Life on the edge: O2 binding in Atlantic cod red blood cells near their southern distribution limit is not sensitive to temperature or haemoglobin genotype

Hidden but revealed: After years of genetic studies behavioural monitoring combined with genomics uncover new insight into the population dynamics of Atlantic cod in Icelandic waters

Stock structure is of paramount importance for sustainable management of exploited resources. In that context, genetic markers have been used for more than two decades to resolve spatial structure of marine exploited resources and to fully fathom stock dynamics and interactions. While genetic markers such as allozymes and RFLP dominated the debate in the early era of genetics, technology advances have provided scientists with new tools every decade to better assess stock discrimination and interactions (i.e. gene flow). Here, we provide a review of genetic studies performed to understand stock structure of Atlantic cod in Icelandic waters, from the early allozyme approaches to the genomic work currently carried out. We further highlight the importance of the generation of a chromosome-anchored genome assembly together with whole-genome population data, which drastically changed our perception of the possible management units to consider. After nearly 60 years of genetic investigation of Atlantic cod structure in Icelandic waters, genetic (and later genomic) data combined with behavioural monitoring using Data Storage Tags shifted the attention from geographical population structures to behavioural ecotypes. This review also demonstrates the need for future research to further disentangle the impact of these ecotypes (and gene flow among them) on the population structure of Atlantic cod in Icelandic waters. It also highlights the importance of whole-genome data to unravel unexpected within-species diversity related to chromosomal inversions and associated supergenes, which are important to consider for future development of sustainable management programmes of the species within the North Atlantic.

The effect of temperature on haemoglobin-oxygen binding affinity in regionally endothermic and ectothermic sharks

Haemoglobin (Hb)-O2 binding affinity typically decreases with increasing temperature, but several species of ectothermic and regionally endothermic fishes exhibit reduced Hb thermal sensitivity. Regionally endothermic sharks, including the common thresher shark (Alopias vulpinus) and lamnid sharks such as the shortfin mako shark (Isurus oxyrinchus), can maintain select tissues and organs warmer than ambient temperature by retaining metabolic heat with vascular heat exchangers. In the ectothermic bigeye thresher shark (Alopias superciliosus), diurnal movements above and below the thermocline subject the tissues, including the blood, to a wide range of operating temperatures. Therefore, blood-O2 transport must occur across internal temperature gradients in regionally endothermic species, and over the range of environmental temperatures encountered by the ectothermic bigeye thresher shark. While previous studies have shown temperature-independent Hb-O2 affinity in lamnid sharks, including shortfin mako, the Hb-O2 affinity of the common and bigeye thresher sharks is unknown. Therefore, we examined the effect of temperature on whole-blood Hb-O2 affinity in common thresher shark and bigeye thresher shark. For comparison, analyses were also conducted on the shortfin mako shark and two ectothermic species, blue shark (Prionace glauca) and spiny dogfish (Squalus acanthias). Blood-O2 binding affinity was temperature independent for common thresher shark and shortfin mako shark, which should prevent internal temperature gradients from negatively affecting blood-O2 transport. Blue shark and spiny dogfish blood-O2 affinity decreased with increasing temperature, as expected, but bigeye thresher shark blood exhibited both a reduced temperature dependence and a high Hb-O2 affinity, which likely prevents large changes in environment temperature and low environmental oxygen from affecting O2 uptake.

Respiratory plasticity improves aerobic performance in hypoxia in a marine teleost

Ocean deoxygenation is a pressing concern in the face of climate change. In response to prolonged hypoxia, fishes have demonstrated the ability to dynamically regulate hemoglobin (Hb) expression to enhance oxygen (O₂) uptake. Here, we examined hypoxia-inducible Hb expression in red drum (Sciaenops ocellatus) and the subsequent implications on Hb-O₂ binding affinity and aerobic scope. Fish were acclimated to 30 % air saturation for 1 d, 4 d, 8 d, 2 w, or 6 w, and red blood cells were collected for gene expression and biochemical profiling. Hypoxia acclimation induced significant up-regulation of one Hb subunit isoform (hbα 2) relative to control by 4 d with consistent upregulation thereafter. Hematocrit increased in hypoxia, with no changes in the allosteric modulator [NTP] at any time point. Changes in Hb expression co-occurred with a reduced Root effect (~26 % in normoxia, ~14 % in hypoxia) at a physiologically relevant pH while increasing O₂ binding affinity (i.e., lower P₅₀). These changes correlated with increased maximum metabolic rate and aerobic scope relative to controls when fish were tested in hypoxia. These results demonstrate an important role for Hb multiplicity in improving O₂ affinity and maximizing respiratory performance in hypoxia.

Genetic variation in haemoglobin is associated with evolved changes in breathing in high-altitude deer mice

Physiological systems often have emergent properties but the effects of genetic variation on physiology are often unknown, which presents a major challenge to understanding the mechanisms of phenotypic evolution. We investigated whether genetic variants in haemoglobin (Hb) that contribute to high-altitude adaptation in deer mice (Peromyscus maniculatus) are associated with evolved changes in the control of breathing. We created F2 inter-population hybrids of highland and lowland deer mice to test for phenotypic associations of α- and β-globin variants on a mixed genetic background. Hb genotype had expected effects on Hb-O2 affinity that were associated with differences in arterial O2 saturation in hypoxia. However, high-altitude genotypes were also associated with breathing phenotypes that should contribute to enhancing O2 uptake in hypoxia. Mice with highland α-globin exhibited a more effective breathing pattern, with highland homozygotes breathing deeper but less frequently across a range of inspired O2, and this difference was comparable to the evolved changes in breathing pattern in deer mouse populations native to high altitude. The ventilatory response to hypoxia was augmented in mice that were homozygous for highland β-globin. The association of globin variants with variation in breathing phenotypes could not be recapitulated by acute manipulation of Hb-O2 affinity, because treatment with efaproxiral (a synthetic drug that acutely reduces Hb-O2 affinity) had no effect on breathing in normoxia or hypoxia. Therefore, adaptive variation in Hb may have unexpected effects on physiology in addition to the canonical function of this protein in circulatory O2 transport.

Multiplicity and Polymorphism of Fish Hemoglobins

The diversity of fish hemoglobins and the association with oxygen availability and physiological requirements during the life cycle has attracted scientists since the first report on multiple hemoglobin in fishes (Buhler and Shanks 1959). The functional heterogeneity of the fish hemoglobins enables many species to tolerate hypoxic conditions and exhausting swimming, but also to maintain the gas pressure in the swim bladder at large depths. The hemoglobin repertoire has further increased in various species displaying polymorphic hemoglobin variants differing in oxygen binding properties. The multiplicity of fish hemoglobins as particularly found in the tetraploid salmonids strongly contrasts with the complete loss of hemoglobins in Antarctic icefishes and illustrates the adaptive radiation in the oxygen transport of this successful vertebrate group.

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.

ATP-induced reversed thermal sensitivity of O2 binding in both major haemoglobin polymorphs of the non-endothermic Atlantic cod, Gadus morhua

Atlantic cod is a species that is affected by climate change, with some populations being exposed to higher temperatures than others. The two polymorphs of its major haemoglobin type (HbI) show an inverse change in frequency along a latitudinal temperature cline in the North East Atlantic, which has been associated with differences in population performance and behavioural traits. An earlier study at the northern distribution limit of the species reported differential temperature sensitivities of red blood cell oxygen (O₂) affinity between the northern cold-water HbI-2 polymorph and its southern, warm-water HbI-1 counter-part, which has since widely been held as adaptive for the species across its distributional range. The present study critically re-examined this hypothesis by comparing the thermal sensitivity of O₂ binding in both purified HbI polymorphs from the southern, high-temperature distribution limit of the species under controlled conditions of allosteric modifiers of Hb function. Contrary to the prevailing view, the O₂ affinity of the major HbI polymorphs did not differ from each other under any of the tested conditions. Depending on pH and ATP concentration, the temperature-sensitive and temperature-insensitive Hb-O₂ affinity phenotypes - previously exclusively ascribed to HbI-1 and HbI-2, respectively - could be induced in both HbI polymorphs. These results are the first to establish a molecular mechanism behind a reversed temperature dependence of red blood cell O₂ affinity in a non-endotherm fish and lay the basis for future studies on alternative mechanisms behind the differences in distribution, performance and behavioural traits associated with the different HbI polymorphs of Atlantic cod.

Dissecting cause from consequence: a systematic approach to thermal limits

Thermal limits mark the boundaries of ectotherm performance, and are increasingly appreciated as strong correlates and possible determinants of animal distribution patterns. The mechanisms setting the thermal limits of ectothermic animals are under active study and rigorous debate as we try to reconcile new observations in the lab and field with the knowledge gained from a long history of research on thermal adaptation. Here, I provide a perspective on our divided understanding of the mechanisms setting thermal limits of ectothermic animals. I focus primarily on the fundamental differences between high and low temperatures, and how animal form and environment can place different constraints on different taxa. Together, complexity and variation in animal form drive complexity in the interactions within and among levels of biological organization, creating a formidable barrier to determining mechanistic cause and effect at thermal limits. Progress in our understanding of thermal limits will require extensive collaboration and systematic approaches that embrace this complexity and allow us to separate the causes of failure from the physiological consequences that can quickly follow. I argue that by building integrative models that explain causal links among multiple organ systems, we can more quickly arrive at a holistic understanding of the varied challenges facing animals at extreme temperatures.

Long-read sequence capture of the haemoglobin gene clusters across codfish species

Combining high-throughput sequencing with targeted sequence capture has become an attractive tool to study specific genomic regions of interest. Most studies have so far focused on the exome using short-read technology. These approaches are not designed to capture intergenic regions needed to reconstruct genomic organization, including regulatory regions and gene synteny. Here, we demonstrate the power of combining targeted sequence capture with long-read sequencing technology for comparative genomic analyses of the haemoglobin (Hb) gene clusters across eight species separated by up to 70 million years. Guided by the reference genome assembly of the Atlantic cod (Gadus morhua) together with genome information from draft assemblies of selected codfishes, we designed probes covering the two Hb gene clusters. Use of custom-made barcodes combined with PacBio RSII sequencing led to highly continuous assemblies of the LA (~100 kb) and MN (~200 kb) clusters, which include syntenic regions of coding and intergenic sequences. Our results revealed an overall conserved genomic organization of the Hb genes within this lineage, yet with several, lineage-specific gene duplications. Moreover, for some of the species examined, we identified amino acid substitutions at two sites in the Hbb1 gene as well as length polymorphisms in its regulatory region, which has previously been linked to temperature adaptation in Atlantic cod populations. This study highlights the use of targeted long-read capture as a versatile approach for comparative genomic studies by generation of a cross-species genomic resource elucidating the evolutionary history of the Hb gene family across the highly divergent group of codfishes.

Evolution of Hemoglobin Genes in Codfishes Influenced by Ocean Depth

Understanding the genetic basis of adaptation is one of the main enigmas of evolutionary biology. Among vertebrates, hemoglobin has been well documented as a key trait for adaptation to different environments. Here, we investigate the role of hemoglobins in adaptation to ocean depth in the diverse teleost order Gadiformes, with species distributed at a wide range of depths varying in temperature, hydrostatic pressure and oxygen levels. Using genomic data we characterized the full hemoglobin (Hb) gene repertoire for subset of species within this lineage. We discovered a correlation between expanded numbers of Hb genes and ocean depth, with the highest numbers in species occupying shallower, epipelagic regions. Moreover, we demonstrate that the Hb genes have functionally diverged through diversifying selection. Our results suggest that the more variable environment in shallower water has led to selection for a larger Hb gene repertoire and that Hbs have a key role in adaptive processes in marine environments.

Acclimation to prolonged hypoxia alters hemoglobin isoform expression and increases hemoglobin oxygen affinity and aerobic performance in a marine fish

Hemoglobin (Hb) multiplicity is common in fish, yet despite its ubiquitous nature, the functional significance is unclear. Here we explore the hypothesis that Hb multiplicity plays a role in hypoxia tolerance using the red drum (Sciaenops ocellatus). Red drum is an economically and ecologically important species native to coastal regions and estuaries of the Gulf of Mexico – habitats that routinely experience pronounced hypoxic events. Using a transcriptomic approach, we demonstrate that red drum red blood cells express 7 and 5 Hbα and Hbβ isoforms, respectively. Phylogenetic analysis grouped these isoforms into distinct isoHb clades, and provided evidence of lineage specific expression of particular isoHbs. In normoxia, three isoHbs predominated (Hbα-3.1, -3.2, and Hbβ-3.1). A three-week hypoxia acclimation (48 mmHg) resulted in significant up-regulation of Hbα-2, Hbα-3.2, and Hbβ-3.1, effectively switching the predominantly expressed isoforms. Changes in subunit expression were correlated with a decrease in non-stripped hemolysate P₅₀. Similarly, hypoxia acclimation resulted in a 20% reduction in whole animal critical oxygen threshold (P_crit). Hypoxia acclimation was not associated with changes in gill morphology, hematocrit, or relative ventricular mass. Overall, these data provide support for the hypothesis that Hb isoform switching can provide a physiological benefit to counteract environmental stress in fishes.