PHENOTYPIC PLASTICITY INDUCED IN TRANSPLANT EXPERIMENTS IN A MUTUALISTIC ASSOCIATION BETWEEN THE RED ALGA JANIA ADHAERENS (RHODOPHYTA,CORALLINALES) AND THE SPONGE HALICLONA CAERULEA (PORIFERA: HAPLOSCLERIDA): MORPHOLOGICAL RESPONSES OF THE ALGA1

The association between the red macroalga Jania adhaerens J. V. Lamour. and the sponge Haliclona caerulea is the most successful life‐form between 2 and 4 m depth in Mazatlán Bay (Mexican Pacific). J. adhaerens colonizes the rocky intertidal area and penetrates into deeper areas only when it lives in association with H. caerulea. The aposymbiotic form of the sponge has not been reported in the bay. To understand the ecological success of this association, we examined the capacity of J. adhaerens to acclimate in Mazatlán Bay using transplant experiments. The transplanted aposymbiotic J. adhaerens did not survive the first 2 weeks; however, J. adhaerens when living in association with H. caerulea, acclimated easily to depth, showing no sign of mortality during the 103 d of the experiment. We conclude that the ability of J. adhaerens to colonize in deeper areas in this hydrodynamic environment may in part rely on the protection provided by the sponge to the algal canopy. Both species contribute to the shape of the associated form. Nevertheless, the morphological variation in the association appears to be dominated by the variation in J. adhaerens canopy to regulate pigment self‐shading under light‐limited conditions and/or tissue resistance under high hydrodynamics. Consequently, our results are consistent with light as the abiotic controlling factor, which regulates the lower depth distribution of the association in Mazatlán Bay, through limiting the growth rate of J. adhaerens. Hydrodynamics may determine the upper limit of the association by imposing high mass losses.     

Nitric oxide decreases superoxide anion generation by microsomes from soybean embryonic axes

Experiments were carried out to evaluate the effects of exposure to nitric oxide on the ability by NADPH‐dependent microsomal electron transfer to generate oxygen radicals. Such interactions could play a role in the potential antioxidant action of nitric oxide (NO). Isolated microsomes from soybean ( Glycine max [L.] Merr. cv. Hood) embryonic axes were exposed to an exogenously added source of nitric oxide (NO) (S‐nitrosoglutathione + dithiothreitol). The O₂⁻ generation rate by microsomes exposed to NO decreased significantly as compared to the rate measured in microsomes incubated in the absence of NO. The exposure of the microsomes to the NO donor did not alter the microsomal rate of hydroxyl radical generation. Preincubation of the microsomes with the NO donor affected neither iron reduction rate nor activity of cytochrome c reductase. However, cytochrome P₄₅₀ activity was significantly inhibited after exposure to NO. This inhibition was completely prevented by hemoglobin. The data are consistent with the hypothesis that NO exhibits a potential antioxidant role in the plant cell by decreasing the rate of generation of superoxide anion. Since endogenous NO was detected in homogenates of soybean embryonic axes by EPR studies, this interaction between NO and cytochrome P₄₅₀ in soybean embryonic axes could be a factor of relevance for the control of oxidative stress in vivo.     

Active site mutations in DNA topoisomerase I distinguish the cytotoxic activities of camptothecin and the indolocarbazole, rebeccamycin.

DNA topoisomerase I (Top1p) catalyzes topological changes in DNA and is the cellular target of the antitumor agent camptothecin (CPT). Non-CPT drugs that target Top1p, such as indolocarbazoles, are under clinical development. However, whether the cytotoxicity of indolocarbazoles derives from Top1p poisoning remains unclear. To further investigate indolocarbazole mechanism, rebeccamycin R-3 activity was examined in vitro and in yeast. Using a series of Top1p mutants, where substitution of residues around the active site tyrosine has well-defined effects on enzyme catalysis, we show that catalytically active, CPT-resistant enzymes remain sensitive to R-3. This indolocarbazole did not inhibit yeast Top1p activity, yet was effective in stabilizing Top1p-DNA complexes. Similar results were obtained with human Top1p, when Ser or His were substituted for Asn-722. The mutations altered enzyme function and sensitivity to CPT, yet R-3 poisoning of Top1p was unaffected. Moreover, top1delta, rad52delta yeast cells expressing human Top1p, but not catalytically inactive Top1Y723Fp, were sensitive to R-3. These data support hTop1p as the cellular target of R-3 and indicate that distinct drug-enzyme interactions at the active site are required for efficient poisoning by R-3 or CPT. Furthermore, resistance to one poison may potentiate cell sensitivity to structurally distinct compounds that also target Top1p.     

Vanadium compounds

The direct effect of different vanadium compounds upon alkaline phosphatase (ALP) activity was investigated. Vanadate and vanadyl inhibited both the soluble and particulate ALP activity from UMR.106 cells and from bovine intestinal ALP. We have also shown the inhibition of ALP activity in the soluble fraction of osteoblasts by peroxo and hydroperoxo vanadium compounds. ALP activity in the particulate fraction was not inhibited by these species; nor was the bovine intestinal ALP. Using inhibitors of Tyr-phosphatase (PTPases), the soluble ALP was partially characterized as a PTPase. The major activity in the particulate fraction represents the bone-specific ALP-activity. This study demonstrates that different forms of vanadium are direct inhibitors of ALP activity. This effect is dependent on the enzymatic activity investigated and on the origin of the ALP.     

Between-population variation in size-dependent reproduction and reproductive allocation in Pinguiculavulgaris (Lentibulariaceae) and its environmental correlates

Several important fitness components in herbaceous perennial plants are commonly related to plant size: flowering probability, reproductive allocation and fecundity. However, evidence for such size-dependence of fitness components is mostly anecdotal and unconnected to other life history traits. Here we report size-dependence for flowering probability and reproductive allocation in 11 populations of Pinguicula vulgaris and relate it to environmental factors. Flowering probability was size-dependent in all populations of P. vulgaris , and indicated the existence of a threshold size for reproduction. Populations at low altitudes and in wet soils showed a significantly higher threshold size for reproduction. Reproductive mass was also size-dependent in all populations. We found considerable between-population differences in the slope and the intercept of the regression between plant size and reproductive mass. This variation was weakly related to the environmental factors measured. In general, relationships between different size-dependent fitness components were low. Instead of showing a covariation of traits, in line with interpretations in terms of life history "tactics", P. vulgaris seemed to independently vary each size-dependent fitness component in each locality. In particular, no significant relationship was found between threshold size for reproduction and the slope of size-dependent reproductive allocation, as predicted by previous authors. Neither we found a significant influence of somatic cost of reproduction on size-dependent fitness components.     

Immunocytolocalization of glutamine synthetase in mesophyll and phloem of leaves ofSolanum tuberosum L.

Summary Localization of glutamine synthetase inSolanum tuberosum leaves was investigated by techniques of Western tissue printing and immunogold electron microscopy. Anti-GS antibodies used in immunolocalization recognize two peptides (45 kDa and 42 kDa) on Western blots. Antibody stained tissue prints on nitrocellulose membranes allowed low resolution localization of GS. Immunostaining was most evident in the adaxial phloem of the leaf midribs and petiole veins. High-resolution localization of glutamine synthetase by immunogold electron microscopy revealed that this enzyme occurs in both the chloroplasts and the cytosol ofS. tuberosum leaf cells. However, GS was specifically associated with the chloroplasts of mesophyll cells and with the cytoplasm of phloem companion cells. The evidence for cell-specific localization of chloroplast and cytosolic GS presented here agrees with the recently reported cell-specific pattern of expression of GUS reporter gene, directed by promoters for chloroplast and cytosolic GS form in tobacco transgenic plants. These data provide additional clues to the interpretation of the functional role of these different isoenzymes and its relationship with their specific localization.Abbreviations BSA bovine serum albumin - EM electron microscope - GOGAT glutamate synthase - GS glutamine synthetase - GUS -glucuronidase - IgG immunoglobulin - PBS phosphate buffer saline - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis     

Synchronous growth of a freshwater diatom Melosira italica under natural environment

Summary A filamentous diatom Melosira italica was collected at the beginning of rainy season from a shallow lake in the tropical savanna region in Brazil. Even the sample taken from surface water contained empty cells in high percentages. The number of cells per filament of M. italica showed a peculiar pulse-like frequency distribution with peak values at 4, 8, 12 and 16. Evidences of the synchronous cell division in this planktonic diatom under natural environment are discussed.     

Common and rare species respond to similar niche processes in macroinvertebrate metacommunities

Ecologists have long investigated why communities are composed of a few common species and many rare species. Most studies relate rarity to either niche differentiation among species or spatial processes. There is a parallel between these processes and the processes proposed to explain the structure of metacommunities. Based on a metacommunity perspective and on data on stream macroinvertebrates from different regions of Brazil, we answer two questions. 1) Are sets of common and rare species affected by similar niche and spatial processes? 2) How does the community composition of common and of rare species differ? The main hypothesis we test is that common species are mainly affected by environmental factors, whereas rare species are mostly influenced by dispersal limitation. We used variation partitioning to determine the proportion of variation explained by the environment and space in common and rare species matrices. Contrary to our expectations, evidence supported the idea that both common and rare species are affected mainly by environmental factors, even after controlling for the differing information content between common and rare species matrices. Moreover, the abundance of some common species is also a good predictor of variation in rare species matrices. Niche differences are unlikely to be the sole cause of patterns of rarity in these metacommunities. We suggest that sets of common and rare species react to similar major environmental gradients and that rare species also respond to processes that operate at a more fine‐grained spatial scale, particularly biotic interactions. We extend the view that species sorting is the dominant process structuring metacommunities and argue that future studies focusing on rarity would benefit from a metacommunity perspective.     

Functional dissection of osteoprotegerin and its interaction with receptor activator of NF-kappaB ligand

The receptor activator of NF-kappaB (RANK) belongs to the neuregulin/tumor necrosis factor (TNF) receptor superfamily and is activated by RANK ligand (RANK-L), a homotrimeric, TNF-like cytokine. RANK is present on the surface of osteoclast cell precursors, where its interaction with RANK-L induces their terminal differentiation into osteoclasts, thus increasing bone breakdown. The secreted, soluble receptor osteoprotegerin (OPG) interrupts this activation by binding directly to RANK-L. Therefore, osteoclast maturation (and bone homeostasis) is regulated in vivo by OPG levels of expression. We have studied the assembly state and affinity of OPG for RANK-L by sedimentation analyses and surface plasmon resonance (Biacore). Full-length, homodimeric OPG binds to RANK-L with a KD of 10 nM. OPG is also a member of the TNF receptor superfamily and contains four disulfide-rich ligand-binding domains, yet lacks a transmembrane region separating the ligand-binding region from the two death domains, as observed for other receptor family members. We showed that dimerization of OPG results from noncovalent interactions mediated by the death domains and to a lesser extent by a C-terminal heparin-binding region. In contrast, a C-terminal intermolecular disulfide bond does not contribute to the formation or stability of OPG dimers. A truncate of osteoprotegerin, containing the ligand-binding domains but lacking the dimerization domains, bound RANK-L with a KD of approximately 3 microM, indicating that monomer oligomerization for the OPG provides an increase of 3 orders of magnitude in the affinity for RANK-L. Therefore, OPG dimer formation is required for the mechanism of inhibition of the RANK-L/RANK receptor interaction.