Adaptations and lifestyle in polar marine environments: a biological challenge for the study of fi sh evolution Cinzia Verde, Ennio Cocca, Donatella de Pascale, Elio Parisi & Guido di Prisco The climatic features of Antarctic waters are more extreme and constant than in the Arctic. The Antarctic has been isolated and cold longer than the Arctic. The polar ichthyofaunas differ in age, endemism, taxonomy, zoogeographic distinctiveness and physiological tolerance to environmen- tal parameters. The Arctic is the connection between the Antarctic and the temperate–tropical systems. Paradigmatic comparisons of the path- ways of adaptive evolution of fi sh from both poles address the oxygen- transport system and the antifreezes of northern and southern species. (i) Haemoglobin evolution has included adaptations at the biochemical, physiological and molecular levels. Within the study of the molecular bases of fi sh cold adaptation, and taking advantage of the information on haemoglobin amino acid sequence, we analysed the evolutionary history of the α and β globins of Antarctic, Arctic and temperate haemoglobins as a basis for reconstructing phylogenetic relationships. In the trees, the constant physico-chemical conditions of the Antarctic waters are matched by clear grouping of globin sequences, whereas the variability typical of the Arctic ecosystem corresponds to high sequence variation, refl ected by scattered intermediate positions between the Antarctic and non-Ant- arctic clades. (ii) Antifreeze (glyco)proteins and peptides allow polar fi sh to survive at sub-zero temperatures. In Antarctic Notothenioidei the anti- freeze gene evolved from a trypsinogen-like serine protease gene. In the Arctic polar cod the genome contains genes which encode nearly identi- cal proteins, but have evolved from a different genomic locus—a case of convergent evolution. C. Verde, E. Cocca, D. de Pascale, E. Parisi & G. di Prisco, Institute of Protein Biochemistry, CNR, Via Marconi 12, I-80125 Naples, Italy, [email protected]. Although high latitudes and cold climates are The colonized terrestrial portions are extensive; common to both the Antarctic and the Arctic, in they are directly linked to temperate areas, great- many respects the two regions are more dissimi- ly facilitating adaptation and redistribution of ter- lar than similar. Due to the Antarctic Polar Front restrial organisms on one hand and, on the other, (APF), an oceanographic system producing a bar- producing wide and complex terrestrial mecha- rier responsible for the isolation of marine organ- nisms of feedback to the climate, which add to isms, the climatic features of the Antarctic waters those originating from ocean and atmosphere cir- are more extreme and constant than those of the culation. In addition, the anthropogenic impact Arctic. In the Arctic isolation is less pronounced, is greater. Tectonic and oceanographic events and the range of temperature variations is wider. played a key role in delimiting the two polar eco- Verde et al. 2004: Polar Research 23(1), 3–10 3 systems and in infl uencing the evolution of their Recent studies address the evolution of AFGP faunas, whose composition and diversity are genes in Antarctic Notothenioidei. The AFGP strongly linked to geological history. The Ant- gene evolved from a functionally unrelated pan- arctic has been isolated and cold longer than the creatic trypsinogen-like serine protease gene, Arctic, with ice sheet development preceding that through a molecular mechanism by which the in the Arctic by at least 10 My. The modern polar ancestral gene provided the front and tail of the faunas differ in age, endemism, zoogeographic emerging AFGP gene (Chen et al. 1997a). The distinctiveness, taxonomy and physiological tol- fi nding in the notothenioid genome of a chimer- erance to various environmental parameters. ic AFGP-protease gene intermediate, a protease The differences in the two polar environ- gene still bearing the incipient coding element, ments are refl ected in global changes. Accord- and independent AFGP genes, reveals a fascinat- ingly, studies of ecosystems are likely to provide ing case of “evolution in action” (Cheng & Chen answers to different questions, often complemen- 1999). It was deduced that the conversion of the tary to one another. In summary, the Arctic is the ancestral gene to the fi rst AFGP gene occurred connection between the more extreme, simpler 5 - 14 Mya. This value agrees well with the gen- Antarctic system and the more complex temper- erally accepted time frame (10 - 14 Mya) in which ate and tropical systems. the Antarctic water reached the present freezing “Evolution is the major unifying principle of conditions. biology, and evidence of evolutionary processes Analysis of AFGP in the polar cod (Boreo- pervades all levels of biological organisation from gadus saida, family Gadidae) showed that the molecules to ecosystems” (Eastman 2000: 276). genome of this species (phylogenetically unre- In studying evolution, the comparative approach lated to notothenioids, which belong to differ- permits examination of convergent and parallel ent superorder and order) contains genes which evolutionary trends at levels ranging from mol- encode nearly identical proteins. This would sug- ecules to organisms. gest a common ancestry. However the genes of Key aspects of evolution comprise molecu- the two fi sh groups are not homologous, hence lar, physiological and behavioural mechanisms have not followed the same evolutionary pathway. of adaptation by which organisms are enabled to Assuming an endogenous, yet unknown genetic survive, grow and reproduce. Current investiga- origin, the cod AFGP genes have evolved from a tions are increasingly exploring links between different, certainly not trypsinogen-like, genomic evolutionary processes, lifestyle and molecular/ locus (Chen et al. 1997b). An example of conver- organ system adaptations. gent evolution has thus been discovered. Two paradigmatic topics have been selected for this article, namely antifreeze compounds and haemoglobin, including loss of expression of The haemoglobin system globin genes and systematics. Genes coding iden- tical antifreeze compounds in northern cods and The Antarctic suborder Notothenioidei comprises southern Notothenioidei have different evolution- eight families. Ninety-six of the 213 (45 %) spe- ary histories; recent studies show that Arctic fi sh cies of the continental shelf, and 122 (including globins diverge from those of notothenioids and non-Antarctic species) of the 313 (39 %) Southern display paralogous relationships. Ocean species described to date are notothenio- ids (Gon & Heemstra 1990; Eastman 1993, 2000). The dominance of a single taxonomic group of Antifreeze compounds in Antarctic fi sh is unparalled in any other oceanic ecosys- and Arctic fi sh tem. The coastal Antarctic waters are cold and The biosynthesis of antifreeze (glyco)proteins oxygen-rich. The metabolic demand of fi sh for and peptides (AFGPs, AFPs), which allows polar oxygen is low, the solubility of oxygen in their fi sh to survive at sub-zero temperatures, is one plasma is high, and the energetic cost associated of the most intriguing evolutionary adaptations with blood circulation is large. With the selective (reviewed in Cheng & DeVries 1991) and meets pressure for erythrocytes and haemoglobin (Hb) the criteria for a “key innovation” (Eastman relaxed and with the cells posing a rheological 2000). disadvantage in the temperature-driven increase 4 Adaptations and lifestyle in polar marine environments: fi sh evolution Fig. 1. Abrogation of Hb synthesis as a result of a single, large-scale deletional event that removed all globin genes with the exception of the 3′ end of the α1-globin gene of adult Hb 1. The latter gene of N. coriiceps is represented in the upper part. in blood viscosity, over evolutionary time noto- tively, loss of Hb production may have occurred thenioids have developed reduced haematocrits independently of the trend as a nonadaptive, yet (Hct), Hb concentration/multiplicity and oxygen non-lethal mutation in the oxygen-rich Antarctic affi nity. waters. In this regard, one of the most unusual pheno- types of vertebrates was reported by Ruud (1954): Globin genes in notothenioids; evolutionary the colourless blood of the “icefi sh” species of the loss of expression in icefi sh family Channichthyidae, the most phyletically derived notothenioids, was devoid of Hb. Icefi shes The globin-gene organization in red-blooded noto- maintain normal metabolic function by deliver- thenioids has been characterized, the status of ing oxygen physically dissolved in the blood to globin genes in channichthyid genomes has been tissues. Hct reduction to near zero appears selec- studied, and potential evolutionary mechanisms tively advantageous because it diminishes the leading to the Hb-less phenotype have been eval- energetic cost associated with circulation of a uated (Cocca et al. 1995, 1997, 2000; Zhao et al. highly viscous, corpuscular blood fl uid (Wells 1998; di Prisco et al. 2002). It was found that ice- et al. 1990; di Prisco et al. 1991; Eastman 1993). fi sh retain genomic DNA sequences closely relat- The remaining seven notothenioid families are ed to the adult α-globin gene(s) of its red-blood- red-blooded. ed notothenioid ancestors and contemporaries, Channichthyids diverged from other notothen- whereas its ancestral β-globin-gene sequences ioids approximately 7 - 15 Mya, but the radiation have either been deleted or have diverged beyond of species within the icefi