Our lab strives to understand how the respiratory and metabolic systems of vertebrates adjust to challenging environments and how they adapt over evolutionary time. We use a comparative evolutionary approach in mammals, birds, and fish to understand the mechanistic basis for variation within and between vertebrate groups. Integrative approaches explore the causal links between levels of organization – from gene/genome to physiological system to organism. Some of our ongoing projects are discussed below. If you'd like to learn more, check out our Publications page where you can find links to various journal articles and features in the popular media.
High-Altitude Hypoxia

The decline in oxygen (‘hypoxia’) at high altitudes is extremely challenging to terrestrial vertebrates. Oxygen levels atop the highest mountains in the world are scarcely sufficient to support life in many species – they are so low that unacclimatized lowland animals can be rendered comatose within minutes. However, every mountain range contains species that are uniquely adapted to thrive at high altitudes. Our research strives to explain the physiological mechanisms of genotypic adaptation and phenotypic plasticity (e.g., acclimatization) to high-altitude hypoxia in mammals and birds. The emphasis of much of our work has been the O2 transport pathway (shown at right), the physiological system important for supplying O2 from the air to every cell in the body.

Our current work in this area focusses on deer mice (Peromyscus maniculatus), the species with the broadest altitudinal distribution of any North American mammal. Highland deer mice can sustain impressive rates of O2 consumption to support locomotion and thermogenesis (heat production to maintain body temperature) while facing a diminished O2 supply. We maintain a lab colony of deer mice derived from wild populations at high altitude (Peak of Mount Evans, Colorado, at 4300 m elevation) and low altitude (Great Plains of Nebraska), which we use to explore how both adaptation and plasticity affect performance and O2 transport in hypoxia. We have found that maximal rates of O2 consumption (VO2max) during exercise in hypoxia are higher in highland deer mice than in lowland deer mice. We have also found that hypoxia acclimatization improves VO2max in each population. Our ongoing work suggests that several changes across the O2 transport pathway underlie these differences.

The oxygen transport pathway is composed of a cascading series of processes that deliver O2 from atmospheric air to the mitochondria in tissue cells (e.g., muscle fibres). The pathway above is for air breathers (mammals and birds), but is similar for water-breathing fish if pulmonary diffusion at the lung is replaced by branchial diffusion at the gills.
Our early work on high-altitude hypoxia examined the bar-headed goose, a bird that flies over the Himalayas on its biannual migration between south and central Asia. The ability of this species to support the high metabolic rates needed for flight during hypoxia is largely explained byseveral enhancements in the O2 transport pathway, including an enhanced hypoxic ventilatory response, large lungs, a high haemoglobin O2-affinity, and an increase in capillarity in the heart and flight muscle. This increase in O2 supply capacity is complemented by modifications in some of the traits that influence cellular O2 demands (e.g., kinetics of cytochrome c oxidase, mitochondrial regulation by creatine), whereas other features of O2 utilization are largely unaltered (e.g., metabolic capacity, mitochondrial O2 kinetics). These mechanistic discoveries were made by comparing closely related species raised in common conditions at sea level, suggesting that genotypic adaptations enable flight performance in hypoxia. Ongoing work is exploring the molecular basis for these adaptations.
The flight muscle of bar-headed geese has mitochondria that are located very close to the cell membrane, and thus closer to the source of O2 in capillaries. A transmission electron micrograph of an aerobic muscle fibre in a bar-headed goose (top left) shows a strong clustering of mitochondria (which appear dark) near the sarcolemma (arrow). Aerobic fibres in barnacle geese (bottom left) have more mitochondria throughout the sarcoplasm (arrowhead). Bar-headed geese also have an amino-acid mutation (green residue below) in the terminal O2 acceptor of the electron transport chain in their mitochondria, cytochrome c oxidase (COX), which is related to a change in enzyme function. Have a look at our papers on the Publications page (Scott et al. 2009 Proc R Soc B; Scott et al. 2011 Mol Biol Evol) for the details.
Thermal image of a bar-headed goose during flight (courtesy of Sally Ward)

We have begun working with a broad range of highland species to better appreciate common strategies of high-altitude adaptation. Independent high-altitude lineages of Andean waterfowl are being used to understand the mechanistic basis of parallel evolution in highland taxa.
Aquatic Hypoxia
Hypoxia is prevalent in many aquatic environments. Freshwater, seawater, and estuarine environments can all exhibit variation in O2 levels as a result of both natural and anthropogenic causes. Nevertheless, hypoxia tolerant species can thrive in a range of hypoxic habitats in the wild. We use fish as a parallel system for studying the mechanisms of genotypic adaptation and phenotypic plasticity, which allows us to appreciate the coping strategies that unify or distinguish vertebrates from distinct environments (i.e., air versus water) and with different respiratory systems (e.g., gills versus lungs).

We are studying hypoxia tolerance in fish from the family Centrarchidae (bass and sunfish), a group that is widely represented across eastern North America. We use both lab and field experiments to understand the physiological mechanisms of hypoxia adaptation and acclimatization, and how they contribute to variation in tolerance and distribution in nature. We have uncovered substantial variation in hypoxia tolerance and exercise performance across the family. Sunfish appear to have evolved greater hypoxia tolerance than other centrarchid species, and some species of sunfish (pumpkinseed) are more tolerant than others (bluegill). Bass have instead evolved greater exercise performance and maximal rates of O2 consumption. Our ongoing work is examining the respiratory, metabolic, and molecular mechanisms that underlie this variation.

Pumpkinseed sunfish (Lepomis gibbosus)
Largemouth bass (Micropterus salmoides)
We are also examining the mechanisms of hypoxia acclimation in killifish (Fundulus heteroclitus). This species lives in estuaries along the east coast of North America, where daily fluctuations in many stressors are common. Intermittent hypoxia (particularly at night) is routinely encountered by killifish in the wild, yet we know very little about the effects of intermittent hypoxia in fish. Nevertheless, intermittent hypoxia has attracted substantial attention in biomedical research on mammals, and has been shown to have distinct and harmful cellular and systemic effects compared to constant hypoxia. We are comparing the responses of killifish to constant and intermittent hypoxia to determine whether these distinct patterns of hypoxia exposure favour different strategies for matching O2 supply and O2 demand.
Diffusive oxygen transport from water to blood occurs across the gills. Zebrafish gills on the left (sectioned, stained with toluidine blue and imaged with dark field illumination) show the long axis of the gill filaments from which gill lamellae extend. The killifish gills on the right (scanning electron micrograph) shows the pavement cells and ionocytes covering the gas-exchange surface (trailing edge of the filament is shown).
Hypoxia acclimation reduces the rate at which mitochondria emit reactive oxygen species (ROS), possibly to reduce oxidative stress or alter ROS-mediated signalling in hypoxia (Du et al., 2016). Illustration courtesy of Dilshaayee Prabaharan.
Other Stressors
Hypoxia does not always occur in isolation, but often occurs concurrent with other stressors. This is particularly true in aquatic ecosystems, where hypoxia can occur in conjuction with temperature change, acidification, elevated CO2, salinity fluctuation, or a range of other challenges. We are therefore examining how hypoxia interacts with other stressors to affect the physiology of fish. Our main work in this area focusses on the effects of temperature and CO2 on hypoxia tolerance and the cardiorespiratory responses to hypoxia in multiple species of bass and sunfish. Our main goal is to understand the mechanisms that underlie the susceptibility of different species to combinations of multiple stressors.
Other work is examining the physiological and molecular responses of fish to challenging temperatures. Although the mechanisms of temperature acclimation have been studied for decades, this research area is gaining renewed and vital importance as we seek to understand the biological effects of global warming. Our research in this area uses the model species zebrafish as a useful tool to understand the physiological, molecular, and genomic bases of developmental plasticity and thermal acclimation as they relate to exercise performance. The mechanisms of temperature's effects are examined in the physiological systems that support energy supply and demand during exercise (respiration, metabolism, muscle contraction, etc.).

Our recent research has shown that temperature during embryonic development can having striking and persistent effects on thermal sensitivity and thermal acclimation capacity, as reflected by alterations in aerobic swimming performance
Icebergs in Newfoundland
(Ucrit). There appear to be multiple causes of these effects that span levels of biological organization, including influences of embryonic temperature on muscle plasticity (see figure below), the function and capacity of metabolic enzymes, and genome-wide patterns of mRNA expression in pathways involved in energy metabolism, angiogenesis, cell stress, and muscle remodelling (detected using whole-transcriptome shotgun sequencing, ‘RNA-Seq’).
Fish restructure the fibre type composition of their muscle in response to temperature acclimation. Zebrafish muscle is shown, sectioned and stained for slow oxidative fibres (red stain for S58 protein), fast oxidative fibres (brown stain of alkaline-resistant myosin ATPase activity), and aerobic capacity (purple stain of succinate dehydrogenase activity). On the right is a representation of an entire transverse section of fish swimming muscle, showing the areas dominated by the slow (red), intermediate (pink), and fast (white) muscle (the blue box depicts the area shown in the stained sections). Check out our paper on the Publications page (Scott & Johnston. 2012 Proc Natl Acad Sci) for the details.