Nutrient stress and performance of invasive Hieracium lepidulum and co-occurring species in New Zealand

Many studies have highlighted the relationship between nutrient fluctuations and enrichment in the process of plant invasion. However, invasion may be also associated with conditions of plant stress, either nutrient depletion or toxicity, in the environment. In this study, we investigate the possible role of nutrient stress in the invasion of Hieracium lepidulum (Stenstroem) Omang, in South Island, New Zealand. We do this by comparing several performance attributes, and their plasticity, for H. lepidulum and a number of co-occurring species, across a series of nutrient depletion and toxicity tests. H. lepidulum had intermediate yields, high root:shoot ratios and high tissue nutrient contents at control nutrient concentrations. H. lepidulum differed in edaphic tolerance from all but Chionochloa flavescens var. brevis, in being insensitive to nutrient dilutions other than nitrogen. The significance of performance in terms of edaphic tolerance and adaptations are discussed. Intermediate yields at control nutrient levels suggest that H. lepidulum should not be competitive compared to high yielding species including Agrostis stolonifera and Poa cita, but should be competitive with lower yielding Coprosma rugosa and C. flavescens var. brevis. Conversely, significant yield decreases under nitrogen limitation stress suggests that H. lepidulum will not likely occupy very nutrient poor sites. H. lepidulum, along with C. flavescens var. brevis, were found to be tolerant of ammonium as an alternative source of nitrogen, while other species were not. These data suggest that H. lepidulum and C. flavescens var. brevis would be relatively tolerant of the stresses associated with acidic soils compared to the other species, but not to stresses associated with absolute shortage of nitrogen. Combined results point to the likely occurrence of H. lepidulum at sites of intermediate fertility. The possible roles of ammonium stress and disturbance reliance in further defining H. lepidulum ecology is discussed.     

Viscoelastic study of the mechanical unfolding of a protein by AFM

We have applied a dynamic force modulation technique to the mechanical unfolding of a homopolymer of immunoglobulin (Ig) domains from titin, (C47S C63S I27)5, [(I27)5] to determine the viscoelastic response of single protein molecules as a function of extension. Both the stiffness and the friction of the homopolymer system show a sudden decrease when a protein domain unfolds. The decrease in measured friction suggests that the system is dominated by the internal friction of the (I27)5 molecule and not solvent friction. In the stiffness-extension spectrum we detected an abrupt feature before each unfolding event, the amplitude of which decreased with each consecutive unfolding event. We propose that these features are a clear indication of the formation of the known unfolding intermediate of I27, which has been observed previously in constant velocity unfolding experiments. This simple force modulation AFM technique promises to be a very useful addition to constant velocity experiments providing detailed viscoelastic characterization of single molecules under extension.     

Mechanical resistance of proteins explained using simple molecular models

Recent experiments have demonstrated that proteins unfold when two atoms are mechanically pulled apart, and that this process is different to when heated or when a chemical denaturant is added to the solution. Experiments have also shown that the response of proteins to external forces is very diverse, some of them being "hard," and others "soft." Mechanical resistance originates from the presence of barriers on the energy landscape; together, experiment and simulation have demonstrated that unfolding occurs through alternative pathways when different pairs of atoms undergo mechanical extension. Here we use simulation to probe the mechanical resistance of six structurally diverse proteins when pulled in different directions. For this, we use two very different models: a detailed, transferable one, and a coarse-grained, structure-based one. The coarse-grained model gives results that are surprisingly similar to the detailed one and qualitatively agree with experiment; i.e., the mechanical resistance of different proteins or of a single protein pulled in different directions can be predicted by simulation. The results demonstrate the importance of pulling direction relative to the local topology in determining mechanical stability, and rationalize the effect of the location of importation/degradation tags on the rates of mitochondrial import or protein degradation in vivo.     

A predictive model of avian natal dispersal distance provides prior information for investigating response to landscape change

1. Informative Bayesian priors can improve the precision of estimates in ecological studies or estimate parameters for which little or no information is available. While Bayesian analyses are becoming more popular in ecology, the use of strongly informative priors remains rare, perhaps because examples of informative priors are not readily available in the published literature. 2. Dispersal distance is an important ecological parameter, but is difficult to measure and estimates are scarce. General models that provide informative prior estimates of dispersal distances will therefore be valuable. 3. Using a world-wide data set on birds, we develop a predictive model of median natal dispersal distance that includes body mass, wingspan, sex and feeding guild. This model predicts median dispersal distance well when using the fitted data and an independent test data set, explaining up to 53% of the variation. 4. Using this model, we predict a priori estimates of median dispersal distance for 57 woodland-dependent bird species in northern Victoria, Australia. These estimates are then used to investigate the relationship between dispersal ability and vulnerability to landscape-scale changes in habitat cover and fragmentation. 5. We find evidence that woodland bird species with poor predicted dispersal ability are more vulnerable to habitat fragmentation than those species with longer predicted dispersal distances, thus improving the understanding of this important phenomenon. 6. The value of constructing informative priors from existing information is also demonstrated. When used as informative priors for four example species, predicted dispersal distances reduced the 95% credible intervals of posterior estimates of dispersal distance by 8-19%. Further, should we have wished to collect information on avian dispersal distances and relate it to species' responses to habitat loss and fragmentation, data from 221 individuals across 57 species would have been required to obtain estimates with the same precision as those provided by the general model.     

Mechanism and function of Drosophila capa GPCR: a desiccation stress-responsive receptor with functional homology to human neuromedinU receptor

The capa peptide receptor, capaR (CG14575), is a G-protein coupled receptor (GPCR) for the D. melanogaster capa neuropeptides, Drm-capa-1 and -2 (capa-1 and -2). To date, the capa peptide family constitutes the only known nitridergic peptides in insects, so the mechanisms and physiological function of ligand-receptor signalling of this peptide family are of interest. Capa peptide induces calcium signaling via capaR with EC₅₀ values for capa-1 = 3.06 nM and capa-2 = 4.32 nM. capaR undergoes rapid desensitization, with internalization via a b-arrestin-2 mediated mechanism but is rapidly re-sensitized in the absence of capa-1. Drosophila capa peptides have a C-terminal -FPRXamide motif and insect-PRXamide peptides are evolutionarily related to vertebrate peptide neuromedinU (NMU). Potential agonist effects of human NMU-25 and the insect -PRLamides [Drosophila pyrokinins Drm-PK-1 (capa-3), Drm-PK-2 and hugin-gamma [hugg]] against capaR were investigated. NMU-25, but not hugg nor Drm-PK-2, increases intracellular calcium ([Ca²⁺]i) levels via capaR. In vivo, NMU-25 increases [Ca²⁺]i and fluid transport by the Drosophila Malpighian (renal) tubule. Ectopic expression of human NMU receptor 2 in tubules of transgenic flies results in increased [Ca²⁺]i and fluid transport. Finally, anti-porcine NMU-8 staining of larval CNS shows that the most highly immunoreactive cells are capa-producing neurons. These structural and functional data suggest that vertebrate NMU is a putative functional homolog of Drm-capa-1 and -2. capaR is almost exclusively expressed in tubule principal cells; cell-specific targeted capaR RNAi significantly reduces capa-1 stimulated [Ca²⁺]i and fluid transport. Adult capaR RNAi transgenic flies also display resistance to desiccation. Thus, capaR acts in the key fluid-transporting tissue to regulate responses to desiccation stress in the fly.     

Consistent abundance distributions of marine fishes in an old,climatically buffered,infertile seascape

Aim Macroecological theory predicts that along direct physiological gradients there will be unimodal abundance distributions of species and consistent rates of assemblage turnover. However, the majority of marine studies that have investigated the realized distribution of species along latitudinal or temperature gradients have generally found unimodal distributions to be rare. We assess fish distributions along a temperature gradient in a stable oligotrophic seascape and suggest that unimodal distributions will be more common. Location Nearshore demersal fish habitat extending 1500 km along the coast of south‐western Australia. Methods The relative abundances of demersal fish species were sampled off the coast of south‐western Australia along a temperature gradient. The confounding influence of other environmental variables was tested, and the assemblage was found to be highly correlated with temperature. For the 20 most abundant species, quantile regression spline models were used to construct a model within which 95% of their abundance was expected to fall. We compared the results from this study with the proportion of unimodal species abundance distributions observed in other studies. Results Of the 20 most abundant species, 19 displayed patterns that indicated temperature was an important factor influencing their range and relative abundance; with 15 species exhibiting unimodal abundance distributions, four having ramped distribution to one end of the sampled range and one showing no consistent pattern. Main conclusions The high diversity and percentage of endemic species in terrestrial and marine habitats of south‐western Australia is likely to be due to the stable geological and oceanographic history of the region. In comparison, studies of abundance distribution in other marine systems have been conducted in relatively heterogeneous and productive environments. The old, climatically buffered, oligotrophic seascape of south‐western Australia has provided a simple system in which the consistent influence of physiological gradients on the abundance distribution of fish species can be observed.     

Temporal patterns in ambient noise of biological origin from a shallow water temperate reef

A systematic study of the ambient noise in the shallow coastal waters of north-eastern New Zealand shows large temporal variability in acoustic power levels between seasons, moon phase and the time of day. Ambient noise levels were highest during the new moon and the lowest during the full moon. Ambient noise levels were also significantly higher during summer and lower during winter. Bandpass filtering (700-2,000 Hz and 2-15 kHz), combined with snap counts and data from other studies show that the majority of the sound intensity increases could be attributed to two organisms: the sea urchin and the snapping shrimp. The increased intensity of biologically produced sound during dusk, new moon and summer could enhance the biological signature of a reef and transmit it further offshore. Ambient noise generated from the coast, especially reefs, has been implicated as playing a role in guiding pelagic post-larval fish and crustaceans to settlement habitats. Determining a causal link between temporal increases in ambient noise and higher rates of settlement of reef fish and crustaceans would provide support for the importance of ambient underwater sound in guiding the settlement of these organisms.     

Translational control by cytoplasmic polyadenylation in Xenopus oocytes

Elongation of the poly(A) tails of specific mRNAs in the cytoplasm is a crucial regulatory step in oogenesis and early development of many animal species. The best studied example is the regulation of translation by cytoplasmic polyadenylation elements (CPEs) in the 3' untranslated region of mRNAs involved in Xenopus oocyte maturation. In this review we discuss the mechanism of translational control by the CPE binding protein (CPEB) in Xenopus oocytes as follows: Finally we discuss some of the remaining questions regarding the mechanisms of translational regulation by cytoplasmic polyadenylation and give our view on where our knowledge is likely to be expanded in the near future.