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.
Background: Temperature affects essentially every aspect of the biology of poikilothermic animals including the energy and mass budgets, activity, growth, and reproduction. While thermal effects in ecologically important groups such as daphnids have been intensively studied at the ecosystem level and at least partly at the organismic level, much less is known about the molecular mechanisms underlying the acclimation to different temperatures. By using 2D gel electrophoresis and mass spectrometry, the present study identified the major elements of the temperatureinduced subset of the proteome from differently acclimated Daphnia pulex.
The function of hemoglobin (Hb), which is a key protein of many species, was analyzed in Daphnia from the field. Temperature and oxygen content; abundance and composition of phytoplankton; and abundance, body, and clutch sizes of the Daphnia assemblage were measured in the epilimnion and around the thermocline of a reservoir for 2 yr. In addition, carbohydrate, lactate, and hemoglobin concentrations were determined in whole Daphnia. In Daphnia from the thermocline, hypoxia-induced Hb expression was responsible for an increase in Hb concentration in parallel with a decrease in oxygen content. In Daphnia from oxygen-rich water (epilimnion), however, Hb concentration unexpectedly showed a negative relationship to temperature and to the animals' temperature-dependent metabolic rate. Furthermore, the seasonal minima of Hb concentration coincided with low food (phytoplankton) availability and maxima of Daphnia's reproductive activity. These data suggest that Hb serves a dual function as respiratory protein and protein store. Laboratory experiments supported this hypothesis: starvation caused the concentration of Hb and other hemolymph proteins to decrease during normoxia, whereas the hemolymph protein store remained stable during hypoxia. Hemolymph proteins obviously function as a buffer between food (protein) availability and protein demand, with the latter increasing with temperature mainly because of elevated growth and reproduction rates. Accordingly, the size of the hemolymph protein store can indicate the state of nutrition of Daphnia (and possibly other species) from oxygen-rich water, which can easily be assessed by spectroscopic measurement of Hb in whole animals.
The function of hemoglobin (Hb), which is a key protein of many species, was analyzed in Daphnia from the field. Temperature and oxygen content; abundance and composition of phytoplankton; and abundance, body, and clutch sizes of the Daphnia assemblage were measured in the epilimnion and around the thermocline of a reservoir for 2 yr. In addition, carbohydrate, lactate, and hemoglobin concentrations were determined in whole Daphnia. In Daphnia from the thermocline, hypoxia‐induced Hb expression was responsible for an increase in Hb concentration in parallel with a decrease in oxygen content. In Daphnia from oxygen‐rich water (epilimnion), however, Hb concentration unexpectedly showed a negative relationship to temperature and to the animals’ temperature‐dependent metabolic rate. Furthermore, the seasonal minima of Hb concentration coincided with low food (phytoplankton) availability and maxima of Daphnia’s reproductive activity. These data suggest that Hb serves a dual function as respiratory protein and protein store. Laboratory experiments supported this hypothesis: starvation caused the concentration of Hb and other hemolymph proteins to decrease during normoxia, whereas the hemolymph protein store remained stable during hypoxia. Hemolymph proteins obviously function as a buffer between food (protein) availability and protein demand, with the latter increasing with temperature mainly because of elevated growth and reproduction rates. Accordingly, the size of the hemolymph protein store can indicate the state of nutrition of Daphnia (and possibly other species) from oxygen‐rich water, which can easily be assessed by spectroscopic measurement of Hb in whole animals.
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