Hemoglobin Physiology in Vertebrate Animals: a Cautionary Approach to Adaptationist Thinking. In: Boutilier RG, editor. Vertebrate Gas Exchange. Berlin: Springer Berlin Heidelberg; 1990. pp. 143-61. [DOI: 10.1007/978-3-642-75380-0_5]

Evolutionary suppression of erythropoiesis via the modulation of TGF-β signalling in an Antarctic icefish

The Antarctic icefish, a family (Channichthyidae) of teleosts within the perciform suborder Notothenioidei, are the only known vertebrates without oxygen-transporting haemoglobins and that are largely devoid of circulating erythrocytes. To elucidate the evo-devo mechanisms underpinning the suppressed erythropoiesis in the icefish, we conducted comparative studies on the transcriptomes and microRNAomes of the primary haematopoietic tissues between an icefish (Chionodraco hamatus) and two red-blooded notothenioids (Trematomus bernacchii and Gymnodraco acuticeps). We identified substantial remodelling of the haematopoietic programs in the icefish through which erythropoiesis is selectively suppressed. Experimental verification showed that erythropoietic suppression in the icefish may be attributable to the upregulation of TGF-β signalling, which coincides with reductions in multiple transcription factors essential for erythropoiesis and the upregulation of hundreds of microRNAs, the majority (> 80%) of which potentially target erythropoiesis regulating factors. Of the six microRNAs selected for verification, three miRNAs (miR-152, miR-1388 and miR-16b) demonstrated suppressive functions on GATA1 and ALAS2, which are two factors important for erythroid differentiation, resulting in reduced numbers of erythroids in microinjected zebra fish embryos. Codon substitution analyses of the genes of the TGF-β superfamily revealed signs of positive selection in TGF-β1 and endoglin in the lineages leading to Antarctic notothenioids. Both genes are previously known to function in erythropoietic suppression. These findings implied a general trend of erythropoietic suppression in the cold-adapted notothenioid lineages through evolutionary modulation of the multi-functional TGF-β signalling pathway. This trend is more pronounced in the haemoglobin-less icefish, which may pre-emptively hinder the otherwise defective erythroids from production.

Phenylhydrazine-induced anemia causes nitric-oxide-mediated upregulation of the angiogenic pathway in Notothenia coriiceps

Antarctic icefishes possess several cardiovascular characteristics that enable them to deliver oxygen adequately in the absence of hemoglobin (Hb). To gain insight into mechanisms driving development of these cardiovascular characteristics of icefish, we chemically induced severe anemia in a red-blooded notothenioid, Notothenia coriiceps. After 10 days of treatment with phenylhydrazine HCl, the hematocrit and Hb concentration of N. coriiceps decreased by >90% and >70%, respectively. Anemic fish exhibited a significantly higher concentration of nitric oxide (NO) metabolites in their plasma compared with that of control animals, indicating that corporeal levels of NO are higher in anemic animals than in control fish. The activity of nitric oxide synthase (NOS) was measured in brain, retina, pectoral muscle and ventricle of control and anemic animals. With the exception of retina, no significant differences in NOS activities were observed, indicating that the increase in plasma NO metabolites is due to loss of Hb, which normally plays a major role in the degradation of NO, and not due to an overall increase in the capacity for NO production. To determine whether loss of Hb can stimulate remodeling of the cardiovascular system, we measured expression of HIF-1alpha, PHD2 and VEGF mRNA in retinae of control and anemic fish. Expression of all three genes was higher in anemic animals compared with control N. coriiceps, suggesting a causative relationship between loss of Hb and induction of angiogenesis that probably is mediated through nitric oxide signaling.

The Antarctic hemoglobinless icefish, fifty five years later: a unique cardiocirculatory interplay of disaptation and phenotypic plasticity

The teleostean Channichthyidae (icefish), endemic stenotherms of the Antarctic waters, perennially at or near freezing, represent a unique example of disaptation among adult vertebrates for their loss of functional traits, particularly hemoglobin (Hb) and, in some species, cardiac myoglobin (Mb), once considered to be essential-life oxygen-binding chromoproteins. Conceivably, this stably frigid, oxygen-rich habitat has permitted high tolerance of disaptation, followed by subsequent adaptive recovery based on gene expression reprogramming and compensatory responses, including an alternative cardio-circulatory design, Hb-free blood and Mb-free cardiac muscle. This review revisits the functional significance of the multilevel cardio-circulatory compensations (hypervolemia, near-zero hematocrit and low blood viscosity, large bore capillaries, increased vascularity with great capacitance, cardiomegaly with very large cardiac output, high blood flow with low systemic pressure and systemic resistance) that counteract the challenge of hypoxemic hypoxia by increasing peripheral oxygen transcellular movement for aerobic tissues, including the myocardium. Reconsidered in the context of recent knowledge on both polar cold adaptation and the new questions related to the advent of nitric oxide (NO) biology, these compensations can be interpreted either according to the "loss-without-penalty" alternative, or in the context of an excessive environmental oxygen supply at low cellular cost and oxygen requirement in the cold. Therefore, rather than reflecting oxygen limitation, several traits may indicate structural overcompensation of oxygen supply reductions at cell/tissue levels. At the multilevel cardio-circulatory adjustments, NO is revealing itself as a major integrator, compensating disaptation with functional phenotypic plasticity, as illustrated by the heart paradigm. Beside NOS-dependent NO generation, recent knowledge concerning Hb/Mb interplay with NO and nitrite has revealed unexpected functions in addition to the classical respiratory role of these proteins. In fact, nitrite, a major biologic reservoir of NO, generates it through deohyHb- and deoxyMb-dependent nitrite reduction, thereby regulating hypoxic vasodilation, cellular respiration and signalling. We suggest that both Hb and Mb are involved as nitrite reductases under hypoxic conditions in a number of cardiocirculatory processes. On the whole, this opens new horizons in environmental and evolutionary physiology.

When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes

The Antarctic icefishes (Family Channichthyidae) provide excellent examples of unique traits that can arise in a chronically cold and isolated environment. Their loss of hemoglobin (Hb) expression, and in some cases, loss of myoglobin (Mb) expression, has taught us much about the function of these proteins. Although absences of the proteins are fixed traits in icefishes, the losses do not appear to be of adaptive value. Contrary to some suggestions, loss of Hb has led to higher energetic costs for circulating blood, and losses of Mb have reduced cardiac performance. Moreover, losses of Hb and Mb have resulted in extensive modifications to the cardiovascular system to ensure adequate oxygen delivery to working muscles. Recent studies suggest that losses of Hb and Mb, and their associated nitric oxide (NO)-oxygenase activities, may have accelerated the development and evolution of these cardiovascular modifications. The high levels of NO that should occur in the absence of Hb and Mb have been shown in other animal groups to lead to an increase in tissue vascularization, an increase in the lumenal diameter of blood vessels, and an increase in mitochondrial densities. These characteristics are all hallmark traits of Antarctic icefishes. Homeostatic feedback mechanisms thus may have accelerated evolution of the pronounced cardiovascular traits of Antarctic icefishes.

Brain and sense organ anatomy and histology in hemoglobinless Antarctic icefishes (Perciformes: Notothenioidei: Channichthyidae)

The Channichthyidae, one of five Antarctic notothenioid families, includes 16 species and 11 genera. Most live at depths of 200-800 m and are a major component of fish biomass in many shelf areas. Channichthyids are unique among adult fishes in possessing pale white blood containing a few vestigal erythrocytes and no hemoglobin. Here we describe the brains of seven species and special sense organs of eight species of channichthyids. We emphasize Chionodraco hamatus and C. myersi, compare these species to other channichthyids, and relate our findings to what is known about brains and sense organs of red-blooded notothenioids living sympatrically on the Antarctic shelf. Brains of channichthyids generally resemble those of their bathydraconid sister group. Among channichthyids the telencephalon is slightly regressed, resulting in a stalked appearance, but the tectum, corpus cerebellum, and mechanoreceptive areas are well developed. Interspecific variation is present but slight. The most interesting features of channichthyid brains are not in the nervous tissue but in support structures: the vasculature and the subependymal expansions show considerable elaboration. Channichthyids have large accessory nasal sacs and olfactory lamellae are more numerous than in other notothenioids. The eyes are relatively large and laterally oriented with similar duplex (cone and rod) retinae in all eight species. Twin cones are the qualitatively dominant photoreceptor in histological sections and, unlike bathydraconids, there are no species with rod-dominated retinae. Eyes possess the most extensive system of hyaloid arteries known in teleosts. Unlike the radial pattern seen in red-blooded notothenioids and most other teleosts, channichthyid hyaloid arteries arise from four or five main branches and form a closely spaced anastomosing series of parallel channels. Cephalic lateral line canals are membranous and some exhibit extensions (canaliculi), but canals are more ossified than those of deeper-living bathydraconids. We conclude that, with respect to the anatomy and histology of the neural structures, the brain and sensory systems show little that is remarkable compared to other fishes, and exhibit little diversification within the family. Thus, the unusual habitat and a potentially deleterious mutation resulting in a hemoglobinless phenotype are reflected primarily in expansion of the vasculature in the brain and eye partially compensating for the absence of respiratory pigments. Neural morphology gives the impression that channichthyids are a homogeneous and little diversified group.

Genetic variation and functional properties of Atlantic cod hemoglobins: introducing a modified tonometric method for studying fragile hemoglobins

Hemoglobin polymorphism in Atlantic cod has been investigated with respect to physiological performance at 10, 15 and 20 degrees C applying a modified tonometric method for O2 equilibrium analysis with full control of the equilibrating gas mixture. The results did not indicate any dissociation of the hemoglobins by a reduction in cooperativity and a parallel increase in affinity during the analytical procedure in contrast to the original tonometric method. With the applied preparation technique, we could store the hemolysate for 70 days at -25 degrees C without any significant changes in the O2 binding properties (P < 0.05) demonstrating the high quality of this procedure for analysing fragile fish hemoglobins. The present investigation demonstrates that the oxygen affinity of the hemoglobins varied between the genotypes. At all temperatures, except 20 degrees C and pH 8.0, Hb-I(2/2) had a higher O2 affinity than Hb-I(1/1). These results conform with previous results (16), suggesting Hb-I(2/2), the genotype which is the dominant allele in northern areas, to be the most efficient O2 carrier at low temperatures. The highest O2 affinity, however, was found for Hb-I(2/2b), supporting the results of Fyhn et al. (9), that this genotype is more restricted to coastal and warmer water and thus a better marker of the coastal population. Our results further suggest a correlation between genotype specific growth rates and oxygen affinities at all temperatures studied, with the highest growth rates observed in those genotypes having the highest O2 affinities. In conclusion, the hemoglobin polymorphism of cod seems to be correlated with physiological performance.

Cooperativity and allosteric regulation in non-mammalian vertebrate haemoglobins

1. This review illustrates the vast range of molecular functions expressed in non-mammalian vertebrate haemoglobins; with particular reference to the degree of aggregation of haemoglobin subunits and their interactions with allosteric effectors. 2. In at least the broadest sense, these properties suggest that haemoglobin function in non-mammalian vertebrates can be viewed against the evolutionary hierarchy of organisms rather than from a purely adaptive perspective.