Can tropical seaweeds reduce herbivory by growing at night? Diel patterns of growth,nitrogen content,herbivory, and chemical versus morphological defenses

Summary Tropical seaweeds in the genus Halimeda reduce losses to grazing by capitalizing on diel patterns of herbivore activity. These seaweeds produce new, more herbivoresusceptible growth at night when herbivorous reef fishes are inactive. Plant portions more than 48 h old are low in food value, well defended morphologically (calcified and high in ash content), and relatively resistant to herbivory. Younger plant portions represent 3–4.5 times the food value (nitrogen or organic content) of older portions but are only moderately more susceptible to herbivores due to their high concentrations of the terpenoid feeding deterrents halimedatrial and halimedatetraacetate. Halimedatrial significantly deters grazing by both parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) and occurs in high concentrations (2–4.5% of plant ash-free dry mass) in plant portions that are 4–12 h old, intermediate concentrations (0.3–2.3%) in portions that are 16–26 h old, and low concentrations (0.3%) in older plant portions. The related compound halimedatetraacetate is absent from the youngest plant portions, shows a rapid increase in concentration (from 0 to 1%) in plant material that is approximately 16 h old, and then rapidly declines to low levels (0.1 to 0.2%) in older plant portions. Thus, newly produced tissues are nutritionally valuable but contain high concentrations of defensive chemicals. As these tissues age, morphological defenses increase, the tissue becomes less valuable as a food for herbivores, and chemical defenses decrease. Additionally, new growth of Halimeda remains unpigmented until just before sunrise. Thus, the valuable, nitrogen-containing molecules associated with photosynthesis are not placed in the new, and more herbivore susceptible, growth until lights is available and they can start producing income for the plant.Experiments in a coral-reef microcosm, where diel patterns of light and water chemistry could be altered, indicated that Halimeda's growth pattern is cued by the timing of light-dark cycles rather than by co-occurring diel changes in water chemistry. Although the growth patterns of Halimeda seem unusual, similar patterns appear to occur in numerous other seaweeds and in microalgae such as diatoms and dinoflagellates.     

Fetal CO2 kinetics

Knowledge of CO2 kinetics in the fetus is important for the design and interpretation of fetal metabolic studies that use carbon-labelled tracers. To study fetal CO2 kinetics, four fetal sheep were infused at constant rate with NaH14CO3 to simulate a constant rate of fetal 14CO2 production from the metabolism of a 14C-labelled substrate. Uterine and umbilical blood flows, and concentrations of 14CO2 and total CO2 in umbilical arterial and venous blood and in uterine arterial and venous blood were measured. During steady state, the excretion of 14CO2 via the umbilical circulation was 99.6 +/- 1.0 (SEM)% of the NaH14CO3 infusion rate. The irreversible disposal rate of CO2 molecules from the fetal CO2 pool was approximately 5 times greater than the metabolic production of CO2 by the fetus. This evidence demonstrates that measurements of fetal 14CO2 excretion via the umbilical circulation can provide an accurate measurement of fetal 14CO2 production and that the exchange rate of CO2 molecules between placenta and fetal blood is much greater than the net rate of excretion of CO2 molecules from fetus to placenta.     

Toxoplasma infection and response to novelty in mice

Three groups of mice were infected with Toxoplasma and used for behavioral testing using a Y-maze. One group was infected when adult and two groups congenitally, one of these born to dams infected during gestation, the other to dams chronically infected prior to mating. In an initial habituation period each mouse was exposed to a black arm and stem of the maze, entrance to a white arm being blocked by a transparent door. In a subsequent free-choice trial both arms were black and the mouse was free to explore all parts of the maze. During both periods infected mice were more active than controls. Infected mice engaged in less grooming behavior indicative of less approach-avoidance conflict than controls prior to entry into a choice arm at the beginning of the free-choice trial. Infected mice spent more time in the familiar than in the novel (previously blocked) arm during the free-choice trial; conversely, uninfected mice spent more time in the novel than in the familiar arm. It is suggested that the reported behavioural changes would lead to dissemination of the infection in the environment by ultimately making infected mouse intermediate hosts more susceptible to predation by domestic cats, the definitive hosts of Toxoplasma.     

Predictable spatial escapes from herbivory: how do these affect the evolution of herbivore resistance in tropical marine communities?

Summary Between-habitat differences in macrophyte consumption by herbivorous fishes were examined on three Caribbean and two Indian Ocean coral reefs. Transplanted sections of seagrasses were used as a bioassay to compare removal rates in reef-slope, reef-flat, sand-plain, and lagoon habitats. Herbivore susceptibility of fifty-two species of seaweeds from these habitats was also measured in the field. Seagrass consumption on shallow reef slopes was always significantly greater than on shallow reef flats, deep sand plains, or sandy lagoons. Reef-slope seaweeds were consistently resistant to herbivory while reef-flat seaweeds were consistently very susceptible to herbivory. This pattern supports the hypothesis that defenses against herbivores are costly in terms of fitness and are selected against in habitats with predictably low rates of herbivory.Sand-plain and lagoon seaweeds showed a mixed response when placed in habitats with high herbivore pressure; most fleshy red seaweeds were eaten rapidly, most fleshy green seaweeds were eaten at intermediate rates, and most calcified green seaweeds were avoided or eaten at very low rates. Differences in susceptibility between red and green seaweeds from sand-plain or lagoon habitats may result from differential competitive pressures experienced by these seaweed groups or from the differential probability of being encountered by herbivores. The susceptibility of a species to removal by herbivorous fishes was relatively consistent between reefs. Preferences of the sea urchin Diadema antillarum were also similar to those of the fish guilds.Unique secondary metabolites were characteristic of almost all of the most herbivore resistant seaweeds. However, some of the herbivore susceptible species also contain chemicals that have been proposed as defensive compounds. Genera such as Sargassum, Turbinaria, Thalassia, Halodule, and Thalassodendron, which produce polyphenolics or phenolic acids, were consumed at high to intermediate rates, suggesting that these compounds are not effective deterrents for some herbivorous fishes. Additionally, potential for the production of the compounds caulerpin, caulerpicin and caulerpenyne in various species of Caulerpa did not assure low susceptibility to herbivory.Heavily calcified seaweeds were very resistant to herbivory, but all of these species also produce toxic secondary metabolites which makes it difficult to distinguish between the effects of morphological and chemical defenses. Predictions of susceptibility to herbivory based on algal toughness and external morphology were of limited value in explaining differing resistances to herbivory.     

Interaction of the C-terminal region of p105 with the nuclear localisation signal of p50 is required for inhibition of NF-kappa B DNA binding activity.

DNA binding of the homodimeric p50 subunit of NF-kappa B was inhibited by a bacterially expressed protein containing the ankyrin repeats present in the C-terminus of the p105 precursor but not by the I kappa B protein MAD-3. However p50 was retained on protein affinity matrices containing either the C-terminal ankyrin repeats of p105 or MAD-3. To investigate the interaction between p50 and proteins containing ankyrin repeats we have used a number of approaches to probe the accessibility of the p50 nuclear localisation signal in the protein complex. A monoclonal antibody recognising a linear epitope either very close to, or including, the nuclear localisation signal of the p50 protein could immunoprecipitate p50 homodimers but was unable to precipitate the protein when it was bound to the C-terminal region of p105. A close association between the nuclear localisation signal of p50 and the C-terminal region of p105 was also suggested by protease accessibility experiments. While the nuclear localisation signal of free p50 is extremely susceptible to cleavage with trypsin the same site is masked in the presence of the C-terminal ankyrin repeats of p105 and, to a lesser extent MAD-3. Removal of the nuclear localisation signal by trypsin digestion generates a protein that is fully competent for DNA binding but is refractile to inhibition by the C-terminal ankyrin repeats of p105. Addition of DNA destabilises complexes between p50 and ankyrin repeat containing proteins, increasing the susceptibility of the nuclear localisation signal to trypsin cleavage. The data suggest that there is a rapid exchange of p50 between complexes containing DNA or I kappa B proteins via a metastable complex containing DNA, p50 and I kappa B.     

Activation of adenovirus-coded protease and processing of preterminal protein.

Adenoviruses code for a protease that is essential for infectivity and is activated by a disulfide-linked peptide, derived from the C terminus of the virus structural protein pVI (pVI-CT). The protease was synthesized at relatively high levels late in infection and was detected in both cytoplasmic and nuclear fractions of adenovirus-infected cells. DNA was not found to be a cofactor of the protease, as previously proposed (W. F. Mangel, W. J. McGrath, D. Toledo, and C. W. Anderson, Nature [London] 361:274-275, 1993), but a role for DNA in facilitating the activation of the protease by pVI-CT in vivo cannot be ruled out. Adenovirus preterminal protein is a substrate for the virus-coded protease, with digestion to the mature terminal protein proceeding via the formation of two intermediates. Each of the three cleavage sites in the preterminal protein was identified by N-terminal sequencing and shown to conform to the substrate specificity of adenovirus protease, (M,L,I)XGX-X. Functional studies revealed that preterminal protein and intermediates but not mature terminal protein associated with adenovirus polymerase, while only the intact preterminal protein and none of its digestion products bound to DNA. These results suggest that the virus-coded protease may influence viral DNA replication by cleavage of both genome-bound and freely soluble preterminal protein, with consequent alterations to their functional properties.     

Childhood eczema: disease of the advantaged?

OBJECTIVE--To determine whether the increased prevalence of childhood eczema in advantaged socioeconomic groups is due to increased parental reporting. DESIGN--Comparison of parental reports of eczema with visible eczema recorded by medical officers during a detailed physical examination. SETTING--National birth cohort study. SUBJECTS--8279 children from England, Wales, and Scotland born during 3-9 March 1958 and followed up at the ages of 7, 11, and 16. MAIN OUTCOME MEASURES--Prevalence of eczema according to parental report compared with medical officer''s examination at the ages of 7, 11, and 16. RESULTS--Prevalence of both reported and examined eczema increased with rising social class at the ages of 7, 11, and 16 years. The point prevalence of examined eczema at age 7 was 4.8%, 3.6%, 3.6%, 2.4%, 2.2%, and 2.4% in social classes I, II, III non-manual, III manual, IV, and V respectively (chi 2 value for linear trend 12.6, P < 0.001). This trend persisted after adjustment for potential confounders such as region and family size and was not present for examined psoriasis or acne. CONCLUSIONS--Eczema is more prevalent among British schoolchildren in social classes I and II than those in lower classes. Exposures associated with social class are probably at least as important as genetic factors in the expression of childhood eczema.     

Characterization and molecular genetic complementation of mutants affecting dimorphism in the fungus ustilago maydis

Ustilago maydis, the causal agent of corn smut disease, displays dimorphic growth in which it alternates between a unicellular, nonpathogenic yeast-like form and a dikaryotic, pathogenic filamentous form. Previously, a constitutively filamentous haploid mutant was obtained. Complementation of this mutant led to the isolation of the gene encoding adenylate cyclase, uac1. Secondary mutagenesis of a uac1 disruption strain allowed the isolation of a large number of suppressor mutants, termed ubc, for Ustilago bypass of cyclase, lacking the filamentous phenotype. Analysis of one of these suppressor mutants previously led to the identification of the ubc1 gene, encoding the regulatory subunit of cAMP-dependent protein kinase. In this report we describe the isolation of cosmids containing three new ubc genes, termed ubc2, ubc3, and ubc4. We also describe the morphology of the ubc2, ubc3, and ubc4 mutants in a uac1- background as well as in a background with a functional uac1 gene. In addition, we describe several mutant strains not complemented with any of the genes currently in hand and that are thus presumed to possess mutations in additional ubc genes. Copyright 1998 Academic Press.     

Simple high-performance liquid chromatographic method for the determination of metformin in human plasma

A simple high-performance liquid chromatographic method using ultraviolet detection was developed for the determination of metformin in human plasma. The method entailed direct injection of the plasma sample after deproteination using perchloric acid. The mobile phase comprised 0.01 M potassium dihydrogen orthophosphate (pH 3.5) and acetonitrile (60:40, v/v). Analyses were run at a flow-rate of 1.0 ml/min with the detector operating at a detection wavelength of 234 nm. The method is specific and sensitive, with a quantification limit of approximately 60 ng/ml and a detection limit of 15 ng/ml at a signal-to-noise ratio of 3:1. The mean absolute recovery value was about 97%, while the within-day and between-day coefficient of variation and percent error values of the assay method were all less than 8%. The calibration curve was linear over a concentration range of 62.5–4000 ng/ml.