Floral colour diversity in plant communities,bee colour space and a null model

Evolutionary biologists have long hypothesized that the diversity of flower colours we see is in part a strategy to promote memorization by pollinators, pollinator constancy, and therefore, a directed and efficient pollen transfer between plants. However, this hypothesis has never been tested against a biologically realistic null model, nor were colours assessed in the way pollinators see them. Our intent here is to fill these gaps. Throughout one year, we sampled floral species compositions at five ecologically distinct sites near Berlin, Germany. Bee-subjective colours were quantified for all 168 species. A model of colour vision was used to predict how similar the colours of sympatric and simultaneously blooming flowers were for bees. We then compared flower colour differences in the real habitats with those of random plant communities. We did not find pronounced deviations from chance when we considered common plants. When we examined rare plants, however, we found significant divergence in two of the five plant communities. At one site, similarly coloured species were found to be more frequent than expected, and at the other two locations, flower colours were indistinguishable from a random distribution. These results fit theoretical considerations that rare plants are under stronger selective pressure to secure pollination than common plants. Our study illustrates the power of linking such distinct biological traditions as community ecology and the neuroethology of bee vision.     

Expression of pH-sensitive green fluorescent protein in Arabidopsis thaliana

This is the first report on using green fluorescent protein (GFP) as a pH reporter in plants. Proton fluxes and pH regulation play important roles in plant cellular activity and therefore, it would be extremely helpful to have a plant gene reporter system for rapid, non‐invasive visualization of intracellular pH changes. In order to develop such a system, we constructed three vectors for transient and stable transformation of plant cells with a pH‐sensitive derivative of green fluorescent protein. Using these vectors, transgenic Arabidopsis thaliana and tobacco plants were produced. Here the application of pH‐sensitive GFP technology in plants is described and, for the first time, the visualization of pH gradients between different developmental compartments in intact whole‐root tissues of A. thaliana is reported. The utility of pH‐sensitive GFP in revealing rapid, environmentally induced changes in cytoplasmic pH in roots is also demonstrated.     

Spontaneous fusion of phosphatidylcholine small unilamellar vesicles in the fluid phase

Using a high-sensitivity differential scanning microcalorimeter capable of performing cooling scans, we have examined the phase behavior of small unilamellar vesicles (SUV) as a function of time of storage above their order-disorder phase transition. Vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were examined. Cooling scans on fresh (5-7-h postsonication) samples revealed broad, relatively simple heat capacity peaks (peak temperatures: 19.9 degrees C for DMPC, 37.8 degrees C for DPPC) free of high-temperature spikes or shoulders. Subsequent heating scans displayed a sharp peak characteristic of previously described fusion products formed below the phase transition. SUV samples stored for 1 or more days above their phase transition displayed a moderately broad, high-temperature shoulder (23.8 degrees C for DMPC and 40.2 degrees C for DPPC) in the cooling profile. For DMPC, the enthalpy associated with this peak increased in a first-order fashion with time. Hydrolysis products were not detected until 12-20 days of storage. Both the rate and extent of shoulder appearance increased with temperature (k = 0.0017 h-1, fraction of total enthalpy = 0.1 at 36 degrees C; k = 0.0037 h-1, fraction = 0.2 at 42 degrees C). Freeze-fracture electron micrographs confirmed that an intermediate-sized vesicle population (diameters 400-500 A) appeared in SUV samples stored above their phase transition. Also, the trapped volume of DMPC SUV increased from 0.26 microL/mumol after 17 h of storage to 0.54 microL/mumol after storage for 16 days at 36 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Quantifying the ion selectivity of the Ca2+ site in photosystem II: evidence for direct involvement of Ca2+ in O2 formation.

Calcium is an essential cofactor in the oxygen-evolving complex (OEC) of photosystem II (PSII). The removal of Ca2+ or its substitution by any metal ion except Sr2+ inhibits oxygen evolution. We used steady-state enzyme kinetics to measure the rate of O2 evolution in PSII samples treated with an extensive series of mono-, di-, and trivalent metal ions in order to determine the basis for the affinity of metal ions for the Ca2+-binding site. Our results show that the Ca2+-binding site in PSII behaves very similarly to the Ca2+-binding sites in other proteins, and we discuss the implications this has for the structure of the site in PSII. Activity measurements as a function of time show that the binding site achieves equilibrium in 4 h for all of the PSII samples investigated. The binding affinities of the metal ions are modulated by the 17 and 23 kDa extrinsic polypeptides; their removal decreases the free energy of binding of the metal ions by 2.5 kcal/mol, but does not significantly change the time required to reach equilibrium. Monovalent ions are effectively excluded from the Ca2+-binding site, exhibiting no inhibition of O2 evolution. Di- and trivalent metal ions with ionic radii similar to that of Ca2+ (0.99 A) bind competitively with Ca2+ and have the highest binding affinity, while smaller metal ions bind more weakly and much larger ones do not bind competitively. This is consistent with a size-selective Ca2+-binding site that has a rigid array of coordinating ligands. Despite the large number of metal ions that competitively replace Ca2+ in the OEC, only Sr2+ is capable of partially restoring activity. Comparing the physical characteristics of the metal ions studied, we identify the pK(a) of the aqua ion as the factor that determines the functional competence of the metal ion. This suggests that Ca2+ is directly involved in the chemistry of water oxidation and is not only a structural cofactor in the OEC. We propose that the role of Ca2+ is to act as a Lewis acid, binding a substrate water molecule and tuning its reactivity.     

Two remarkable new species of scuttle-fly (Diptera: Phoridae) that parasitize termites (Isoptera) in Sulawesi

ABSTRACT. Diplonevra mortimeri sp.nov. and D.watsoni. sp.nov. are described from specimens caught at Nasutitermitinae termite colonies in Sulawesi. The female of both species has a remarkable pre-oviposition behaviour in which she lures a worker termite into following her away from the colony. She then oviposits in the termite's abdomen. Pupation takes place within the remains of the host's abdomen. Both species only mature a single egg at a time. The female probably guards the comatose host termite during the period of larval development. The morphology of the male hypopygia of the new species allows reinterpretation of the peculiar anal tube in typical male Diplonevra. Furthermore the suggested homologies reinforce the view that the epandrium (tergite 9) has not been replaced by a periandrium (fused gonocoxites) in the Cyclorrhapha and consequently the latter cannot be derived from the Empidoidea.     

Viscoelasticity of packed erythrocyte suspensions subjected to low amplitude oscillatory deformation.

Concentrated adult erythrocyte suspensions were subjected to low amplitude oscillatory shear in a Weissenberg rheogoniometer equipped with a cone-and-plate assembly. The dynamic viscoelastic properties of the suspension were measured over a broad range of frequency by a numerical solution that accounted for fluid inertia. Variation of shear amplitude and cell volume percent, and comparison of buffered saline, plasma, and dextran as suspending media showed that the cellular elements had undergone small bending and shearing deformations. Studies of normal adult erythrocytes, hypotonically swollen cells, temperature-altered cells, and erythrocyte ghosts suggested that the method was evaluating membrane material properties. The normal membrane was found to exhibit a shear rate dependent elastic modulus that increased by more than a factor of 20 over a frequency range from 0.0076 Hz to 60 Hz. The membrane viscosity showed a substantial drop with frequency indicative of a frequency thinning phenomenon. At high frequency of deformation the viscous response of normal erythrocytes was no longer indicative of a membrane property due to the dominant influence of the internal hemoglobin solution. The studies generally supported the ability of the method to quantify relative membrane material properties and detect changes in membrane structure.