Thermal tolerance of Antarctic notothenioid fishes correlates with level of circulating hemoglobin

The West Antarctic Peninsula region is experiencing some of the most rapid elevations in temperature of any marine environment. We assessed thermal tolerance of white- and red-blooded Antarctic notothenioid fishes inhabiting these waters, using a modified critical thermal maximum (CT(max)) design. Temperature was elevated acutely from ambient at a constant rate of 3.6°C h(-1), and CT(max) was defined as the temperature where animals lost righting response. CT(max) temperatures of white-blooded icefishes Chionodraco rastrospinosus (13.3° ± 0.2°C) and Chaenocephalus aceratus (13.9° ± 0.4°C) were significantly lower than those of red-blooded fishes Gobionotothen gibberifrons (15.5° ± 0.2°C) and Notothenia coriiceps (17.1° ± 0.2°C). Lepidonotothen squamifrons, a red-blooded species with low hematocrit, exhibited a CT(max) (14.2° ± 0.4°C) that was significantly lower than that of the other red-blooded animals and similar to that of icefishes. A strong relationship between CT(max) and hematocrit (r(2) = 0.76) suggests that the oxygen-carrying capacity of blood may partially dictate acute lethal temperature. Despite a short treatment duration, we detected a rise in the mRNA level of hypoxia response gene HIF-1α in N. coriiceps heart tissue. One-week exposure to 4°C had no effect on the CT(max) of N. coriiceps, indicating an inability to compensate for rising temperature under these experimental conditions. Our results suggest that icefishes are particularly sensitive to temperature elevation because of a lack of hemoglobin and may be a sentinel taxon for climate change.     

Positive Darwinian selection operating on the immunoglobulin heavy chain of Antarctic fishes

The cooling of the Southern Ocean to the freezing point of seawater (-1.9 degrees C) over the past 25 million years played a dominant selective role in the evolution of the Antarctic fish fauna. During this period, the perciform suborder Notothenioidei, which is largely endemic to the Antarctic, diversified and developed numerous cold-adapted characters. In this report, we provide compelling evidence that the immunoglobulin heavy chain (IgH) of the notothenioid fishes has undergone adaptive selection. Two and four IgH clones were isolated, respectively, from spleen cDNA libraries prepared from the Antarctic icefish Chaenocephalus aceratus and the yellowbelly rockcod Notothenia coriiceps. The transmembrane region of the membrane form of the rockcod IgM heavy chain was located at the end of the second constant (C(H)) domain, in contrast to other teleost IgMs in which the transmembrane region is located at the end of the third constant domain. Phylogenetic analyses of C(H) regions revealed that rates of nonsynonymous nucleotide substitution were higher than rates of synonymous nucleotide substitution. Many of the nonsynonymous substitutions introduced charge changes, consistent with positive Darwinian selection acting to adapt the structure of the notothenioid immunoglobulins. The rates of nonsynonymous nucleotide substitutions were higher than the rates of synonymous nucleotide substitutions in complementarity determining regions of variable regions, suggesting that diversity at antigen binding sites is enhanced by genomic and/or somatic selection. Results of Southern blot hybridization experiments were consistent with a translocon type of IgH gene organization reminiscent of bony fishes and tetrapods.     

Fatty acyl CoA synthetase from Antarctic notothenioid fishes may influence substrate specificity of fat oxidation

Antarctic notothenioid fishes possess large lipid stores that are important fuels for aerobic metabolism. Oxidative muscle tissues of these animals oxidize long-chain mono-unsaturated fatty acids more readily than saturated fatty acids. The mechanistic basis(es) for the substrate specificity of their fatty acid-oxidizing pathway is unknown. We examined the substrate specificity of fatty acyl coenzyme A synthetase (FACS) to determine whether the enzyme contributes to targeting unsaturated fatty acids for preferential transport into mitochondria as fuels for beta-oxidation. Maximal activities of FACS were measured in isolated mitochondria from Notothenia coriiceps and Chaenocephalus aceratus oxidative skeletal muscles in the presence of fatty acids differing in chain lengths and degrees of unsaturation. With the exception of C(22:6), maximal activities were greater with unsaturated substrates than with C(16:0), a saturated fatty acid. Monoenoic fatty acids did not produce the highest activities. Predicted amino acid sequences of FACS from Antarctic C. aceratus, Gobionotothen gibberifrons, and N. coriiceps and sub-Antarctic Notothenia angustata and Eleginops maclovinus were determined to identify amino acid candidates that may be important for determining the substrate specificity of FACS. Substitutions cysteine548 and polar threonine552 within the putative fatty acid binding pocket may contribute to preference for unsaturated fatty acyl substrates compared to saturated fatty acids.     

南极鱼类多样性和适应性进化研究进展

南极地区是地球上唯一未被人类活动大量影响的地区, 其极端寒冷的环境为南极生物的进化提供了“温床”。过去三千万年间, 南极鱼亚目鱼类在南极海洋逐渐变冷的过程中快速进化, 从一个温暖海域的底栖祖先分化成南极海域最为多样化的鱼类类群。由于其在南极圈内和南极圈外的各种温度区间都有分布, 因而成为研究鱼类适应性进化和耐寒机制的良好生物模型。本文综述了有关南极海域鱼类区系组成与物种多样性现状, 南极鱼亚目鱼类适应低温的一系列特化的生物学性状及其关键的遗传进化机制。现有研究表明: 南极鱼类在几千万年零度以下低温环境的进化中发生了大量基因的大规模扩增和基因表达的改变, 如铁调素、卵壳蛋白和逆转座子等118个基因发生了显著的扩增。另外, 有些从南极鱼中获得的抗寒基因已经用于提高动植物低温抗性的研究并取得了良好的效果。在今后的几年中, 将会有多个南极鱼物种的全基因组得到破译, 在低温适应相关基因的功能和进化方面的研究也会更加深入, 这些研究将深入揭示低温压力下基因组的进化规律以及鱼类低温适应的分子机制。     

Subcellular distribution and characterization of gill carbonic anhydrase and evidence for a plasma carbonic anhydrase inhibitor in Antarctic fish

This main purpose of this study was to examine the subcellular distribution and isozyme characteristics of branchial carbonic anhydrase (CA) in Chaenocephalus aceratus, an Antarctic icefish that lacks erythrocytes. The Antarctic fish, Notothenia coriiceps, which possesses erythrocytes, was also studied for comparative purposes. The gills of both species were found to have measurable activity of CA. N. coriiceps also had normal levels of blood CA activity. In contrast, the icefish, C. aceratus, lacked blood CA activity, but was found to possess an endogenous plasma CA inhibitor. The large majority of branchial CA in the gills of these species was located in the cytoplasmic fraction whereas less than 3% was associated with the membrane fraction. In both species, CA from the cytoplasmic gill fraction and membrane fraction differed markedly in terms of their sensitivity to the plasma CA inhibitor from C. aceratus. In addition, treatment with the cleaving enzyme phosphatidylinositol-specific phospholipase C indicated that CA from the branchial membrane fraction of both species is anchored to the membrane via a phosphatidylinositol-glycan linkage. Taken together, these results provide evidence for a CA IV-like isozyme in the gills of Antarctic fish. At present, the functional significance of this membrane-bound CA is unknown, but the relative amount of this isozyme appeared to be greater in the gills of C aceratus, the species that lacked erythrocytes.     

Mitochondrial function in Antarctic nototheniids with ND6 translocation

Fish of the suborder Notothenioidei have successfully radiated into the Southern Ocean and today comprise the dominant fish sub-order in Antarctic waters in terms of biomass and species abundance. During evolution in the cold and stable Antarctic climate, the Antarctic lineage of notothenioids developed several unique physiological adaptations, which make them extremely vulnerable to the rapid warming of Antarctic waters currently observed. Only recently, a further phenomenon exclusive to notothenioid fish was reported: the translocation of the mitochondrial gene encoding the NADH Dehydrogenase subunit 6 (ND6), an indispensable part of complex I in the mitochondrial electron transport system.This study investigated the potential physiological consequences of ND6 translocation for the function and thermal sensitivity of the electron transport system in isolated liver mitochondria of the two nototheniid species Notothenia coriiceps and Notothenia rossii, with special attention to the contributions of complex I (NADH DH) and complex II (Succinate DH) to oxidative phosphorylation. Furthermore, enzymatic activities of NADH:Cytochrome c Oxidoreductase and Cytochrome C Oxidase were measured in membrane-enriched tissue extracts.During acute thermal challenge (0-15°C), capacities of mitochondrial respiration and enzymatic function in the liver could only be increased until 9°C. Mitochondrial complex I (NADH Dehydrogenase) was fully functional but displayed a higher thermal sensitivity than the other complexes of the electron transport system, which may specifically result from its unique amino acid composition, revealing a lower degree of stability in notothenioids in general. We interpret the translocation of ND6 as functionally neutral but the change in amino acid sequence as adaptive and supportive of cold stenothermy in Antarctic nototheniids. From these findings, an enhanced sensitivity to ocean warming can be deduced for Antarctic notothenioid fish.     

Evolution of genome size in the angiosperms

Genome size varies extensively across the flowering plants, which has stimulated speculation regarding the ancestral genome size of these plants and trends in genome evolution. We investigated the evolution of C-values across the angiosperms using a molecular phylogenetic framework and C-values not previously available for crucial basal angiosperms, including Amborella, Illiciaceae, and Austrobaileya. Reconstructions of genome size across the angiosperms and extant gymnosperms indicate that the ancestral genome size for angiosperms is very small (1C ≤ 1.4 pg), in agreement with an earlier analysis of Leitch et al. (1998). Furthermore, a very small genome size (1C ≤ 1.4 pg) is ancestral not only for the angiosperms in general, but also for most major clades of flowering plants, including the monocots and the eudicots. The ancestral genome of core eudicots may also have been very small given that very low 1C-values appear to be ancestral for major clades of core eudicots, such as Caryophyllales, Saxifragales, and asterids. Very large genomes occur in clades that occupy derived positions within the monocots and Santalales.     

Antarctic fish tubulins: heterogeneity, structure, amino acid compositions and charge

1. Tubulins purified from the brain tissues of three Antarctic fishes (Notothenia gibberifrons, Notothenia coriiceps neglecta, and Chaenocephalus aceratus) contain equimolar quantities of the alpha and beta chains and are free of microtubule-associated proteins (MAPs) and other non-tubulin proteins. 2. When examined by isoelectric focusing and by two-dimensional electrophoresis, brain tubulins from the Antarctic fishes were found to be highly heterogeneous; each was resolved into 15-20 distinct variants. The range of isoelectric points displayed by the Antarctic fish tubulins (5.30-5.75) is slightly more basic than that of bovine brain tubulin (5.25-5.60). 3. Peptide mapping demonstrated that tubulins from the Antarctic fishes and the cow differ in structure. 4. The amino acid compositions of piscine and mammalian tubulins are similar, but the Antarctic fish tubulins apparently contain fewer glutamyl and/or glutaminyl residues than do tubulins from the temperate channel catfish (Ictalurus punctatus) and the cow. 5. Native tubulin from N. coriiceps neglecta possesses 1-2 fewer net negative charges per tubulin dimer than does bovine tubulin. 6. We suggest that the enhanced assembly of Antarctic fish tubulins at low temperatures (-2 to +2 degrees C) results from adaptive, perhaps subtle, changes in their tubulin subunits.     

Large genomes among caridean shrimp.

Recent genome size estimates for Arctic amphipods have revealed the largest genomes known in the Crustacea. Here we provide additional data for 7 species of caridean shrimp collected from the Canadian Arctic and the Gulf of St. Lawrence. Genome sizes were estimated by flow cytometry and haploid C-values ranged from 8.53 +/- 0.30 pg in Pandalus montagui (Pandalidae) to 40.89 +/- 1.23 pg in Sclerocrangon ferox (Crangonidae). The value for S. ferox represents the largest decapod genome yet recorded and indicates a 38-fold variation in genome size within this order. These data suggest that large genomes may be relatively common in Arctic crustaceans, and underline the need for further comparative studies.     

Nuclear DNA amounts in Macaronesian angiosperms

Nuclear DNA contents for 104 Macaronesian angiosperms, with particular attention on Canary Islands endemics, were analysed using propidium iodide flow cytometry. Prime estimates for more than one-sixth of the whole Canarian endemic flora (including representatives of 11 endemic genera) were obtained. The resulting 1C DNA values ranged from 0.19 to 7.21 pg for Descurainia bourgeauana and Argyranthemum frutescens, respectively (about 38-fold difference). The majority of species, however, possessed (very) small genomes, with C-values <1.6 pg. The tendency towards small nuclear DNA contents and genome sizes was confirmed by comparing average values for Macaronesian and non-Macaronesian representatives of individual families, genera and major phylogenetic lineages. Our data support the hypothesis that the insular selection pressures in Macaronesia favour small C-values and genome sizes. Both positive and negative correlations between infrageneric nuclear DNA amount variation and environmental conditions on Tenerife were also found in several genera.     

Nuclear DNA Amounts in Pteridophytes

DNA amounts (C-value and genome size) are much-used biodiversitycharacters. A workshop held at Kew in 1997 identified majorgaps in our knowledge of plant DNA amounts, recommending targetsfor new work to fill them. Murray reviewed non-angiosperm plantsnoting that representation of pteridophyte species (approx.0.42%) was poor, while locating C-value data for them was verydifficult. The workshop confirmed the need to make data forother groups besides angiosperms accessible for reference purposes.This paper pools DNA C-values for 48 pteridophyte species fromeight original sources into one reference source, and fulfilsa key workshop recommendation for this group. Comparing thesedata shows that nuclear 1C-values in pteridophytes vary approx.1000-fold, from 0.055 pg in Selaginella species to about 55pg in Ophioglossum petiolatum. Genome size estimates for 25pteridophytes vary approx. 200-fold from 0.055 to 10.7 pg, andthe mean genome sizes in diploids and polyploids (5.15 and 4.59pg, respectively) are not significantly different. Wider comparisonsshow that ranges of genome sizes in the major groups of landplants are very different. Those in bryophytes and pteridophytesare narrow compared with those in gymnosperms and angiosperms.The data indicate that the origin of land plants possibly involveda first major increase in genome size in the evolution of vascularplants, while a second such increase occurred later in gymnosperms.C-values for pteridophytes remain very few, but conversely opportunitiesfor new work on them are many. Copyright 2001 Annals of BotanyCompany Pteridophyte DNA amounts, DNA C-values, nuclear genome sizes     

Two species of antarctic icefishes (genus <Emphasis Type="Italic">Champsocephalus</Emphasis>) share a common genetic lesion leading to the loss of myoglobin expression

Species of the suborder Notothenioidei dominate the fish fauna of coastal Antarctic waters. Members of one notothenioid family, Channichthyidae (Antarctic icefishes), are unique among all vertebrates in lacking the circulating oxygen-binding protein hemoglobin. Icefish species also do not uniformly express the intracellular oxygen-binding protein myoglobin (Mb) in their oxidative muscles. Our laboratory previously characterized the pattern of cardiac Mb expression in 13 of the 16 known icefish species. In this paper, we complete the survey of cardiac Mb expression among all 16 known species of icefishes. Using PAGE and immunoblot analyses, we demonstrate that both Channichthys rhinoceratus and Cryodraco atkinsoni express Mb in heart ventricle, while Champsocephalus esox does not express the protein. We report Mb gene sequences from Channichthys rhinoceratus and Champsocephalus esox genomic DNA. The Mb gene of C. esox contains the identical 5-bp duplication/insertion to that observed in congeneric Champsocephalus gunnari, a species that also does not produce Mb. This duplication in exon 2 of the Champsocephalus spp. gene causes a shift in reading frame at a position normally encoding for amino acid 91 and also results in a premature stop codon, thus disrupting translation of the normal protein. Thus, 6 of the 16 known icefish species do not express cardiac Mb. These results confirm earlier conclusions that losses of Mb expression have occurred via at least four independent events during the evolution of the icefish family. Extreme similarity of Mb genes in Champsocephalus congeners further suggests recent speciation despite early divergence of this group from the lineage leading to more derived icefishes.     

The 19 genomes of Drosophila: a BAC library resource for genus-wide and genome-scale comparative evolutionary research

The genus Drosophila has been the subject of intense comparative phylogenomics characterization to provide insights into genome evolution under diverse biological and ecological contexts and to functionally annotate the Drosophila melanogaster genome, a model system for animal and insect genetics. Recent sequencing of 11 additional Drosophila species from various divergence points of the genus is a first step in this direction. However, to fully reap the benefits of this resource, the Drosophila community is faced with two critical needs: i.e., the expansion of genomic resources from a much broader range of phylogenetic diversity and the development of additional resources to aid in finishing the existing draft genomes. To address these needs, we report the first synthesis of a comprehensive set of bacterial artificial chromosome (BAC) resources for 19 Drosophila species from all three subgenera. Ten libraries were derived from the exact source used to generate 10 of the 12 draft genomes, while the rest were generated from a strategically selected set of species on the basis of salient ecological and life history features and their phylogenetic positions. The majority of the new species have at least one sequenced reference genome for immediate comparative benefit. This 19-BAC library set was rigorously characterized and shown to have large insert sizes (125-168 kb), low nonrecombinant clone content (0.3-5.3%), and deep coverage (9.1-42.9×). Further, we demonstrated the utility of this BAC resource for generating physical maps of targeted loci, refining draft sequence assemblies and identifying potential genomic rearrangements across the phylogeny.     

A comparative study of protein synthesis in nototheniids and icefish at Palmer Station, Antarctica

Protein synthetic rates were measured in tissues of Notothenia corriceps, N. gibberifrons and Chaenocephalus aceratus in vivo at 2 degrees C by a method in which high doses of 14C-phenylalanine are used for stabilization of specific radioactivity. Rates in N. coriiceps, as per cent of tissue protein synthesized per day, were: liver 10.4, head kidney 3.5, testis 2.6, spleen 2.1, kidney 1.9, gill 1.6, heart 1.4, pectoral muscle 1.0, epaxial muscle 0.37, brain 0.42. With the exception of liver and head kidney (9.8 and 3.4, respectively) all rates in the icefish C. aceratus were significantly reduced compared to the nototheniids, consistent with the dependence of protein synthesis on oxidative metabolism. Icefish lack hemoglobin in the blood. The effects of two-week starvation were tissue-specific. Rates declined markedly in pectoral and epaxial muscle, were unchanged in liver, kidney, brain, heart and testis, and were increased in gill and head kidney. The results are discussed in relation to cold adaptation of Antarctic fishes and to the adaptation of metabolism required during non-feeding periods and for species which lack an oxygen-binding pigment in their blood.     

Sex determination in Antarctic notothenioid fish: chromosomal clues and evolutionary hypotheses

In fish, the determination of sex can be controlled by genetic factors, environmental factors or a combination of both. The presence of heteromorphic sex-related chromosomes is widely acknowledged as strongly indicative of genetic control of sex determination (GSD) acting over other sex control systems. Heteromorphic sex-related chromosomes have been observed in a minority of teleosts (approximately 4 %). However, when looking at the fishes of the suborder Notothenioidei the frequency of sex-related chromosomes increases substantially, reaching 26.67 % of the cytogenetically studied species. Noteworthy, sex chromosomes were observed only in cold-adapted species which live in the Antarctic coastal waters, whereas morphologically differentiated sex chromosomes were never observed in the temperate non-Antarctic notothenioid families. Recent evidence suggests that the sex-linked chromosomes across the Antarctic notothenioid families may not share a common origin, but likely originated more than once during notothenioid evolutionary history, thus implying the presence of selection pressures operating toward fixation of GSD system. On the whole, the cytogenetic evidences suggest the Antarctic-specific fixation of differentiated heteromorphic sex-related chromosomes and of a prominent GSD across Antarctic notothenioids that may be an additional manifestation of notothenioid evolution in thermally stable cold environment.