The quantity and quality of the haemoglobin (Hb) of Daphnia magna is related to oxygen partial pressure in the water. Both the dynamics of hypoxia-induced Hb gene transcription, as well as Hb properties in animals incubated long-term at hyperoxia, normoxia and hypoxia, were investigated. Examination of Hb gene (dhb1-dhb3) transcription showed the expression of dhb2 and especially dhb3 to increase markedly approximately one hour after the onset of hypoxia, whereas dhb1 was expressed more or less constitutively. At an incubation close to anoxia, an onset of dhb3 transcription was found already after two minutes. In long-term incubated animals, concentration and oxygen affinity of Hb were lower at higher oxygen partial pressures. With decreasing oxygen availability, the subunit composition of Hb macromolecules changed. The share of the dhb2-encoded subunit, DHbF, increased already during moderate hypoxia. The increase of dhb3 mRNA (encoding DHbC) may be related to a transient increase of DHbC in the first days of hypoxia and/or to an additional coding of dhb3 for DHbD. The rise of DHbD, and particularly DHbA, only at severe hypoxia coincided with the increase of Hb oxygen affinity. The dhb1-encoded subunits DHbB and DHbE showed either a relatively moderate increase or even a decrease in concentration at hypoxia. In small animals with restricted homeostasis capabilities such as Daphnia, adaptation of the protein equipment seems to be a more effective strategy than allosteric modulator control.
To evaluate the role of temperature in oxygen transport in the water flea Daphnia magna, temperaturerelated behaviour as well as oxygen-transport physiology and biochemistry were investigated in animals long-term acclimated to moderate temperatures: 10, 20, or 30°C (normoxia) or 20°C (hypoxia). Study of the behaviour of animal groups within a normoxic thermal gradient showed their preferred temperatures and distribution patterns to be connected with acclimation conditions; for example, cold-acclimated individuals, with their relatively high metabolic rates and low oxygen-transport capacities, crowded at low temperatures where the oxygen concentration was high. One key to explaining these data is to assume a strategy of avoiding oxygen shortage. Both temperature and hypoxia acclimation also modified oxygen-transport variables such as oxygen consumption, ventilation and circulation rates, or the concentration and oxygen affinity of haemoglobin. Characteristic features of cold-acclimated D. magna were relatively high oxygen-consumption and heart rates and a low concentration and oxygen affinity of haemoglobin. Hypoxia-acclimated D. magna showed relatively low convective rates (frequency of thoracic-limb movements, heart rate) but a high concentration and oxygen affinity of haemoglobin. Additional experiments showed that independently of each other, temperature and ambient oxygen concentration modify haemoglobin quantity and quality, the former possibly acting via a temperature-induced hypoxia. The varying oxygen affinity is probably due to alterations of haemoglobin subunit composition, with the particular participation of subunits A, E, and G as revealed by two-dimensional gel electrophoresis.Résumé : L'étude du comportement relié à la température ainsi que de la physiologie et de la biochimie du transport de l'oxygène chez des cladocères Daphnia magna acclimatés à long terme à des températures modérées -10, 20, 30°C (en normoxie) ou 20 EC (en anoxie) -nous a permis d'évaluer le rôle de la température en rapport avec le transport de l'oxygène. Le comportement de groupes d'animaux dans un gradient thermique normoxique montre que les choix de température et les patterns de répartition sont reliés aux conditions d'acclimatation; par exemple, les individus acclimatés au froid, qui ont des taux métaboliques relativement élevés et des capacités réduites de transport d'oxygène, se regroupent dans les zones de température préférentielle basse où les concentrations d'oxygène sont éle-vées. Une façon d'expliquer ces données est d'assumer qu'il y a une stratégie d'évitement des pénuries d'oxygène. L'acclimatation tant à la température qu'à l'hypoxie modifie aussi les variables reliées au transport de l'oxygène, telles que la consommation d'oxygène, les taux de ventilation et de circulation ou alors la concentration de l'hémoglobine et son affinité pour l'oxygène. Les caractéristiques des cladocères acclimatés au froid comprennent une consommation d'oxygène et un rythme cardiaque relativement élevés et une concentration basse ...
1. In a combined field and laboratory study, seasonal relationships between water temperature and oxygen content, genetic structure (composition of MultiLocus Genotypes, MLGs) of a Daphnia assemblage (D. galeata-hyalina hybrid species complex), and the physiological properties of clones of frequent MLGs were studied. In accordance with the oxygen-limited thermal tolerance hypothesis, essential physiological variables of oxygen transport and supply were measured within the tolerable temperature range. 2. A few MLGs (types T1-T4) were frequent during early spring and late autumn at surface temperatures below 10°C. Clones of T1-T4 showed a low tolerance towards higher temperatures (above 20°C) and a high phenotypic plasticity under thermal acclimation in comparison to clones derived from frequent MLGs from later seasons, and stored highmedium quantities of carbohydrates at 12 and 18°C. 3. Another MLG (T6) succeeded the MLGs T1-T4. T6 was frequent over most of the year at temperatures above 10°C and below 20°C. A clone derived from T6 exhibited a high tolerance towards warm temperatures and a more restricted phenotypic plasticity. It stored high-medium quantities of carbohydrates at 12, 18 and 24°C and showed a high capacity for acclimatory adjustments based on haemoglobin expression. 4. During the summer period at temperatures ‡20°C, the MLG T6 was found mainly near to the thermocline, where temperature and oxygen content were distinctly lower, and to a lesser extent in surface water. At the surface, another MLG (T19) was predominant during this period. A clone of this MLG showed a very high tolerance towards warm temperatures, minimal phenotypic plasticity, low carbohydrate stores and a high capacity for circulatory adjustments to improve oxygen transport at higher temperatures. 5. This study provides evidence for connections between the spatio-temporal genetic heterogeneity of a Daphnia assemblage and the seasonal changes of water temperature and oxygen content. The data also suggest that not only the actual temperature but also the dynamics of temperature change may influence the genetic structure of Daphnia populations and assemblages.
BackgroundFreshwater planktonic crustaceans of the genus Daphnia show a remarkable plasticity to cope with environmental changes in oxygen concentration and temperature. One of the key proteins of adaptive gene control in Daphnia pulex under hypoxia is hemoglobin (Hb), which increases in hemolymph concentration by an order of magnitude and shows an enhanced oxygen affinity due to changes in subunit composition. To explore the full spectrum of adaptive protein expression in response to low-oxygen conditions, two-dimensional gel electrophoresis and mass spectrometry were used to analyze the proteome composition of animals acclimated to normoxia (oxygen partial pressure [Po2]: 20 kPa) and hypoxia (Po2: 3 kPa), respectively.ResultsThe comparative proteome analysis showed an up-regulation of more than 50 protein spots under hypoxia. Identification of a major share of these spots revealed acclimatory changes for Hb, glycolytic enzymes (enolase), and enzymes involved in the degradation of storage and structural carbohydrates (e.g. cellubiohydrolase). Proteolytic enzymes remained constitutively expressed on a high level.ConclusionAcclimatory adjustments of the D. pulex proteome to hypoxia included a strong induction of Hb and carbohydrate-degrading enzymes. The scenario of adaptive protein expression under environmental hypoxia can be interpreted as a process to improve oxygen transport and carbohydrate provision for the maintenance of ATP production, even during short episodes of tissue hypoxia requiring support from anaerobic metabolism.
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