Zooplankton of Lake Peipsi-Pihkva in 1909–1987

164 taxa were identified in the net zooplankton of the pelagial of L. Peipsi-Pihkva in 1909–1987, including 3 species of protozoans, 74 species of rotifers, 58 species of cladocerans, 28 species of copepods and 1 mollusc. One rotifer species, Ploesoma peipsiense Mäemets et Kutikova, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Estonia: Limnosida frontosa, Drepanothrix dentata, Bythotrephes longimanus, B. cederstroemi etc. Due to its large surface area, L. Peipsi-Pihkva provides a large scale of biotopes of a diverse trophic state and humic content, which support species with different ecological requirements. Most of the aquatory of the lake has lately been mesotrophic, favouring the coexistence of indicators of oligo- and mesotrophic state and species preferring a higher trophic state. The occurrece of 10 species of the genus Bosmina including B. berolinensis, B. gibbera, B. lilljeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, is the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lilljeborgi etc. which were earlier regarded as subspecies of B. c. coregoni proves that they are different species producing usually no hybrids. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zooplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909–1964, but was lacking in later samples.     

The Chromosomal Organization of the Human Endogenous Retrovirus-like Sequence HERV-H: Clustering of the HERV-H Sequences in a 300-kb Region Close to the GRPR Locus on the X Chromosome

Within the haploid genome there are approximately 1,000 copiesof the human endogenous retroviruslike sequence, HERV-H. Althoughthese sequences are scattered throughout the entire genome,in situ hybridization experiments revealed that there are discreteclusters positioned on chromosomes 1p and 7q. In this study,we have located three HERV-H sequences which were unexpectedlyclustered within a 300-kilobase region close to the GRPR locuson the X chromosome. In previous studies, no clusteringof thissequence has been reported at this locus. Our finding demonstratesthat, like other repetitive sequences, clustering of HERV-Hoccurs in the human genome, although these sequences may notalways be detected by in situ hybridization methods.     

Expression of Human Glucose Transporter Type 1 and Rat Hexokinase Type II Complementary DNAs in Rainbow Trout Embryos: Effects on Glucose Metabolism

Sugars are utilized poorly in fish mainly because of low rates of transport across plasma membrane and phosphorylation. To evaluate whether it is possible to augment carbohydrate metabolism in fish using heterologous genes, expression of human glucose transporter type 1 (hGLUT1) and rat hexokinase type II (rHKII) complementary DNAs cloned with cytomegalovirus promoter was followed in rainbow trout embryos. Both genes were transcribed. Hexokinase activity, undetectable in control, was found in transformed blastulas. Increased rates of ¹⁴C-methylglucose uptake and sensitivity to cytochalasin B indicated the presence of facilitative hexose transport due to hGLUT1 expression. Effect of hGLUT1 on production of ¹⁴CO₂ from glucose was greater than that of rHKII. Coexpression of the genes did not increase the rate of glucose oxidation compared with expression of hGLUT1 alone. Received; accepted June 30, 1998.     

Mannose inhibits hyaluronan synthesis by down-regulation of the cellular pool of UDP-N-acetylhexosamines

We found that d-mannose dose-dependently decreases hyaluronan synthesis in cultured epidermal keratinocytes to approximately 50%, whereas glucose, galactose, and fructose up to 20 mm concentration had no effect. The full inhibition occurred within 3 h following introduction of mannose and did not involve down-regulation of hyaluronan synthase (Has1-3) mRNA. Following introduction of mannose, there was an approximately 50% reduction in the cellular concentration of UDP-N-acetylhexosamines (UDP-HexNAc, i.e. UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine). On the other hand, 2 mm glucosamine in the culture medium increased UDP-HexNAc content, stimulated hyaluronan secretion, and negated the effect of mannose, supporting the notion that the inhibition by mannose on hyaluronan synthesis was because of down-regulated UDP-HexNAc content. The content of UDP-glucuronic acid, the other building block for hyaluronan synthesis, was not reduced by mannose but declined from 39 to 14% of controls by 0.2-1.0 mm 4-methylumbelliferone, another compound that inhibits hyaluronan synthesis. Applying 4-methylumbelliferone and mannose together produced the expected reductions in both UDP sugars but no additive reduction in hyaluronan production, indicating that the concentration of each substrate alone can limit hyaluronan synthesis. Mannose is a potentially useful tool in studies on hyaluronan-dependent cell functions, as demonstrated by reduced rates of keratinocyte proliferation and migration, functions known to depend on hyaluronan synthesis.     

Frequency and density-dependent selection on life-history strategies--a field experiment

Negative frequency-dependence, which favors rare genotypes, promotes the maintenance of genetic variability and is of interest as a potential explanation for genetic differentiation. Density-dependent selection may also promote cyclic changes in frequencies of genotypes. Here we show evidence for both density-dependent and negative frequency-dependent selection on opposite life-history tactics (low or high reproductive effort, RE) in the bank vole (Myodes glareolus). Density-dependent selection was evident among the females with low RE, which were especially favored in low densities. Instead, both negative frequency-dependent and density-dependent selection were shown in females with high RE, which were most successful when they were rare in high densities. Furthermore, selection at the individual level affected the frequencies of tactics at the population level, so that the frequency of the rare high RE tactic increased significantly at high densities. We hypothesize that these two selection mechanisms (density- and negative frequency-dependent selection) may promote genetic variability in cyclic mammal populations. Nevertheless, it remains to be determined whether the origin of genetic variance in life-history traits is causally related to density variation (e.g. population cycles).     

CX3CR1 protein signaling modulates microglial activation and protects against plaque-independent cognitive deficits in a mouse model of Alzheimer disease

Aberrant microglial activation has been proposed to contribute to the cognitive decline in Alzheimer disease (AD), but the underlying molecular mechanisms remain enigmatic. Fractalkine signaling, a pathway mediating the communication between microglia and neurons, is deficient in AD brains and down-regulated by amyloid-β. Although fractalkine receptor (CX3CR1) on microglia was found to regulate plaque load, no functional effects have been reported. Our study demonstrates that CX3CR1 deficiency worsens the AD-related neuronal and behavioral deficits. The effects were associated with cytokine production but not with plaque deposition. Ablation of CX3CR1 in mice overexpressing human amyloid precursor protein enhanced Tau pathology and exacerbated the depletion of calbindin in the dentate gyrus. The levels of calbindin in the dentate gyrus correlated negatively with those of tumor necrosis factor α and interleukin 6, suggesting neurotoxic effects of inflammatory factors. Functionally, removing CX3CR1 in human amyloid precursor protein mice worsened the memory retention in passive avoidance and novel object recognition tests, and their memory loss in the novel object recognition test is associated with high levels of interleukin 6. Our findings identify CX3CR1 as a key microglial pathway in protecting against AD-related cognitive deficits that are associated with aberrant microglial activation and elevated inflammatory cytokines.     

Polyketide derivatives active against Botrytis cinerea in Gerbera hybrida

A previously isolated cDNA molecule from Gerbera hybrida (Asteraceae) codes for a new chalcone synthase-like polyketide synthase, 2-pyrone synthase (2PS). 2PS is able to synthesise 4-hydroxy-6-methyl-2-pyrone (triacetolactone), a putative precursor for gerberin and parasorboside, two abundant glucosides in gerbera. In this study, we show that gerbera plants transformed with the gene for 2PS in an antisense orientation and unable to synthesise gerberin and parasorboside are susceptible to Botrytis cinerea infection. In addition to the preformed glucosides, the transgenic plants also lack several compounds that are induced in control plants when infected with the mould. Some of these induced substances are effective in inhibiting fungal growth both in vitro and in vivo. Two of the phytoalexins were identified as the aglycones of gerberin and trans-parasorboside. The third phytoalexin is a rare coumarin, 4-hydroxy-5-methylcoumarin; however, it is typical of many plants of the sunflower family Asteraceae. The coumarin cannot be structurally derived from either gerberin or parasorboside, but may be derived from a related polyketide intermediate.     

Modelling Nutrient Uptake by Individual Hyphae of Arbuscular Mycorrhizal Fungi: Temporal and Spatial Scales for an Experimental Design

Arbuscular mycorrhizas, associations between plant roots and soil fungi, are ubiquitous among land plants. Arbuscular mycorrhizas can be beneficial for plants by overcoming limitations in nutrient supply. Hyphae, which are long and thin fungal filaments extending from the root surface into the soil, increase the volume of soil accessible for plant nutrient uptake. However, no models so far specifically consider individual hyphae. We developed a mathematical model for nutrient uptake by individual fungal hyphae in order to assess suitable temporal and spatial scales for a new experimental design where fungal uptake parameters are measured on the single hyphal scale. The model was developed based on the conservation of nutrients in an artificial cylindrical soil pore (capillary tube) with adsorbing wall, and analysed based on parameter estimation and non-dimensionalisation. An approximate analytical solution was derived using matched asymptotic expansion. Results show that nutrient influx into a hypha from a small capillary tube is characterized by three phases: Firstly, uptake rapidly decreases as the hypha takes up nutrients, secondly, the depletion zone reaches the capillary wall and thus uptake is sustained by desorption of nutrients from the capillary wall, and finally, uptake goes to zero after nutrients held on the capillary wall have been completely depleted. Simulating different parameter regimes resulted in recommending the use of capillaries filled with hydrogel instead of water in order to design an experiment operating over measurable time scales.     

Micronuclei, hemoglobin adducts and respiratory tract irritation in mice after inhalation of toluene diisocyanate (TDI) and 4,4'-methylenediphenyl diisocyanate (MDI)

Toluene diisocyanate (TDI) and 4,4'-methylenediphenyl diisocyanate (MDI), used in the production of polyurethane foam, are well known for their irritating and sensitizing properties. Contradictory results have been obtained on their genotoxicity. We investigated the genotoxicity and protein binding of inhaled TDI and MDI in mice by examining micronucleated polychromatic erythrocytes (PCEs) in bone marrow and peripheral blood and TDI- and MDI-derived adducts in hemoglobin. Male C57Bl/6J mice (8 per group) were exposed head-only to TDI vapour (mean concentrations 1.1, 1.5, and 2.4mg/m(3); the mixture of isomers contained, on the average, 63% 2,4-TDI and 37% 2,6-TDI) or MDI aerosol (mean concentrations 10.7, 20.9 and 23.3mg/m(3)), during 1h/day for 5 consecutive days. Bone marrow and peripheral blood were collected 24h after the last exposure. Inhalation of TDI caused sensory irritation (SI) in the upper respiratory tract, and cumulative effects were observed at the highest exposure level. Inhalation of MDI produced SI and airflow limitation, and influx of inflammatory cells into the lungs. Hemoglobin adducts detected in the exposed mice resulted from direct binding to globin of 2,4- and 2,6-TDI and MDI, and dose-dependent increases were observed especially for 2,4-TDI-derived adducts. Adducts originating from the diamines of TDI (toluene diamine) or MDI (methylene dianiline) were not observed. No significant increase in the frequency of micronucleated PCEs was detected in the bone marrow or peripheral blood of the mice exposed to TDI or MDI. The ratio of PCEs and normochromatic erythrocytes (NCEs) was reduced at the highest concentration of MDI, and a slight reduction of the PCE/NCE ratio, dependent on cumulative inhaled dose, was also seen with TDI. Our results indicate that inhalation of TDI or MDI (1h/day for 5 days), at levels that induce toxic effects and formation of TDI- or MDI-specific adducts in hemoglobin, does not have detectable genotoxic effects in mice, as studied with the micronucleus assay.