Dinoflagellate-cyanobacterium communication may determine the composition of phytoplankton assemblage in a mesotrophic lake

The reasons for annual variability in the composition of phytoplankton assemblages are poorly understood but may include competition for resources and allelopathic interactions. We show that domination by the patch-forming dinoflagellate, Peridinium gatunense, or, alternatively, a bloom of a toxic cyanobacterium, Microcystis sp., in the Sea of Galilee may be accounted for by mutual density-dependent allelopathic interactions. Over the last 11 years, the abundance of these species in the lake displayed strong negative correlation. Laboratory experiments showed reciprocal, density-dependent, but nutrient-independent, inhibition of growth. Application of spent P. gatunense medium induced sedimentation and, subsequently, massive lysis of Microcystis cells within 24 hr, and sedimentation and lysis were concomitant with a large rise in the level of McyB, which is involved in toxin biosynthesis by Microcystis. P. gatunense responded to the presence of Microcystis by a species-specific pathway that involved a biphasic oxidative burst and activation of certain protein kinases. Blocking this recognition by MAP-kinase inhibitors abolished the biphasic oxidative burst and affected the fate (death or cell division) of the P. gatunense cells. We propose that patchy growth habits may confer enhanced defense capabilities, providing ecological advantages that compensate for the aggravated limitation of resources in the patch. Cross-talk via allelochemicals may explain the phytoplankton assemblage in the Sea of Galilee.     

Flight Durations in Bumblebees Under Manipulation of Feeding Choices

Foraging bees spend less time flying between flowers of the same species than between individuals of different species. This time saving has been suggested as a possible advantage of flower-constant foraging. We hypothesized that the time required to switch flower type increases if (a) such switches are infrequent and (b) the bees need to decide whether to switch or not. Bumblebees were taught to forage on artificial feeders that were identical in morphology and reward schedule but differed in the color of their landing surface. In the first two experiments bees foraged alternatively between two feeders. In Experiment 1 the colors of the landing surfaces were switched every two or three visits, while in Experiment 2 they were switched every six or seven visits. In the third experiment, the bees were required to decide whether to make a color-constant or a color-shift flight. Intervisit time was defined as time elapsed between consecutive visits to feeders. When feeder colors were changed frequently (Experiment 1), we detected no difference between color-constant and color-shift intervisit times. When bees were repeatedly exposed to one color (Experiment 2), color shifts required a significantly longer time. When allowed to choose (Experiment 3), bees performed more color-constant flights than color-shift flights. Intervisit times were similar for color-constant and color-shift flights in this experiment. Intervisit times in Experiment 3 were significantly longer than in Experiment 2 and slightly but nonsignificantly longer than in Experiment 1. The results suggest that bees indeed save time though flower-constant foraging but that this time savings is a small (1 s/flower visit) under laboratory conditions, and appears only when switches between flower types are infrequent. The time saved may be more significant over long foraging trips, and when morphological differences between flower species are large, as often happens under natural conditions, providing a selective advantage to flower-constant foraging.     

Comparison of the effects of mechanical and osmotic pressures on the collagen fiber architecture of intact and proteoglycan-depleted articular cartilage

One of the functions of articular cartilage is to withstand recurrent pressure applied in everyday life. In previous studies, osmotic pressure has been used to mimic the effects of mechanical pressure. In the present study, the response of the collagen network of intact and proteoglycans (PG)-depleted cartilage to mechanical and osmotic pressures is compared. The technique used is one-dimensional ²H double quantum filtered spectroscopic MRI, which gives information about the degree of order and the density of the collagen fibers at the different locations throughout the intact tissue. For the nonpressurized plugs, the depletion had no effect on these parameters. Major differences were found in the zones near the bone between the effects of the two types of application of pressure for both intact and depleted plugs. While the order is lost in these zones as a result of mechanical load, it is preserved under osmotic pressure. For both intact and PG-depleted plugs under osmotic stress most of the collagen fibers become disordered. Our results indicate that different modes of strain are produced by unidirectional mechanical load and the isotropic osmotic stress. Thus, osmotic stress cannot serve as a model for the effect of load on cartilage in vivo. This paper is presented in part in ISMRM 11th Scientific Meeting, p. 55, 2003 and ISMRM 14th Scientific Meeting, p. 59, 2006. Dedicated to Prof. K. Arnold on the occasion of his 65th birthday.     

Design, synthesis and properties of novel powerful antioxidants, glutathione analogues

Glutathione (GSH) is the major low-molecular weight antioxidant in mammalian cells. Thus, its analogues carrying similar and/or additional positive properties might have clinical perspectives. Here, we report the design and synthesis of a library of tetrapeptidic GSH analogues called UPF peptides. Compared to cellular GSH our designed peptidic analogues showed remarkably higher hydroxyl radical scavenging ability (EC(50) of GSH: 1,231.0 +/- 311.8 microM; EC(50) of UPF peptides: from 0.03 to 35 microM) and improved antiradical efficiency towards a stable alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical. The best of UPF peptides was 370-fold effective hydroxyl radical scavengers than melatonin (EC(50): 11.4 +/- 1.0 microM). We also found that UPF peptides do not influence the viability and membrane integrity of K562 human erythroleukemia cells even at 200 microM concentration. Dimerization of GSH and UPF peptides was compared in water and in 0.9% saline solutions. The results, together with an earlier finding that UPF1 showed protective effects in global cerebral ischemia model in rats, suggest that UPF peptides might serve both as potent antioxidants as well as leads for design of powerful non-peptidic antioxidants that correct oxidative stress-driven events.     

Genome methylation patterns across castes and generations in a parasitoid wasp

Environmental influences shape phenotypes within and across generations, often through DNA methylations that modify gene expression. Methylations were proposed to mediate caste and task allocation in some eusocial insects, but how an insect's environment affects DNA methylation in its offspring is yet unknown. We characterized parental effects on methylation profiles in the polyembryonic parasitoid wasp Copidosoma koehleri, as well as methylation patterns associated with its simple caste system. We used methylation‐sensitive amplified fragment length polymorphism (MS‐AFLP) to compare methylation patterns, among (1) reproductive and soldier larvae; and (2) offspring (larvae, pupae, and adults) of wasps that were reared at either high or low larval density and mated in the four possible combinations. Methylation frequencies were similar across castes, but the profiles of methylated fragments differed significantly. Parental rearing density did not affect methylation frequencies in the offspring at any developmental stage. Principal coordinate analysis indicated no significant differences in methylation profiles among the four crossbreeding groups and the three developmental stages. Nevertheless, a clustering analysis, performed on a subset of the fragments, revealed similar methylation patterns in larvae, pupae, and adults in two of the four parental crosses. Nine fragments were methylated at two cytosine sites in all larvae, and five others were methylated at two sites in all adults. Thus, DNA methylations correlate with within‐generation phenotypic plasticity due to caste. However, their association with developmental stage and with transgenerational epigenetic effects is not clearly supported.     

Long-Lasting Maintenance of Learning-Induced Enhanced Neuronal Excitability: Mechanisms and Functional Significance

Pyramidal neurons in the piriform cortex of olfactory discrimination trained rats show enhanced intrinsic neuronal excitability that lasts for several days after learning. Such enhanced intrinsic excitability is mediated by long-term reduction in the postburst after hyperpolarization which is generated by repetitive spike firing. The molecular machinery underlying such long-lasting modulation of intrinsic excitability, as well as its exceptional durability, is yet to be fully described. In this review, we present recent advancements that reveal the identity of the current that is modulated after learning and the second messenger system by which enhanced excitability is maintained. We also discuss the significance of such long-lasting modulation to the local network’s sensitivity to noradrenaline, a major learning-relevant neuromodulator.