The Molecular Basis of High-Altitude Adaptation in Deer Mice

Citation: Storz JF, Sabatino SJ, Hoffmann FG, Gering EJ, Moriyama H, et al. ( The Molecular Basis of High-Altitude Adaptation in Deer Mice Jay F. Storz 0 1 Stephen J. Sabatino 0 1 Federico G. Hoffmann 0 1 Eben J. Gering 0 1 Hideaki Moriyama 0 1 Nuno Ferrand 0 1 Bruno Monteiro 0 1 Michael W. Nachman 0 1 0 Editor: Molly Przeworski, University of Chicago , United States of America 1 1 School of Biological Sciences, University of Nebraska, Lincoln, Nebraska, United States of America, 2 Department of Chemistry, University of Nebraska, Lincoln, Nebraska, United States of America, 3 Centro de Investigac a o em Biodiversidade e Recursos Gene ticos, Campus Agra rio de Vaira o, Universidade do Porto , Vaira o, Portugal , 4 Departamento de Zoologia e Anthropologia, Faculdade de Ciencias do Porto , Porto , Portugal , 5 Department of Ecology and Evolutionary Biology, University of Arizona , Tucson, Arizona , United States of America Elucidating genetic mechanisms of adaptation is a goal of central importance in evolutionary biology, yet few empirical studies have succeeded in documenting causal links between molecular variation and organismal fitness in natural populations. Here we report a population genetic analysis of a two-locus a-globin polymorphism that underlies physiological adaptation to high-altitude hypoxia in natural populations of deer mice, Peromyscus maniculatus. This system provides a rare opportunity to examine the molecular underpinnings of fitness-related variation in protein function that can be related to a well-defined selection pressure. We surveyed DNA sequence variation in the duplicated a-globin genes of P. maniculatus from high- and low-altitude localities (i) to identify the specific mutations that may be responsible for the divergent fine-tuning of hemoglobin function and (ii) to test whether the genes exhibit the expected signature of diversifying selection between populations that inhabit different elevational zones. Results demonstrate that functionally distinct protein alleles are maintained as a long-term balanced polymorphism and that adaptive modifications of hemoglobin function are produced by the independent or joint effects of five amino acid mutations that modulate oxygen-binding affinity. - Many long-standing questions about genetic mechanisms of adaptation remain unanswered due to the difficulty of integrating molecular data with evidence for causal effects on organismal fitness. In principle, progress could be made by identifying key proteins or key components of protein interaction networks that are known to mediate an adaptive response to some specific environmental challenge. Analysis of DNA sequence variation at the underlying genes could then guide the identification of specific nucleotide changes that are responsible for functional modifications of biochemical or physiological pathways, and could also shed light on the role of natural selection in maintaining the observed variation in protein function [1,2]. Although this approach holds much promise, very few studies have successfully documented a mechanistic link between allelic variation in protein function and fitness-related variation in wholeorganism physiology [37]. Hemoglobin polymorphism in the deer mouse, Peromyscus maniculatus, represents an especially promising system for examining the molecular underpinnings of physiological adaptation to different environments. P. maniculatus has the broadest altitudinal range of any North American mammal, as the species is continuously distributed from sea-level environments to alpine environments at elevations above 4,300 m. At 4,300 m, the partial pressure of oxygen (PO2) is approximately 55% of the sea-level value, and the resultant hypoxia imposes severe constraints on aerobic metabolism. Experimental evidence indicates that adaptive variation in blood biochemistry among mice from different elevations is associated with a complex hemoglobin polymorphism [811]. Specifically, experimental crosses involving wild-derived strains of P. maniculatus revealed that variation in blood oxygen affinity is strongly associated with allelic variation at two closely linked gene duplicates that encode the a-chain subunits of adult hemoglobin [12,13]. In P. maniculatus, the two a-globin gene duplicates, Hba and Hbc, are each polymorphic for two main classes of electrophoretically detectable protein alleles, Hba0, Hba1, Hbc0, and Hbc1 [8,9,14,15]. These loci are closely linked, and because of strong linkage disequilibrium, nearly all a-globin haplotypes fall into two main classes: a0c0 and a1c1. The three nonrecombinant genotypes exhibit a highly consistent rankorder of blood oxygen affinities when tested under both highand low-altitude conditions: mice with the a0c0/a0c0 genotype exhibit the highest affinity (the most left-shifted oxygen dissociation curve), mice with the a1c1/a1c1 genotype exhibit the lowest affinity (the most right-shifted dissociation curve), and the a0c0/a1c1 double heterozygotes are intermediate [12,13]. In these experiments, the wild-derived strains of mice carried different a-globin haplotypes in identical-byA major goal in evolutionary biology is to identify the specific genetic mechanisms that have enabled organisms to adapt to their environments. Variation in deer mouse hemoglobin represents an especially promising system for examining the molecular underpinnings of adaptation because it has been possible to establish a mechanistic link between allelic variation in protein function and fitness-related variation in physiological performance. Specifically, adaptive variation in blood biochemistry and aerobic metabolism among mice from different elevations is associated with allelic variation at two closely linked gene duplicates that encode the achain subunits of adult hemoglobin. In this study, we report an analysis of DNA sequence variation in the two a-globin gene duplicates of deer mice in order to identify the specific mutations that underlie adaptation to high-altitude hypoxia. The study revealed that allelic differences in hemoglobin-oxygen affinity are attributable to the independent or joint effects of substitutions in five exterior amino acid residues that line the opening of the heme pocket. Additionally, patterns of DNA sequence variation indicate that functionally distinct a-globin alleles are maintained by natural selection that favors different genotypes in different elevational zones. descent condition, and the effects of the two genes were isolated against a randomized genetic background. In addition to the effects on blood biochemistry, the phenotypic effects of these a-globin genes are also manifest at the level of whole-organism physiology. In the context of adaptation to high-altitude hypoxia, one especially important measure of physiological performance is VO2max, which is defined as the maximal rate of oxygen consumption elicited by aerobic exercise or cold exposure. VO2max sets the upper limit (...truncated)