Phenotypic plasticity in blood-oxygen transport in highland and lowland deer mice

Hummingbird blood traits track oxygen availability across space and time

Predictable trait variation across environments suggests shared adaptive responses via repeated genetic evolution, phenotypic plasticity or both. Matching of trait-environment associations at phylogenetic and individual scales implies consistency between these processes. Alternatively, mismatch implies that evolutionary divergence has changed the rules of trait-environment covariation. Here we tested whether species adaptation alters elevational variation in blood traits. We measured blood for 1217 Andean hummingbirds of 77 species across a 4600-m elevational gradient. Unexpectedly, elevational variation in haemoglobin concentration ([Hb]) was scale independent, suggesting that physics of gas exchange, rather than species differences, determines responses to changing oxygen pressure. However, mechanisms of [Hb] adjustment did show signals of species adaptation: Species at either low or high elevations adjusted cell size, whereas species at mid-elevations adjusted cell number. This elevational variation in red blood cell number versus size suggests that genetic adaptation to high altitude has changed how these traits respond to shifts in oxygen availability.

The hypoxia tolerance of eight related African mole-rat species rivals that of naked mole-rats, despite divergent ventilatory and metabolic strategies in severe hypoxia

AIMS

Burrowing mammals tend to be more hypoxia tolerant than non-burrowing mammals and rely less on increases in ventilation and more on decreases in metabolic rate to tolerate hypoxia. Naked mole-rats (Heterocephalus glaber, NMRs), eusocial mammals that live in large colonies, are among the most hypoxia-tolerant mammals, and rely almost solely on decreases in metabolism with little change in ventilation during hypoxia. We hypothesized that the remarkable hypoxia tolerance of NMRs is an evolutionarily conserved trait derived from repeated exposure to severe hypoxia owing to their burrow environment and eusocial colony organization.

METHODS

We used whole-body plethysmography and indirect calorimetry to measure the hypoxic ventilatory and metabolic responses of eight mole-rat species closely related to the NMR.

RESULTS

We found that all eight species examined had a strong tolerance to hypoxia, with most species tolerating 3 kPa O₂ , Heliophobius emini tolerating 2 kPa O₂ and Bathyergus suillus tolerating 5 kPa O₂ . All species examined employed a combination of increases in ventilation and decreases in metabolism in hypoxia, a response midway between that of the NMR and that of other fossorial species (larger ventilatory responses, lesser reductions in metabolism). We found that eusociality is not fundamental to the physiological response to hypoxia of NMRs as Fukomys damarensis, another eusocial species, was among this group.

CONCLUSIONS

Our data suggest that, while the NMR is unique in the pattern of their physiological response to hypoxia, eight closely related mole-rat species share the ability to tolerate hypoxia like the current "hypoxia-tolerant champion," the NMR.

Evolution of physiological performance capacities and environmental adaptation: insights from high-elevation deer mice (Peromyscus maniculatus)

Analysis of variation in whole-animal performance can shed light on causal connections between specific traits, integrated physiological capacities, and Darwinian fitness. Here, we review and synthesize information on naturally occurring variation in physiological performance capacities and how it relates to environmental adaptation in deer mice (Peromyscus maniculatus). We discuss how evolved changes in aerobic exercise capacity and thermogenic capacity have contributed to adaptation to high elevations. Comparative work on deer mice at high and low elevations has revealed evolved differences in aerobic performance capacities in hypoxia. Highland deer mice have consistently higher aerobic performance capacities under hypoxia relative to lowland natives, consistent with the idea that it is beneficial to have a higher maximal metabolic rate (as measured by the maximal rate of O₂ consumption, VO_2max) in an environment characterized by lower air temperatures and lower O₂ availability. Observed differences in aerobic performance capacities between highland and lowland deer mice stem from changes in numerous subordinate traits that alter the flux capacity of the O₂-transport system, the oxidative capacity of tissue mitochondria, and the relationship between O₂ consumption and ATP synthesis. Many such changes in physiological phenotype are associated with hypoxia-induced changes in gene expression. Research on natural variation in whole-animal performance forms a nexus between physiological ecology and evolutionary biology that requires insight into the natural history of the study species.

Neutral and functionally important genes shed light on phylogeography and the history of high-altitude colonization in a widespread New World duck

Phylogeographic studies often infer historical demographic processes underlying species distributions based on patterns of neutral genetic variation, but spatial variation in functionally important genes can provide additional insights about biogeographic history allowing for inferences about the potential role of adaptation in geographic range evolution. Integrating data from neutral markers and genes involved in oxygen (O2)-transport physiology, we test historical hypotheses about colonization and gene flow across low- and high-altitude regions in the Ruddy Duck (Oxyura jamaicensis), a widely distributed species in the New World. Using multilocus analyses that for the first time include populations from the Colombian Andes, we also examined the hypothesis that Ruddy Duck populations from northern South America are of hybrid origin. We found that neutral and functional genes appear to have moved into the Colombian Andes from both North America and southern South America, and that high-altitude Colombian populations do not exhibit evidence of adaptation to hypoxia in hemoglobin genes. Therefore, the biogeographic history of Ruddy Ducks is likely more complex than previously inferred. Our new data raise questions about the hypothesis that adaptation via natural selection to high-altitude conditions through amino acid replacements in the hemoglobin protein allowed Ruddy Ducks to disperse south along the high Andes into southern South America. The existence of shared genetic variation with populations from both North America and southern South America as well as private alleles suggests that the Colombian population of Ruddy Ducks may be of old hybrid origin. This study illustrates the breadth of inferences one can make by combining data from nuclear and functionally important loci in phylogeography, and underscores the importance of complete range-wide sampling to study species history in complex landscapes.

Life-history characteristics influence physiological strategies to cope with hypoxia in Himalayan birds

Hypobaric hypoxia at high elevation represents an important physiological stressor for montane organisms, but optimal physiological strategies to cope with hypoxia may vary among species with different life histories. Montane birds exhibit a range of migration patterns; elevational migrants breed at high elevations but winter at low elevations or migrate further south, while high-elevation residents inhabit the same elevation throughout the year. Optimal physiological strategies to cope with hypoxia might therefore differ between species that exhibit these two migratory patterns, because they differ in the amount time spent at high elevation. We examined physiological parameters associated with blood-oxygen transport (haemoglobin concentration and haematocrit, i.e. the proportion of red blood cells in blood) in nine species of elevational migrants and six species of high-elevation residents that were sampled along a 2200 m (1000-3200 m) elevational gradient. Haemoglobin concentration increased with elevation within species regardless of migratory strategy, but it was only significantly correlated with haematocrit in elevational migrants. Surprisingly, haemoglobin concentration was not correlated with haematocrit in high-elevation residents, and these species exhibited higher mean cellular haemoglobin concentration than elevational migrants. Thus, alternative physiological strategies to regulate haemoglobin concentration and blood O₂ carrying capacity appear to differ among birds with different annual elevational movement patterns.

O2 binding and CO2 sensitivity in haemoglobins of subterranean African mole rats

Inhabiting deep and sealed subterranean burrows, mole rats exhibit a remarkable suite of specializations, including eusociality (living in colonies with single breeding queens), extraordinary longevity, cancer immunity and poikilothermy, and extreme tolerance of hypoxia and hypercapnia. With little information available on adjustments in haemoglobin (Hb) function that may mitigate the impact of exogenous and endogenous constraints on the uptake and internal transport of O₂, we measured haematological characteristics, as well as Hb-O₂ binding affinity and sensitivity to pH (Bohr effect), CO₂, temperature and 2,3-diphosphoglycerate (DPG, the major allosteric modulator of Hb-O₂ affinity in red blood cells) in four social and two solitary species of African mole rats (family Bathyergidae) originating from different biomes and soil types across Central and Southern Africa. We found no consistent patterns in haematocrit (Hct) and blood and red cell DPG and Hb concentrations or in intrinsic Hb-O₂ affinity and its sensitivity to pH and DPG that correlate with burrowing, sociality and soil type. However, the results reveal low specific (pH independent) effects of CO₂ on Hb-O₂ affinity compared with humans that predictably safeguard pulmonary loading under hypoxic and hypercapnic burrow conditions. The O₂ binding characteristics are discussed in relation to available information on the primary structure of Hbs from adult and developmental stages of mammals subjected to hypoxia and hypercapnia and the molecular mechanisms underlying functional variation in rodent Hbs.

Hematocrit of mammals (Artiodactyla, Carnivora, Primates) at 1500m and 2100m altitudes

The rise in hematocrit (Hct) is one of the hallmarks of human acclimatization to high altitude and, in chronic conditions, reflects the hypoxia-induced polycythemia. However, it is not a uniform response among domestic species and it is not found in Andean camelids, species long adapted to high altitudes. Hence, we asked to what extent the polycythemia of humans is common among mammals. Hct data were collected from captive mammals of three orders (Primates, Artiodactyla, Carnivora), 70 specimens of 33 species at ∼1500m altitude (barometric pressure Pb=635mmHg) and 296 specimens of 64 species at ∼2100m (Pb=596mmHg), long-term residents at those altitudes. Sea level values and data in men and women at the corresponding altitudes were from a compilation of literature sources. At either altitude Hct was significantly higher than at sea level both in men and women; the increase (ΔHct) for genders combined averaged 3.4% (±0.7 SEM) at 1500m and 5.4% (±0.3) at 2100m. Differently, among the three mammalian orders studied a significant increase in Hct occurred only in females of Carnivora (at 1500m) and in males of Primates (at 2100m). The average ΔHct of all species combined was 0.8% (±0.7) at 1500m and 1.5% (±0.4) at 2100m, both significantly less than in humans (P<0.001). At 2100m the average ΔHct of nine species long adapted to high altitude was 0.4% (±1), significantly less than in non-adapted species (P<0.001). A polycythemic response like that of men and women at 2100m occurred in less than 10% of the mammals examined. We conclude that, at least for the altitudes studied, a minimal polycythemia is a general feature of both high-altitude adapted and non-adapted species, and the magnitude of the human response is exceptional among mammals.

Functional roles of globin proteins in hypoxia-tolerant ectothermic vertebrates

Globins are heme-containing proteins ubiquitously expressed in vertebrates, where they serve a broad range of biological functions, directly or indirectly related to the tight control of oxygen levels and its toxic products in vivo. Perhaps the most investigated of all proteins, hemoglobin and myoglobin are primarily involved in oxygen transport and storage, but also in facilitating arterial vasodilation, suppressing mitochondrial respiration, and preventing tissue oxidative damage via accessory redox enzymatic activities during hypoxia. By contrast, the more recently discovered neuroglobin and cytoglobin do not seem to function as reversible oxygen carriers and are instead involved in redox activities, although their exact biological roles remain to be clarified. In this context, hypoxia-tolerant ectotherms, such as freshwater turtles and members of the carp family that survive winter in extreme hypoxia, have proven as excellent models to appreciate the diversity of biological functions of globin proteins. Unraveling physiological roles of globin proteins in these extreme animals will clarify an important part of the adaptive mechanisms for surviving extreme fluctuations of oxygen availability that are prohibitive to mammals.

Hematocrit and Hemoglobin Levels of Nonhuman Apes at Moderate Altitudes: A Comparison with Humans

Mortola, Jacopo P. and DeeAnn Wilfong. Hematocrit and hemoglobin levels of nonhuman apes at moderate altitudes: a comparison with humans. High Alt Med Biol. 17:323-335, 2016.-We asked to what extent the hematologic response (increase in hematocrit [Hct] and in blood hemoglobin concentration [Hb]) of humans to altitude hypoxia was shared by our closest relatives, the nonhuman apes. Data were collected from 29 specimens of 7 species of apes at 2073 m altitude (barometric pressure Pb = 598 mm Hg); additional data originated from apes located at a lower altitude (1493 m, Pb = 639 mm Hg). The human altitude profiles of Hct and Hb between sea level and 3000 m were constructed from a compilation of literature sources that (all combined) comprised data sets of 10,000-12,000 subjects for each gender. These human data were binned for 0-250 m altitude (sea level) and for each 500 m of progressively higher altitudes. Values of Hb and Hct of both men and women were significantly higher than at sea level at the 1500 bin (1250-1750 m); hence, the altitude threshold for the human hematological responses must be between 1000 and 1500 m. In the nonhuman apes, no increase in Hct or Hb was apparent at 1500 m; at 2000 m, the increase was significant only for the Hb of females. At either altitude in the group of nonhuman apes, the increase in Hct was much less than in humans, and that of Hb was significantly less at 1500 m. We conclude that lack of, or minimal, hematopoietic response to moderate altitude can occur in mammalian species that are not genetically adapted to high altitudes. Polycythemia is not a common response to altitude hypoxia and, at least at moderate altitudes, the degree of the human response may represent the exception among apes rather than the rule.

Hemoglobin-oxygen affinity in high-altitude vertebrates: is there evidence for an adaptive trend?

In air-breathing vertebrates at high altitude, fine-tuned adjustments in hemoglobin (Hb)-O2 affinity provide an energetically efficient means of mitigating the effects of arterial hypoxemia. However, it is not always clear whether an increased or decreased Hb-O2 affinity should be expected to improve tissue O2 delivery under different degrees of hypoxia, due to the inherent trade-off between arterial O2 loading and peripheral O2 unloading. Theoretical results indicate that the optimal Hb-O2 affinity varies as a non-linear function of environmental O2 availability, and the threshold elevation at which an increased Hb-O2 affinity becomes advantageous depends on the magnitude of diffusion limitation (the extent to which O2 equilibration at the blood-gas interface is limited by the kinetics of O2 exchange). This body of theory provides a framework for interpreting the possible adaptive significance of evolved changes in Hb-O2 affinity in vertebrates that have colonized high-altitude environments. To evaluate the evidence for an empirical generalization and to test theoretical predictions, I synthesized comparative data in a phylogenetic framework to assess the strength of the relationship between Hb-O2 affinity and native elevation in mammals and birds. Evidence for a general trend in mammals is equivocal, but there is a remarkably strong positive relationship between Hb-O2 affinity and native elevation in birds. Evolved changes in Hb function in high-altitude birds provide one of the most compelling examples of convergent biochemical adaptation in vertebrates.

Bohr effect and temperature sensitivity of hemoglobins from highland and lowland deer mice

An important means of physiological adaptation to environmental hypoxia is an increased oxygen (O2) affinity of the hemoglobin (Hb) that can help secure high O2 saturation of arterial blood. However, the trade-off associated with a high Hb-O2 affinity is that it can compromise O2 unloading in the systemic capillaries. High-altitude deer mice (Peromyscus maniculatus) have evolved an increased Hb-O2 affinity relative to lowland conspecifics, but it is not known whether they have also evolved compensatory mechanisms to facilitate O2 unloading to respiring tissues. Here we investigate the effects of pH (Bohr effect) and temperature on the O2-affinity of high- and low-altitude deer mouse Hb variants, as these properties can potentially facilitate O2 unloading to metabolizing tissues. Our experiments revealed that Bohr factors for the high- and low-altitude Hb variants are very similar in spite of the differences in O2-affinity. The Bohr factors of deer mouse Hbs are also comparable to those of other mammalian Hbs. In contrast, the high- and low-altitude variants of deer mouse Hb exhibited similarly low temperature sensitivities that were independent of red blood cell anionic cofactors, suggesting an appreciable endothermic allosteric transition upon oxygenation. In conclusion, high-altitude deer mice have evolved an adaptive increase in Hb-O2 affinity, but this is not associated with compensatory changes in sensitivity to changes in pH or temperature. Instead, it appears that the elevated Hb-O2 affinity in high-altitude deer mice is compensated by an associated increase in the tissue diffusion capacity of O2 (via increased muscle capillarization), which promotes O2 unloading.

Functional Genomic Insights into Regulatory Mechanisms of High-Altitude Adaptation

Recent studies of indigenous human populations at high altitude have provided proof-of-principle that genome scans of DNA polymorphism can be used to identify candidate loci for hypoxia adaptation. When integrated with experimental analyses of physiological phenotypes, genome-wide surveys of DNA polymorphism and tissue-specific transcriptional profiles can provide insights into actual mechanisms of adaptation. It has been suggested that adaptive phenotypic evolution is largely mediated by cis-regulatory changes in genes that are located at integrative control points in regulatory networks. This hypothesis can be tested by conducting transcriptomic analyses of hypoxic signaling pathways in conjunction with experimental measures of vascular oxygen supply and metabolic pathway flux. Such studies may reveal whether the architecture of gene regulatory networks can be used to predict which loci (and which types of loci) are likely to be "hot spots" for adaptive physiological evolution. Functional genomic studies of deer mice (Peromyscus maniculatus) demonstrate how the integrated analysis of variation in tissue-specific transcriptomes, whole-animal physiological performance, and various subordinate traits can yield insights into the mechanistic underpinnings of high-altitude adaptation.

Genetically based low oxygen affinities of felid hemoglobins: lack of biochemical adaptation to high-altitude hypoxia in the snow leopard

Genetically based modifications of hemoglobin (Hb) function that increase blood-O2 affinity are hallmarks of hypoxia adaptation in vertebrates. Among mammals, felid Hbs are unusual in that they have low intrinsic O2 affinities and reduced sensitivities to the allosteric cofactor 2,3-diphosphoglycerate (DPG). This combination of features compromises the acclimatization capacity of blood-O2 affinity and has led to the hypothesis that felids have a restricted physiological niche breadth relative to other mammals. In seeming defiance of this conjecture, the snow leopard (Panthera uncia) has an extraordinarily broad elevational distribution and occurs at elevations above 6000 m in the Himalayas. Here, we characterized structural and functional variation of big cat Hbs and investigated molecular mechanisms of Hb adaptation and allosteric regulation that may contribute to the extreme hypoxia tolerance of the snow leopard. Experiments revealed that purified Hbs from snow leopard and African lion exhibited equally low O2 affinities and DPG sensitivities. Both properties are primarily attributable to a single amino acid substitution, β2His→Phe, which occurred in the common ancestor of Felidae. Given the low O2 affinity and reduced regulatory capacity of feline Hbs, the extreme hypoxia tolerance of snow leopards must be attributable to compensatory modifications of other steps in the O2-transport pathway.

Intraspecific polymorphism, interspecific divergence, and the origins of function-altering mutations in deer mouse hemoglobin

Major challenges for illuminating the genetic basis of phenotypic evolution are to identify causative mutations, to quantify their functional effects, to trace their origins as new or preexisting variants, and to assess the manner in which segregating variation is transduced into species differences. Here, we report an experimental analysis of genetic variation in hemoglobin (Hb) function within and among species of Peromyscus mice that are native to different elevations. A multilocus survey of sequence variation in the duplicated HBA and HBB genes in Peromyscus maniculatus revealed that function-altering amino acid variants are widely shared among geographically disparate populations from different elevations, and numerous amino acid polymorphisms are also shared with closely related species. Variation in Hb-O₂ affinity within and among populations of P. maniculatus is attributable to numerous amino acid mutations that have individually small effects. One especially surprising feature of the Hb polymorphism in P. maniculatus is that an appreciable fraction of functional standing variation in the two transcriptionally active HBA paralogs is attributable to recurrent gene conversion from a tandemly linked HBA pseudogene. Moreover, transpecific polymorphism in the duplicated HBA genes is not solely attributable to incomplete lineage sorting or introgressive hybridization; instead, it is mainly attributable to recurrent interparalog gene conversion that has occurred independently in different species. Partly as a result of concerted evolution between tandemly duplicated globin genes, the same amino acid changes that contribute to variation in Hb function within P. maniculatus also contribute to divergence in Hb function among different species of Peromyscus. In the case of function-altering Hb mutations in Peromyscus, there is no qualitative or quantitative distinction between segregating variants within species and fixed differences between species.

Epistasis constrains mutational pathways of hemoglobin adaptation in high-altitude pikas

A fundamental question in evolutionary genetics concerns the roles of mutational pleiotropy and epistasis in shaping trajectories of protein evolution. This question can be addressed most directly by using site-directed mutagenesis to explore the mutational landscape of protein function in experimentally defined regions of sequence space. Here, we evaluate how pleiotropic trade-offs and epistatic interactions influence the accessibility of alternative mutational pathways during the adaptive evolution of hemoglobin (Hb) function in high-altitude pikas (Mammalia: Lagomorpha). By combining ancestral protein resurrection with a combinatorial protein-engineering approach, we examined the functional effects of sequential mutational steps in all possible pathways that produced an increased Hb–O₂ affinity. These experiments revealed that the effects of mutations on Hb–O₂ affinity are highly dependent on the temporal order in which they occur: Each of three β-chain substitutions produced a significant increase in Hb–O₂ affinity on the ancestral genetic background, but two of these substitutions produced opposite effects when they occurred as later steps in the pathway. The experiments revealed pervasive epistasis for Hb–O₂ affinity, but affinity-altering mutations produced no significant pleiotropic trade-offs. These results provide insights into the properties of adaptive substitutions in naturally evolved proteins and suggest that the accessibility of alternative mutational pathways may be more strongly constrained by sign epistasis for positively selected biochemical phenotypes than by antagonistic pleiotropy.

Integrating evolutionary and functional tests of adaptive hypotheses: a case study of altitudinal differentiation in hemoglobin function in an Andean Sparrow, Zonotrichia capensis

In air-breathing vertebrates, the physiologically optimal blood-O2 affinity is jointly determined by the prevailing partial pressure of atmospheric O2, the efficacy of pulmonary O2 transfer, and internal metabolic demands. Consequently, genetic variation in the oxygenation properties of hemoglobin (Hb) may be subject to spatially varying selection in species with broad elevational distributions. Here we report the results of a combined functional and evolutionary analysis of Hb polymorphism in the rufous-collared sparrow (Zonotrichia capensis), a species that is continuously distributed across a steep elevational gradient on the Pacific slope of the Peruvian Andes. We integrated a population genomic analysis that included all postnatally expressed Hb genes with functional studies of naturally occurring Hb variants, as well as recombinant Hb (rHb) mutants that were engineered through site-directed mutagenesis. We identified three clinally varying amino acid polymorphisms: Two in the α(A)-globin gene, which encodes the α-chain subunits of the major HbA isoform, and one in the α(D)-globin gene, which encodes the α-chain subunits of the minor HbD isoform. We then constructed and experimentally tested single- and double-mutant rHbs representing each of the alternative α(A)-globin genotypes that predominate at different elevations. Although the locus-specific patterns of altitudinal differentiation suggested a history of spatially varying selection acting on Hb polymorphism, the experimental tests demonstrated that the observed amino acid mutations have no discernible effect on respiratory properties of the HbA or HbD isoforms. These results highlight the importance of experimentally validating the hypothesized effects of genetic changes in protein function to avoid the pitfalls of adaptive storytelling.

The relationship between cardiopulmonary size and aerobic performance in adult deer mice at high altitude

Deer mice (Peromyscus maniculatus sonoriensis) populations in the White Mountains of Eastern California are found across a substantial range of partial pressures of oxygen (PO₂). Reduction in PO₂ at high altitude can have a negative impact on aerobic performance. We studied plastic changes in organ mass and volume involved in aerobic respiration in response to acclimation to high altitude, and how those changes are matched with aerobic performance measured by VO₂,max. Adult deer mice born and raised at 340 m were acclimated at either 340 or 3800 m for a period of 9 weeks. Lung volume increased by 9% in mice acclimated to high altitude. VO₂,max was also significantly higher under hypoxic conditions after high altitude acclimation compared with controls. Body mass-corrected residuals of VO₂,max were significantly correlated with an index of cardiopulmonary size (summed standardized residuals of lung volume and heart mass) under both hypoxic and normoxic conditions. These data show that phenotypic plasticity in lung volume and heart mass plays an important role in maintaining aerobic performance under hypoxic conditions, and accounts for up to 55% of the variance in aerobic performance.

Functional genomics of adaptation to hypoxic cold-stress in high-altitude deer mice: transcriptomic plasticity and thermogenic performance

In species that are distributed across steep environmental gradients, adaptive variation in physiological performance may be attributable to transcriptional plasticity in underlying regulatory networks. Here we report the results of common-garden experiments that were designed to elucidate the role of regulatory plasticity in evolutionary adaptation to hypoxic cold-stress in deer mice (Peromyscus maniculatus). We integrated genomic transcriptional profiles with measures of metabolic enzyme activities and whole-animal thermogenic performance under hypoxia in highland (4350 m) and lowland (430 m) mice from three experimental groups: (1) wild-caught mice that were sampled at their native elevations; (2) wild-caught/lab-reared mice that were deacclimated to low-elevation conditions in a common-garden lab environment; and (3) the F(1) progeny of deacclimated mice that were maintained under the same low-elevation common-garden conditions. In each experimental group, highland mice exhibited greater thermogenic capacities than lowland mice, and this enhanced performance was associated with upregulation of transcriptional modules that influence several hierarchical steps in the O(2) cascade, including tissue O(2) diffusion (angiogenesis) and tissue O(2) utilization (metabolic fuel use and cellular oxidative capacity). Most of these performance-related transcriptomic changes occurred over physiological and developmental timescales, suggesting that regulatory plasticity makes important contributions to fitness-related physiological performance in highland deer mice.

Epistasis among adaptive mutations in deer mouse hemoglobin

Epistatic interactions between mutant sites in the same protein can exert a strong influence on pathways of molecular evolution. We performed protein engineering experiments that revealed pervasive epistasis among segregating amino acid variants that contribute to adaptive functional variation in deer mouse hemoglobin (Hb). Amino acid mutations increased or decreased Hb-O2 affinity depending on the allelic state of other sites. Structural analysis revealed that epistasis for Hb-O2 affinity and allosteric regulatory control is attributable to indirect interactions between structurally remote sites. The prevalence of sign epistasis for fitness-related biochemical phenotypes has important implications for the evolutionary dynamics of protein polymorphism in natural populations.