reduced oxygen demand at rest and a modest but significant increase in oxygen uptake and delivery during progressive hypoxia compared with bar-headed geese reared at low altitude.

A novel candidate region for genetic adaptation to high altitude in Andean populations

Humans living at high altitude (≥2,500 meters above sea level) have acquired unique abilities to survive the associated extreme environmental conditions, including hypoxia, cold temperature, limited food availability and high levels of free radicals and oxidants. Long-term inhabitants of the most elevated regions of the world have undergone extensive physiological and/or genetic changes, particularly in the regulation of respiration and circulation, when compared to lowland populations. Genome scans have identified candidate genes involved in altitude adaption in the Tibetan Plateau and the Ethiopian highlands, in contrast to populations from the Andes, which have not been as intensively investigated. In the present study, we focused on three indigenous populations from Bolivia: two groups of Andean natives, Aymara and Quechua, and the low-altitude control group of Guarani from the Gran Chaco lowlands. Using pooled samples, we identified a number of SNPs exhibiting large allele frequency differences over 900,000 genotyped SNPs. A region in chromosome 10 (within the cytogenetic bands q22.3 and q23.1) was significantly differentiated between highland and lowland groups. We resequenced ~1.5 Mb surrounding the candidate region and identified strong signals of positive selection in the highland populations. A composite of multiple signals like test localized the signal to FAM213A and a related enhancer; the product of this gene acts as an antioxidant to lower oxidative stress and may help to maintain bone mass. The results suggest that positive selection on the enhancer might increase the expression of this antioxidant, and thereby prevent oxidative damage. In addition, the most significant signal in a relative extended haplotype homozygosity analysis was localized to the SFTPD gene, which encodes a surfactant pulmonary-associated protein involved in normal respiration and innate host defense. Our study thus identifies two novel candidate genes and associated pathways that may be involved in high-altitude adaptation in Andean populations.

Differences in Hematological Traits between High- and Low-Altitude Lizards (Genus Phrynocephalus)

Phrynocephalus erythrurus (Lacertilia: Agamidae) is considered to be the highest living reptile in the world (about 4500-5000 m above sea level), whereas Phrynocephalus przewalskii inhabits low altitudes (about 1000-1500 m above sea level). Here, we report the differences in hematological traits between these two different Phrynocephalus species. Compared with P. przewalskii, the results indicated that P. erythrurus own higher oxygen carrying capacity by increasing red blood cell count (RBC), hemoglobin concentration ([Hb]) and hematocrit (Hct) and these elevations could promote oxygen carrying capacity without disadvantage of high viscosity. The lower partial pressure of oxygen in arterial blood (PaO2) of P. erythrurus did not cause the secondary alkalosis, which may be attributed to an efficient pulmonary system for oxygen (O2) loading. The elevated blood-O2 affinity in P. erythrurus may be achieved by increasing intrinsic O2 affinity of isoHbs and balancing the independent effects of potential heterotropic ligands. We detected one α-globin gene and three β-globin genes with 1 and 33 amino acid substitutions between these two species, respectively. Molecular dynamics simulation results showed that amino acids substitutions in β-globin chains could lead to the elimination of hydrogen bonds in T-state Hb models of P. erythrurus. Based on the present data, we suggest that P. erythrurus have evolved an efficient oxygen transport system under the unremitting hypobaric hypoxia.

King of the Mountains: Tibetan and Sherpa Physiological Adaptations for Life at High Altitude

Anecdotal evidence surrounding Tibetans' and Sherpas' exceptional tolerance to hypobaric hypoxia has been recorded since the beginning of high-altitude exploration. These populations have successfully lived and reproduced at high altitude for hundreds of generations with hypoxia as a constant evolutionary pressure. Consequently, they are likely to have undergone natural selection toward a genotype (and phenotype) tending to offer beneficial adaptation to sustained hypoxia. With the advent of translational human hypoxic research, in which genotype/phenotype studies of healthy individuals at high altitude may be of benefit to hypoxemic critically ill patients in a hospital setting, high-altitude natives may provide a valuable and intriguing model. The aim of this review is to provide a comprehensive summary of the scientific literature encompassing Tibetan and Sherpa physiological adaptations to a high-altitude residence. The review demonstrates the extent to which evolutionary pressure has refined the physiology of this high-altitude population. Furthermore, although many physiological differences between highlanders and lowlanders have been found, it also suggests many more potential avenues of investigation.

Control of breathing and the circulation in high-altitude mammals and birds

Hypoxia is an unremitting stressor at high altitudes that places a premium on oxygen transport by the respiratory and cardiovascular systems. Phenotypic plasticity and genotypic adaptation at various steps in the O2 cascade could help offset the effects of hypoxia on cellular O2 supply in high-altitude natives. In this review, we will discuss the unique mechanisms by which ventilation, cardiac output, and blood flow are controlled in high-altitude mammals and birds. Acclimatization to high altitudes leads to some changes in respiratory and cardiovascular control that increase O2 transport in hypoxia (e.g., ventilatory acclimatization to hypoxia). However, acclimatization or development in hypoxia can also modify cardiorespiratory control in ways that are maladaptive for O2 transport. Hypoxia responses that arose as short-term solutions to O2 deprivation (e.g., peripheral vasoconstriction) or regional variation in O2 levels in the lungs (i.e., hypoxic pulmonary vasoconstriction) are detrimental at in chronic high-altitude hypoxia. Evolved changes in cardiorespiratory control have arisen in many high-altitude taxa, including increases in effective ventilation, attenuation of hypoxic pulmonary vasoconstriction, and changes in catecholamine sensitivity of the heart and systemic vasculature. Parallel evolution of some of these changes in independent highland lineages supports their adaptive significance. Much less is known about the genomic bases and potential interactive effects of adaptation, acclimatization, developmental plasticity, and trans-generational epigenetic transfer on cardiorespiratory control. Future work to understand these various influences on breathing and circulation in high-altitude natives will help elucidate how complex physiological systems can be pushed to their limits to maintain cellular function in hypoxia.

Structural modulation of brain development by oxygen: evidence on adolescents migrating from high altitude to sea level environment

The present study aimed to investigate structural modulation of brain by high level of oxygen during its peak period of development. Voxel-based morphometry analysis of gray matter (GM) and white matter (WM) volumes and Tract-Based Spatial Statistics analysis of WM fractional anisotropy (FA) and mean diffusion (MD) based on MRI images were carried out on 21 Tibetan adolencents (15-18 years), who were born and raised in Qinghai-Tibetan Plateau (2900-4700 m) and have lived at sea level (SL) in the last 4 years. The control group consisted of matched Tibetan adolescents born and raised at high altitude all the time. SL immigrants had increased GM volume in the left insula, left inferior parietal gyrus, and right superior parietal gyrus and decreased GM in the left precentral cortex and multiple sites in cerebellar cortex (left lobule 8, bilateral lobule 6 and crus 1/2). Decreased WM volume was found in the right superior frontal gyrus in SL immigrants. SL immigrants had higher FA and lower MD at multiple sites of WM tracts. Moreover, we detected changes in ventilation and circulation. GM volume in cerebellum lobule 8 positively correlated with diastolic pressure, while GM volume in insula positively correlated vital capacity and hypoxic ventilatory response. Our finding indicate that the structural modulations of GM by high level of oxygen during its peak period of development are related to respiratory and circulatory regulations, while the modulation in WM mainly exhibits an enhancement in myelin maturation.

Life-long consequences of postnatal normoxia exposure in rats raised at high altitude

We tested the hypothesis that exposure of high-altitude (HA) rats to a period of postnatal normoxia has long-term consequences on the ventilatory and hematological acclimatization in adults. Male and female HA rats (3,600 m, Po(2) ≃ 100 Torr; La Paz, Bolivia) were exposed to normal room air [HA control (HACont)] or enriched oxygen (32% O(2); Po(2) ≃ 160 Torr) from 1 day before to 15 days after birth [HA postnatal normoxia (HApNorm)]. Hematocrit and hemoglobin values were assessed at 2, 12, and 32 wk of age. Cardiac and lung morphology were assessed at 12 wk by measuring right ventricular hypertrophy (pulmonary hypertension index) and lung air space-to-tissue ratio (indicative of alveolarization). Respiratory parameters under baseline conditions and in response to 32% O(2) for 10 min (relieving the ambient hypoxic stimulus) were measured by whole body plethysmography at 12 wk. Finally, we performed a survival analysis up to 600 days of age. Compared with HACont, HApNorm rats had reduced hematocrit and hemoglobin levels at all ages (both sexes); reduced right ventricular hypertrophy (both sexes); lower air space-to-tissue ratio in the lungs (males only); reduced CO(2) production rate, but higher oxygen uptake (males only); and similar respiratory frequency, tidal volume, and minute ventilation. When breathing 32% O(2), HApNorm male rats had a stronger decrease of minute ventilation than HACont. HApNorm rats had a marked tendency toward longer survival throughout the study. We conclude that exposure to ambient hypoxia during postnatal development in HA rats has deleterious consequences on acclimatization to hypoxia as adults.

Phenotypic plasticity and genetic adaptation to high-altitude hypoxia in vertebrates

High-altitude environments provide ideal testing grounds for investigations of mechanism and process in physiological adaptation. In vertebrates, much of our understanding of the acclimatization response to high-altitude hypoxia derives from studies of animal species that are native to lowland environments. Such studies can indicate whether phenotypic plasticity will generally facilitate or impede adaptation to high altitude. Here, we review general mechanisms of physiological acclimatization and genetic adaptation to high-altitude hypoxia in birds and mammals. We evaluate whether the acclimatization response to environmental hypoxia can be regarded generally as a mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to genetic adaptation. In cases in which the acclimatization response to hypoxia is maladaptive, selection will favor an attenuation of the induced phenotypic change. This can result in a form of cryptic adaptive evolution in which phenotypic similarity between high- and low-altitude populations is attributable to directional selection on genetically based trait variation that offsets environmentally induced changes. The blunted erythropoietic and pulmonary vasoconstriction responses to hypoxia in Tibetan humans and numerous high-altitude birds and mammals provide possible examples of this phenomenon. When lowland animals colonize high-altitude environments, adaptive phenotypic plasticity can mitigate the costs of selection, thereby enhancing prospects for population establishment and persistence. By contrast, maladaptive plasticity has the opposite effect. Thus, insights into the acclimatization response of lowland animals to high-altitude hypoxia can provide a basis for predicting how altitudinal range limits might shift in response to climate change.

High-altitude disorders: pulmonary hypertension: pulmonary vascular disease: the global perspective

Globally, it is estimated that > 140 million people live at a high altitude (HA), defined as > 2,500 m (8,200 ft), and that countless others sojourn to the mountains for work, travel, and sport. The distribution of exposure to HA is worldwide, including 35 million in the Andes and > 80 million in Asia, including China and central Asia. HA stress primarily is due to the hypoxia of low atmospheric pressure, but dry air, intense solar radiation, extreme cold, and exercise contribute to acute and chronic disorders. The acute disorders are acute mountain sickness (also known as soroche), HA cerebral edema, and HA pulmonary edema (HAPE). Of these, HAPE is highly correlated with acute pulmonary hypertension. The first chronic syndrome described in HA dwellers in Peru was chronic mountain sickness (Monge disease), which has a large component of relative hypoventilation and secondary erythrocytosis. The prevalence of chronic mountain sickness in HA dwellers ranges from 1.2% in native Tibetans to 5.6% in Chinese Han; 6% to 8% in male residents of La Paz, Bolivia; and 15.6% in the Andes. Subacute mountain sickness is an exaggerated pulmonary hypertensive response to HA hypoxia occurring over months, most often in infants and very young children. Chronic pulmonary hypertension with heart failure but without hypoventilation is seen in Asia. Not only does HA pulmonary hypertension exact health consequences for the millions affected, but also the mechanisms of disease relate to pulmonary hypertension associated with multiple other disorders. Genetic understanding of these disorders is in its infancy.

The genetics of human adaptation: hard sweeps, soft sweeps, and polygenic adaptation

There has long been interest in understanding the genetic basis of human adaptation. To what extent are phenotypic differences among human populations driven by natural selection? With the recent arrival of large genome-wide data sets on human variation, there is now unprecedented opportunity for progress on this type of question. Several lines of evidence argue for an important role of positive selection in shaping human variation and differences among populations. These include studies of comparative morphology and physiology, as well as population genetic studies of candidate loci and genome-wide data. However, the data also suggest that it is unusual for strong selection to drive new mutations rapidly to fixation in particular populations (the 'hard sweep' model). We argue, instead, for alternatives to the hard sweep model: in particular, polygenic adaptation could allow rapid adaptation while not producing classical signatures of selective sweeps. We close by discussing some of the likely opportunities for progress in the field.

Circulatory adaptation to long-term high altitude exposure in Aymaras and Caucasians

About 30 million people live above 2500 m in the Andean Mountains of South America. Among them are 5.5 million Aymaras, an ethnic group with its own language, living on the altiplano of Bolivia, Peru, and northern Chile at altitudes of up to 4400 m. In this high altitude region traces of human population go back for more than 2000 years with constant evolutionary pressure on its residents for genetic adaptation to high altitude. Aymaras as the assumed direct descendents of the ancient cultures living in this region were the focus of much research interest during the last decades and several distinctive adaptation patterns to life at high altitude have been described in this ethnic group. The aim of this article was to review the physiology and pathophysiology of circulatory adaptation and maladaptation to longtime altitude exposure in Aymaras and Caucasians.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Differences in the control of breathing between Andean highlanders and lowlanders after 10 days acclimatization at 3850 m

We used Duffin's isoxic hyperoxic ( mmHg) and hypoxic ( mmHg) rebreathing tests to compare the control of breathing in eight (7 male) Andean highlanders and six (4 male) acclimatizing Caucasian lowlanders after 10 days at 3850 m. Compared to lowlanders, highlanders had an increased non-chemoreflex drive to breathe, characterized by higher basal ventilation at both hyperoxia (10.5 +/- 0.7 vs. 4.9 +/- 0.5 l min(1), P = 0.002) and hypoxia (13.8 +/- 1.4 vs. 5.7 +/- 0.9 l min(1), P < 0.001). Highlanders had a single ventilatory sensitivity to CO(2) that was lower than that of the lowlanders (P < 0.001), whose response was characterized by two ventilatory sensitivities (VeS1 and VeS2) separated by a patterning threshold. There was no difference in ventilatory recruitment thresholds (VRTs) between populations (P = 0.209). Hypoxia decreased VRT within both populations (highlanders: 36.4 +/- 1.3 to 31.7 +/- 0.7 mmHg, P < 0.001; lowlanders: 35.3 +/- 1.3 to 28.8 +/- 0.9 mmHg, P < 0.001), but it had no effect on basal ventilation (P = 0.12) or on ventilatory sensitivities in either population (P = 0.684). Within lowlanders, VeS2 was substantially greater than VeS1 at both isoxic tensions (hyperoxic: 9.9 +/- 1.7 vs. 2.8 +/- 0.2, P = 0.005; hypoxic: 13.2 +/- 1.9 vs. 2.8 +/- 0.5, P < 0.001), although hypoxia had no effect on either of the sensitivities (P = 0.192). We conclude that the control of breathing in Andean highlanders is different from that in acclimatizing lowlanders, although there are some similarities. Specifically, acclimatizing lowlanders have relatively lower non-chemoreflex drives to breathe, increased ventilatory sensitivities to CO(2), and an altered pattern of ventilatory response to CO(2) with two ventilatory sensitivities separated by a patterning threshold. Similar to highlanders and unlike lowlanders at sea-level, acclimatizing lowlanders respond to hypobaric hypoxia by decreasing their VRT instead of changing their ventilatory sensitivity to CO(2).

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Do high-altitude natives have enhanced exercise performance at altitude?

Natives of high altitude (HA) may have enhanced physical work capacity in hypoxia due to growth and development at altitude or, in the case of indigenous Andean and Himalayan residents, due to population genetic factors that determine higher limits to exercise performance. There is a growing scientific literature in support of both hypotheses, although the specific developmental vs. genetic origins of putative population trait differences remain obscure. Considering whole-body measures of exercise performance, a review of the literature suggests that indigenous HA natives have higher mean maximal oxygen consumption (VO(2) (max)) in hypoxia and smaller VO(2) (max) decrement with increasing hypoxia. At present, there is insufficient information to conclude that HA natives have enhanced work economy or greater endurance capacity, although for the former a number of studies indicate that this may be the case for Tibetans. At the physiological level, supporting the hypothesis of enhanced pulmonary gas exchange efficiency, HA natives have smaller alveolar-arterial oxygen partial pressure difference ((A-a)DO(2)), lower pulmonary ventilation (VE), and likely higher arterial O(2) saturation (SaO(2)) during exercise. At the muscle level, a handful of studies show no differences in fiber-type distributions, capillarity, oxidative enzymes, or the muscle response to training. At the metabolic level, a few studies suggest differences in lactate production/removal and (or) lactate buffering capacity, but more work is needed in this area.