Structure and permeability of the egg capsule of the bonnethead shark, Sphyrna tiburo

In the viviparous bonnethead shark, Sphyrna tiburo, a fluid-filled, acellular egg capsule surrounds fertilized eggs and developing embryos throughout gestation. Like other placental shark species, the capsule remains intact even at the placental implantation site. Although its intervention between the uterine and embryonic tissues of the placenta has long been thought to mediate physiological exchange, little information is available concerning even its basic structure or permeability to solutes. The 1 mum thick capsule wall consists of an inner layer of gelatinous material and an outer layer consisting of at least three laminae of orthogonally arranged fibrous material. These fibers are irregular and often branched. Permeability experiments showed that solutes less than 1,355 Da diffuse across the egg capsule whereas those greater than 6,000 Da do not pass through the membrane. Solute movement across the capsule is a concentration-dependent phenomenon indicating diffusion rather than active transport. Experimental data also suggest that there is an increase in the permeability of the egg capsule to low molecular weight materials during mid- and late gestation. These observations are discussed in relation to the function of the egg capsule as a mediator of maternal-embryonic interactions in matrotrophic sharks.     

Characterization and molecular mapping of genes determining semidwarfism in barley

The semidwarf trait is desired in cereal breeding programs for increased lodging resistance. We characterized 27 brachytic (brh) semidwarf mutants in barley (Hordeum vulgare L.) and located the genes on barley chromosome linkage maps. All brachytic genes were transferred into the two-rowed cultivar Bowman by backcrossing four to seven times and selecting for semidwarf plants. The brachytic lines were evaluated for 10 phenotypic traits: plant height, awn, peduncle, and rachis internode length, leaf length and width, lodging, grain yield, number of kernels per spike, and kernel weight. We intercrossed the lines to determine which mutants were at independent loci and which were alleles at the same locus. F2 populations from 18 brh semidwarfs were constructed for genetic mapping using simple sequence repeat (SSR) markers. The brachytic semidwarf near-isogenic lines were significantly shorter than their normal counterparts and most had lower yields (16/27); shorter awns (26/27), peduncles (26/27), and rachis internodes (24/27); and reduced kernel weight (22/27). Twelve of the lines had shorter penultimate leaves and 15 had reduced lodging. Four lines had increased kernels per spike, while one had fewer kernels per spike. Allelism tests and mapping comparisons indicated that the 27 semidwarfs comprise 18 independent genetic loci. SSR mapping placed these loci in five of the seven barley chromosomes. Knowledge of the effects and locations of these brachytic semidwarf genes will help barley breeders select appropriate lines for barley improvement.     

Reconstitution of the photosystem II Ca2+ binding site

The roles of Ca(2+) in H(2)O oxidation may be as a site of substrate binding, and as a structural component of the photosystem II O(2)-evolving complex. One indication of this dual role of the metal is revealed by probing the Mn cluster in the Ca(2+) depleted O(2) evolving complex that retains extrinsic 23- and 17-kDa polypeptides with reductants (NH(2)OH and hydroquinone) [Biochemistry 41 (2002) 958]. Calcium appears to bind to photosystem II at a site where it could bind substrate H(2)O. Equilibration of Ca(2+) with this binding site is facilitated by increased ionic strength, and incubation of Ca(2+) reconstitution mixtures at 22 degrees C accelerates equilibration of Ca(2+) with the site. The Ca(2+) reconstituted enzyme system regains properties of unperturbed photosystem II: Sensitivity to NH(2)OH inhibition is decreased, and Cl(-) binding with increased affinity can be detected. The ability of ionic strength and temperature to facilitate rebinding of Ca(2+) to the intact O(2) evolving complex suggests that the structural environment of the oxidizing side of photosystem II may be flexible, rather than rigid.     

Numerical simulation of tissue differentiation around loaded titanium implants in a bone chamber

The application of a bone chamber provides a controlled environment for the study of tissue differentiation and bone adaptation. The influence of different mechanical and biological factors on the processes can be measured experimentally. The goal of the present work is to numerically model the process of peri-implant tissue differentiation inside a bone chamber, placed in a rabbit tibia. 2D and 3D models were created of the tissue inside the chamber. A number of loading conditions, corresponding to those applied in the rabbit experiments, were simulated. Fluid velocity and maximal distortional strain were considered as the stimuli that guide the differentiation process of mesenchymal cells into fibroblasts, chondrocytes and osteoblasts. Mesenchymal cells migrate through the chamber from the perforations in the chamber wall. This process is modelled by the diffusion equation. The predicted tissue phenotypes as well as the process of tissue ingrowth into the chamber show a qualitative agreement with the results of the rabbit experiments. Due to the limited number of animal experiments (four) and the observed inter-animal differences, no quantitative comparison could be made. These results however are a strong indication of the feasibility of the implemented theory to predict the mechano-regulation of the differentiation process inside the bone chamber.     

Mechanical stabilization of desiccated vegetative tissues of the resurrection grass Eragrostis nindensis: does a TIP 3;1 and/or compartmentalization of subcellular components and metabolites play a role?

During dehydration, numerous metabolites accumulate in vegetative desiccation-tolerant tissues. This is thought to be important in mechanically stabilizing the cells and membranes in the desiccated state. Non-aqueous fractionation of desiccated leaf tissues of the resurrection grass Eragrostis nindensis (Ficalho and Hiern) provided an insight into the subcellular localization of the metabolites (because of the assumptions necessary in the calculations the data must be treated with some caution). During dehydration of the desiccant-tolerant leaves, abundant small vacuoles are formed in the bundle sheath cells, while cell wall folding occurs in the thin-walled mesophyll and epidermal cells, leading to a considerable reduction in the cross-sectional area of these cells. During dehydration, proline, protein, and sucrose accumulate in similar proportions in the small vacuoles in the bundle sheath cells. In the mesophyll cells high amounts of sucrose accumulate in the cytoplasm, with proline and proteins being present in both the cytoplasm and the large central vacuole. In addition to the replacement of water by compatible solutes, high permeability of membranes to water may be critical to reduce the mechanical strain associated with the influx of water on rehydration. The immunolocalization of a possible TIP 3;1 to the small vacuoles in the bundle sheath cells may be important in both increased water permeability as well as in the mobilization of solutes from the small vacuoles on rehydration. This is the first report of a possible TIP 3;1 in vegetative tissues (previously only reported in orthodox seeds).     

Actin cytoskeletal reorganizations and coreceptor-mediated activation of rac during human immunodeficiency virus-induced cell fusion

The membrane fusion events which initiate human immunodeficiency virus type 1 (HIV-1) infection and promote cytopathic syncytium formation in infected cells commence with the binding of the HIV envelope glycoprotein (Env) to CD4 and an appropriate coreceptor. Here, we show that HIV Env-coreceptor interactions activate Rac-1 GTPase and stimulate the actin filament network reorganizations that are requisite components of the cell fusion process. Disrupting actin filament dynamics with jasplakinolide or latrunculin A arrested fusion at a late step in the formation of Env-CD4-coreceptor complexes. Time-lapse confocal microscopy of living cells revealed vigorous activity of actin-based, target cell membrane extensions at the target cell-Env-expressing cell interface. The expression of dominant-negative forms of actin-regulating Rho-family GTPases established that HIV Env-mediated syncytium formation relies on Rac-1 but not on Cdc42 or Rho activation in target cells. Similar dependencies were found when cell fusion was induced by Env expressed on viral or cellular membranes. Additionally, Rac activity was specifically upregulated in a coreceptor-dependent manner in fusion reaction cell lysates. These results define a role for HIV Env-coreceptor interactions in activating the cellular factors essential for virus-cell and cell-cell fusion and provide evidence for the participation of pertussis toxin-insensitive signaling pathways in HIV-induced membrane fusion.     

Stable isotope variation of a highly heterogeneous shallow freshwater system

Food web structure is well known to vary widely among ecosystems. Recent research indicates that there can be a high degree of spatial heterogeneity within ecosystems as well. Xochimilco is a small heterogeneous freshwater system that has been transformed into a network of canals, small lakes, and wetlands. Located within Mexico City, this ecosystem has been intensively managed and highly impacted for more than 50 years. This system receives urban and agricultural runoff, with resulting impacts on water quality. The aquatic community is dominated by exotics such as carp (Cyprinus carpio) and tilapia (Oreocrhomis niloticus), though the system still supports endemic species such as the aquatic salamander, axolotl (Ambystoma mexicanum), and crayfish (Cambarellus montezumae), which are both endangered. In this study, we used carbon and nitrogen stable isotopes for the whole food web and gut content analysis from the exotic fishes to describe food web structure in different canals within Xochimilco. There were significant isotopic differences among canals. These differences may result from isotopic baseline differences as well as differences in actual food web structure: both are related to local spatial variation in water quality driven by nutrient inputs and exotic fishes. Within-ecosystem variability is likely to be seen in other perturbed shallow systems as well, and should be explicitly considered in future food web studies.     

A new dwarf shrubby species of <Emphasis Type="Italic">Baccharis</Emphasis> subg. <Emphasis Type="Italic">Baccharis</Emphasis> (Asteraceae,Astereae) from southeastern Brazil

Baccharis dichotoma, a new dwarf shrubby species, with small leaves and few heads, of high-altitude grasslands from southeastern Brazil, is described, illustrated, and assigned to subgenus Baccharis.