Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study

The ultrastructure and distribution of receptor cells near the mouth and (where present) the pharynx of Hofstenia miamia, Proporus bermudensis, Conaperta thela, and Convoluta convoluta (Acoela) were investigated by transmission electron microscopy and confocal laser scanning microscopy of specimens stained with a fluorescence marker for actin. Five types of monociliary receptors were identified: (1) non‐collared receptors with a single long and narrow ciliary rootlet; (2) non‐collared receptors with a wide main ciliary rootlet and a smaller posterior rootlet; (3) non‐collared receptors with a single wide and hollow ciliary rootlet with a granulated core; (4) Collar (?) receptors with obliquely radial filament bundles in the cell apex and with a single hollow ciliary rootlet composed of numerous strand‐like elements; and (5) Collar receptors lacking a striated rootlet but with a granular body (swallow's nest rootlet). While H. miamia bears the first two receptor types, P. bermudensis has receptors of type 1, 3 and 5, and Cona. thela and Conv. convoluta have receptors of type 3, 4 and 5. The density of receptors is generally highest at the anterior body tip, regardless of where the mouth is located. Most receptor types occur scattered over the whole body but type 2 receptors of H. miamia are restricted to the pharynx and mouth region. The lack of a common receptor type specific for the mouth and pharynx of the investigated species points to an independent origin of the pharynges in Hofsteniidae and in Proporidae and of the mouth tube in Convolutidae. Moreover, the homology of the so‐called collar receptors in Acoela with typical collar receptors in other invertebrates is questioned.     

Effects of pH on bacterial porin function.

Porin is a trimeric channel-forming protein in the outer membrane of Gram-negative bacteria. Functions of the porins OmpF, OmpC, and PhoE from Escherichia coli K12 were analyzed at various pHs. Preliminary results from bilayer lipid membrane and liposome swelling assays indicated that in vitro porin has at least two open-channel configurations with a small and a large size. The small channels were stabilized at low pH while the larger channels were detected under basic conditions. The size switch occurred over a very narrow range near neutral pH, and the two major open-channel configurations responded differently to variations in voltage. The presence of two or more pH-dependent substates of porin could explain the variability in pore diameter measured by others and suggests a more dynamic role for porin in the cell.     

Vertically stratified frugivore community composition and interaction frequency in a liana fruiting across forest strata

Vertical stratification is a key feature of tropical forests and structures plant–frugivore interactions. However, it is unclear whether vertical differences in plant-frugivore interactions are due to differences among strata in plant community composition or inherent preferences of frugivores for specific strata. To test this, we observed fruit removal of a diverse frugivore community on the liana Marcgravia longifolia in a Peruvian rain forest. Unlike most other plants, Marcgravia longifolia produces fruits across forest strata. This enabled us to study effects of vertical stratification on fruit removal without confounding effects of plant species and stratum. We found a high number of visits of a few frugivore species in the understorey and a low number of visits of many different frugivores in the canopy and midstorey. Whereas partial and opportunistic frugivores foraged across strata with differing frequencies, obligate frugivores were only found eating fruits in the higher strata. Avian frugivores foraging in the canopy were mainly large species with pointed wings, whereas under- and midstorey avian foragers were smaller with rounded wings. Our findings suggest a continuous shift in the frugivore community composition along the vertical gradient, from a few generalized frugivores in the understorey to a diverse set of specialized frugivores in the canopy. This shift in the frugivore community leads to correlated, reciprocal changes from specialized to generalized plant-frugivore interactions. Thus, we conclude that vertical niche differentiation between species in tropical forests persists even when food resources are available across strata. This highlights its role for promoting biodiversity and ecosystem functioning.     

Assessment of interleukin 1 and interleukin 2 effects on cycling and noncycling murine thymocytes

When mouse thymocytes are stimulated with PHA, the proliferative response is very low, unless the culture medium is enriched with interleukin 1 (IL-1)- or interleukin 2 (IL-2)-containing supernatants. Cytofluorometric analyses show, however, that PHA stimulation generates a significant number of cells with increased RNA content (transition from the G₀ to G₁ phase of the cell cycle). If IL-2 is added to such cultures, the activated cells complete their process of RNA synthesis and then enter the S phase. The use of IL-2-containing culture medium thus permits one to obtain a high correlation between the number of g₁ cells and [³H]thymidine incorporation (r = 0.97). Enrichment with IL-1-containing supernatants also results in a statistically significant correlation (r = 0.68), but the regression lines are markedly different for the two interleukins (s = 20.3 for IL-2 and s = 9.2 for IL-1), when analyzed after 48 hr of incubation. These observations suggest that the G₁ phase must be divided into two subcompartments, G_1a and G_1b, the G_1a-G_1b transition being an IL-2-dependent event. If the number of G_1b cells is used to establish correlations with [³H]thymidine incorporation, all values fall on the same regression line, regardless of culture conditions and of the addition of interleukins. It is concluded that IL-2 regulates lymphocyte proliferation at the level of RNA synthesis (G_1a-G_1b transition) rather than that of DNA synthesis (G₁-S transition).     

Differences in relative air humidity affect responses to soil salinity in freshwater and salt marsh populations of the dominant grass species Phragmites australis

Hydrobiologia - Soil salinity diminishes the dominance of species and affects their distribution. Phragmites australis is a dominant ecosystem engineer with broad distribution, high intraspecific...     

Intraspecific differences of Asian/Australian Phragmites australis subgroups reveal no potentially invasive traits

Hydrobiologia - Phragmites australis is a cosmopolitan plant species with high intraspecific diversity and phenotypic plasticity. Due to its variability and large ecological niche breadth,...     

Crystal Structure and Pathophysiological Role of the Pneumococcal Nucleoside-binding Protein PnrA

Nucleotides are important for RNA and DNA synthesis and, despite a de novo synthesis by bacteria, uptake systems are crucial. Streptococcus pneumoniae, a facultative human pathogen, produces a surface-exposed nucleoside-binding protein, PnrA, as part of an ABC transporter system. Here we demonstrate the binding affinity of PnrA to nucleosides adenosine, guanosine, cytidine, thymidine and uridine by microscale thermophoresis and indicate the consumption of adenosine and guanosine by ¹H NMR spectroscopy. In a series of five crystal structures we revealed the PnrA structure and provide insights into how PnrA can bind purine and pyrimidine ribonucleosides but with preference for purine ribonucleosides. Crystal structures of PnrA:nucleoside complexes unveil a clear pattern of interactions in which both the N- and C- domains of PnrA contribute. The ribose moiety is strongly recognized through a conserved network of H-bond interactions, while plasticity in loop 27–36 is essential to bind purine- or pyrimidine-based nucleosides.Further, we deciphered the role of PnrA in pneumococcal fitness in infection experiments. Phagocytosis experiments did not show a clear difference in phagocytosis between PnrA-deficient and wild-type pneumococci. In the acute pneumonia infection model the deficiency of PnrA attenuated moderately virulence of the mutant, which is indicated by a delay in the development of severe lung infections. Importantly, we confirmed the loss of fitness in co-infections, where the wild-type out-competed the pnrA-mutant. In conclusion, we present the PnrA structure in complex with individual nucleosides and show that the consumption of adenosine and guanosine under infection conditions is required for virulence.