The effect of a microgravity (space) environment on the expression of expansins from the peg and root tissues of Cucumis sativus

In young cucumber seedlings, the peg is a polar outgrowth of tissue that functions by snagging the seed coat, thereby freeing the cotyledons. The development of the peg is thought to be gravity-dependent and has become a model system for plant-gravity response. Peg development requires rapid cell expansion, a process thought to be catalyzed by α -expansins, and thus was a good system to identify expansins that were regulated by gravity. This study identified 7 new α -expansin cDNAs from cucumber seedlings ( Cucumis sativus L. cv Burpee Hybrid II) and examined their expression patterns. Two α -expansins ( CsExp3 and CsExp4 ) were more highly expressed in the peg and the root. Earlier reports stated that pegs tend not to form in the absence of gravity, so the expression levels were compared in the pegs of seedlings grown in space (STS-95), on a clinostat, and on earth (1 g ). Pegs were observed to form at high frequency on clinostat and space-grown seedlings, yet on clinostats there was more than a 4-fold reduction in the expression of CsExp3 in the pegs of seedlings grown on clinostats vs. those grown at 1 g , while the CsExp4 gene appeared to be turned off (below detection limits). There were no detectable differences in expansin gene expression levels for the pegs of seedlings grown in space or in the orbiter environmental simulator (OES) (1 g ) at NASA. The microgravity environment did not affect the expression of CsExp3 or CsExp4 , and the clinostat did not simulate the microgravity environment well.     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Mechanisms contributing to T cell receptor signaling and assembly revealed by the solution structure of an ectodomain fragment of the CD3 epsilon gamma heterodimer.

The T cell receptor (TCR) consists of genetically diverse disulfide-linked alpha and beta chains in noncovalent association with the invariant CD3 subunits. CD3 epsilon and CD3 gamma are integral components of both the TCR and pre-TCR. Here, we present the solution structure of a heterodimeric CD3 epsilon gamma ectodomain complex. A unique side-to-side hydrophobic interface between the two C2-set immunoglobulin-like domains and parallel pairing of their respective C-terminal beta strands are revealed. Mutational analysis confirms the importance of the distinctive linkage as well as the membrane proximal stalk motif (RxCxxCxE) for domain-domain association. These biochemical and structural analyses offer insights into the modular pairwise association of CD3 invariant chains. More importantly, the findings suggest how the rigidified CD3 elements participate in TCR-based signal transduction.     

Sexual reproduction of Lolium perenne L. and Trifolium repens L. under free air CO2 enrichment (FACE) at two levels of nitrogen application

Field experiments in managed grassland have shown that the response of vegetative growth to elevated CO₂ is nitrogen‐dependent in grasses, but independent in N₂‐fixing legumes. In the present study, we tested whether this is also true for reproduction. We evaluated reproductive growth, flowering phenology, seed development, reproductive success and seed germination in the grass Lolium perenne L. and the legume Trifolium repens L., growing in monocultures in a free air carbon dioxide enrichment (FACE) system at ambient (35 Pa) and elevated (60 Pa) partial pressure of CO₂ and two levels of nitrogen fertilization (14 and 56 g N m^?2 a^?1). In both species, elevated CO₂ had no significant effect on sexual reproduction. In L. perenne, reproduction was mainly nitrogen‐dependent. The weak interactions between CO₂ and mineral N supply (13% more flowers and 8% more grains per spike at high N, 7% less flowers and 8% less grains at low N) were not significant. Under elevated CO₂, grain maturation was slightly enhanced and grain weight tended to decrease. No influence could be ascertained in the date of anthesis, the temporal pattern of grain growth, the rate of grain abortion and germination. Trifolium repens, grown under CO₂ enrichment at both levels of N fertilization, flowered 10 d earlier, tended to form more inflorescences per ground area and more flowers (8–12%) and seeds (>18%) per inflorescence than at ambient CO₂. The temporal pattern of seed growth was about the same in all treatments; embryo development, however, was accelerated in fumigated plants. The number of aborted seeds per pod, seed size, thousand‐seed weight and germinability did not show any influence of CO₂. Fumigated plants at high N were attacked slightly more frequently by seed‐eating weevils, which lowered the seed output per pod. In summary, the reproductive response of L. perenne and T. repens to CO₂ enrichment on the flower and inflorescence level was far weaker than expected from the results on vegetative growth.     

Comparison of Shiga Toxin Production by Hemolytic-Uremic Syndrome-Associated and Bovine-Associated Shiga Toxin-Producing Escherichia coli Isolates

There is considerable diversity among Shiga toxin (Stx)-producing Escherichia coli (STEC) bacteria, and only a subset of these organisms are thought to be human pathogens. The characteristics that distinguish STEC bacteria that give rise to human disease are not well understood. Stxs, the principal virulence determinants of STEC, are thought to account for hemolytic-uremic syndrome (HUS), a severe clinical consequence of STEC infection. Stxs are typically bacteriophage encoded, and their production has been shown to be enhanced by prophage-inducing agents such as mitomycin C in a limited number of clinical STEC isolates. Low iron concentrations also enhance Stx production by some clinical isolates; however, little is known regarding whether and to what extent these stimuli regulate Stx production by STEC associated with cattle, the principal environmental reservoir of STEC. In this study, we investigated whether toxin production differed between HUS- and bovine-associated STEC strains. Basal production of Stx by HUS-associated STEC exceeded that of bovine-associated STEC. In addition, following mitomycin C treatment, Stx2 production by HUS-associated STEC was significantly greater than that by bovine-associated STEC. Unexpectedly, mitomycin C treatment had a minimal effect on Stx1 production by both HUS- and bovine-associated STEC. However, Stx1 production was induced by growth in low-iron medium, and induction was more marked for HUS-associated STEC than for bovine-associated STEC. These observations reveal that disease-associated and bovine-associated STEC bacteria differ in their basal and inducible Stx production characteristics.     

Microbial Community Structure in Midgut and Hindgut of the Humus-Feeding Larva of Pachnoda ephippiata (Coleoptera: Scarabaeidae)

The guts of soil-feeding macroinvertebrates contain a complex microbial community that is involved in the transformation of ingested soil organic matter. In a companion paper (T. Lemke, U. Stingl, M. Egert, M. W. Friedrich, and A. Brune, Appl. Environ. Microbiol. 69:6650-6658, 2003), we show that the gut of our model organism, the humivorous larva of the cetoniid beetle Pachnoda ephippiata, is characterized by strong midgut alkalinity, high concentrations of microbial fermentation products, and the presence of a diverse, yet unstudied microbial community. Here, we report on the community structure of bacteria and archaea in the midgut, hindgut, and food soil of P. ephippiata larvae, determined with cultivation-independent techniques. Clone libraries and terminal restriction fragment length polymorphism analysis of 16S rRNA genes revealed that the intestines of P. ephippiata larvae contain a complex gut microbiota that differs markedly between midgut and hindgut and that is clearly distinct from the microbiota in the food soil. The bacterial community is dominated by phylogenetic groups with a fermentative metabolism (Lactobacillales, Clostridiales, Bacillales, and Cytophaga-Flavobacterium-Bacteroides [CFB] phylum), which is corroborated by high lactate and acetate concentrations in the midgut and hindgut and by the large numbers of lactogenic and acetogenic bacteria in both gut compartments reported in the companion paper. Based on 16S rRNA gene frequencies, Actinobacteria dominate the alkaline midgut, while the hindgut is dominated by members of the CFB phylum. The archaeal community, however, is less diverse. 16S rRNA genes affiliated with mesophilic Crenarchaeota, probably stemming from the ingested soil, were most frequent in the midgut, whereas Methanobacteriaceae-related 16S rRNA genes were most frequent in the hindgut. These findings agree with the reported restriction of methanogenesis to the hindgut of Pachnoda larvae.     

Behavioural interactions between native smokey dace and introduced yellowfin shiner: implications from habitat selection theory for an ongoing invasion

The occupation of adjacent, non‐overlapping positions along environmental gradients by closely related and ecologically similar species has drawn considerable attention from ecologists over the past decades. Condition‐specific competition, wherein competitive superiority varies with the abiotic environmental gradient, has been proposed as the major structuring force behind such distributions. These concepts, however, are generally applied to explain the contemporary distribution of organisms that share an evolutionary history. Our aim was to apply these concepts to the naive interactions between native and introduced fishes. In 1990 yellowfin shiner ( Notropis lutipinnis ) were introduced into the headwaters of the Little Tennessee River of western North Carolina, U.S.A. where it exhibits microhabitat preferences that overlap significantly with those of a threatened native minnow, the smokey dace ( Clinostomus funduloides raneyi ). Previous research has established that these drift‐feeding minnows: (1) differ in their average ability to intercept drifting prey as a function of water velocity; (2) generally occupy focal‐point velocities that maximize the rate of prey capture; and, (3) occasionally defend the forward positions in foraging aggregations that form in high quality patches. We present the results of a series of experiments designed to: (1) evaluate the role of aggression in the establishment and maintenance of preferred foraging positions in drift‐feeding minnows; (2) test the effects of prey availability, group size, and velocity on dominance rank, spatial position in groups, and feeding rates of individuals; and, (3) evaluate the potential for condition‐specific competition to establish competitive refugia for the native within the natural heterogeneity of mountain streams.     

Pulmonary gas exchange and acid-base state at 5,260 m in high-altitude Bolivians and acclimatized lowlanders.

Pulmonary gas exchange and acid-base state were compared in nine Danish lowlanders (L) acclimatized to 5,260 m for 9 wk and seven native Bolivian residents (N) of La Paz (altitude 3,600-4,100 m) brought acutely to this altitude. We evaluated normalcy of arterial pH and assessed pulmonary gas exchange and acid-base balance at rest and during peak exercise when breathing room air and 55% O2. Despite 9 wk at 5,260 m and considerable renal bicarbonate excretion (arterial plasma HCO3- concentration = 15.1 meq/l), resting arterial pH in L was 7.48 +/- 0.007 (significantly greater than 7.40). On the other hand, arterial pH in N was only 7.43 +/- 0.004 (despite arterial O2 saturation of 77%) after ascent from 3,600-4,100 to 5,260 m in 2 h. Maximal power output was similar in the two groups breathing air, whereas on 55% O2 only L showed a significant increase. During exercise in air, arterial PCO2 was 8 Torr lower in L than in N (P < 0.001), yet PO2 was the same such that, at maximal O2 uptake, alveolar-arterial PO2 difference was lower in N (5.3 +/- 1.3 Torr) than in L (10.5 +/- 0.8 Torr), P = 0.004. Calculated O2 diffusing capacity was 40% higher in N than in L and, if referenced to maximal hyperoxic work, capacity was 73% greater in N. Buffering of lactic acid was greater in N, with 20% less increase in base deficit per millimole per liter rise in lactate. These data show in L persistent alkalosis even after 9 wk at 5,260 m. In N, the data show 1) insignificant reduction in exercise capacity when breathing air at 5,260 m compared with breathing 55% O2; 2) very little ventilatory response to acute hypoxemia (judged by arterial pH and arterial PCO2 responses to hyperoxia); 3) during exercise, greater pulmonary diffusing capacity than in L, allowing maintenance of arterial PO2 despite lower ventilation; and 4) better buffering of lactic acid. These results support and extend similar observations concerning adaptation in lung function in these and other high-altitude native groups previously performed at much lower altitudes.     

Secondary reduction of preference for the sword ornament in the pygmy swordtail Xiphophorus nigrensis (Pisces: Poeciliidae)

Female mating preferences can be secondarily lost for a number of reasons. We examined the preference of female pygmy swordtails, Xiphophorus nigrensis, for the sword, a conspicuous extension of the caudal fin in some males. Females failed to show a preference for conspecific males with swords when presented with live males naturally varying in sword length, with live males of manipulated sword length, and with synthetic animations of males expressing natural variation in sword length. Females showed a significant bias against swords when presented with synthetic animations bearing supernormal sword characteristics. The reduced preference for swords, relative to closely related fish, may result from an increase in the cost of choice due to predation risk, selection against mating with heterospecifics, or changes in the spatial and contrast properties of the conspecific signal.     

Induction of TNF in human alveolar macrophages as a potential evasion mechanism of virulent Mycobacterium tuberculosis.

The ability of macrophages to release cytokines is crucial to the host response to intracellular infection. In particular, macrophage-derived TNF plays an important role in the host response to infection with the intracellular pathogen Mycobacterium tuberculosis. In mice, TNF is indispensable for the formation of tuberculous granulomas, which serve to demarcate the virulent bacterium. TNF is also implicated in many of the immunopathological features of tuberculosis. To investigate the role of TNF in the local immune response, we infected human alveolar macrophages with virulent and attenuated mycobacteria. Infection with virulent strains induced the secretion of significantly higher levels of bioactive TNF than attenuated strains correlating with their ability to multiply intracellularly. Treatment of infected macrophages with neutralizing anti-TNF Abs reduced the growth rate of intracellular bacteria, whereas bacterial replication was augmented by addition of exogenous TNF. Infected and uninfected macrophages contributed to cytokine production as determined by double-staining of M. tuberculosis and intracellular TNF. The induction of TNF by human alveolar macrophages at the site of infection permits the multiplication of intracellular bacteria and may therefore present an evasion mechanism of human pathogens.