On the use of variability time-scales as an early classifier of radio transients and variables

We have shown previously that a broad correlation between the peak radio luminosity and the variability time-scales, approximately L ~ t^5, exists for variable synchrotron emitting sources and that different classes of astrophysical source occupy different regions of luminosity and time-scale space. Based on those results, we investigate whether the most basic information available for a newly discovered radio variable or transient - their rise and/or decline rate - can be used to set initial constraints on the class of events from which they originate. We have analysed a sample of ~ 800 synchrotron flares, selected from light-curves of ~ 90 sources observed at 5-8 GHz, representing a wide range of astrophysical phenomena, from flare stars to supermassive black holes. Selection of outbursts from the noisy radio light-curves has been done automatically in order to ensure reproducibility of results. The distribution of rise/decline rates for the selected flares is modelled as a Gaussian probability distribution for each class of object, and further convolved with estimated areal density of that class in order to correct for the strong bias in our sample. We show in this way that comparing the measured variability time-scale of a radio transient/variable of unknown origin can provide an early, albeit approximate, classification of the object, and could form part of a suite of measurements used to provide early categorisation of such events. Finally, we also discuss the effect scintillating sources will have on our ability to classify events based on their variability time-scales.Comment: Accepted for publication in MNRA

Microbial Endemism and Biogeography

The topic of microbial biogeography is almost 100 years old, however, when confronted with questions about the existence and extent of endemism in the microbial world, many microbiologists respond with opinions and theoretical arguments rather than examples of well-conducted studies. We begin this chapter with an overview of this debate as it applies to free-living prokayotes in part because there are relatively few good microbial biogeography studies. Furthermore, the arguments help to frame microbial biogeography in the larger context of biodiversity in that if endemism is common, then many more species exist

The effects of drought stress and type of fertiliser on generalist and specialist herbivores and their natural enemies

Abiotic stresses such as drought and nutrient availability can affect invertebrate herbivores feeding on plants, and potentially cascade up to impact their predators and parasitoids. Although these two factors separately been the subject of many studies, there are few tests of their combined effects in the context of pest species and their natural enemies on cultivated plants. Climate change models predict an increase in the frequency and severity of droughts, while the type and amount of fertiliser applied to crops is more under the control of growers. Understanding how these two abiotic factors may interact is key to utilising the potential of natural enemies to control pests under a future climate. To address this, a range of drought and fertiliser type treatments were applied to a model Brassica system in a factorial design, and the performance of two ubiquitous aphid species and their parasitoids was assessed. One aphid species was a specialist on Brassicas (Brevicoryne brassicae, with parasitoid Diaeretiella rapae) and the second a generalist aphid species (Myzus persicae, with parasitoid Aphidius colemani). The performance of both aphid species responded in a similar way to the treatments, and was maximised on plants growing in organic fertilisers under medium levels of drought stress. The strongest effects of drought and fertiliser cascaded up to affect parasitoids. Parasitoid performance responded in a broadly similar way to their aphid host performance in relation to fertiliser type. Some of the smaller effects of fertiliser treatments on aphid performance were not found for parasitoid performance. Aphid performance was greatest on plants under medium drought stress, but the parasitoids only responded consistently to the high drought stress treatment, on which their performance was reduced. Interactions between the drought and fertiliser did not have a large effect on aphid or parasitoid performance, compared with the strong main effects found for each treatment. These results are discussed in the context of previous and future research on the impacts of abiotic stresses on invertebrate herbivores and their natural enemies

Drought and root herbivory interact to alter the response of above-ground parasitoids to aphid infested plants and associated plant volatile signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/belowground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40–55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and belowground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought

Reclassification of the polyphyletic genus Prosthecomicrobium to form two novel genera, Vasilyevaea gen. nov. and Bauldia gen. nov. with four new combinations: Vasilyevaea enhydra comb. nov., Vasilyevaea mishustinii comb. nov., Bauldia consociata comb. nov. and Bauldia litoralis comb. nov.

Species of the genus Prosthecomicrobium are noted for their numerous cellular appendages or prosthecae that extend from the cells. This investigation confirms that the genus is polyphyletic based on an extensive analysis of the 16S rRNA gene sequences of several named species of the genus. The analyses indicate that some Prosthecomicrobium species are more closely related to non-prosthecate genera, including Devosia, Labrenzia, Blastochloris, Methylosinus, Mesorhizobium and Kaistia, than they are to other species of the genus Prosthecomicrobium. For this reason, two of the Prosthecomicrobium clades which are polyphyletic with the type species, Prosthecomicrobium pneumaticum, are renamed as new genera. The currently named species Prosthecomicrobium enhydrum, Prosthecomicrobium mishustinii, Prosthecomicrobium consociatum and Prosthecomicrobium litoralum are reclassified in two new genera, Vasilyevaea gen. nov. and Bauldia gen. nov. with four new combinations: Vasilyevaea enhydra comb. nov. (the type species) and Vasilyevaea mishustinii comb. nov., and Bauldia consociata comb. nov. and Bauldia litoralis comb. nov. (the type species). The type strain of Vasilyevaea enhydra is strain 9bT (=ATCC 23634T =VKM B-1376T). The type strain of the other species in this genus is Vasilyevaea mishustinii strain 17T (=VKM B-2499T =CCM 7569T). The type strain of Bauldia litoralis is strain 524-16T (= NCIB 2233T =ATCC 35022T). The type strain of the other species in this genus is Bauldia consociata strain 11T (=VKM B-2498T =CCM 7594T)

In vitro assembly and GTP hydrolysis by bacterial tubulins BtubA and BtubB

Arecent study identified genuine tubulin proteins, BtubA and BtubB, in the bacterial genus Prosthecobacter. We have expressed BtubA and BtubB in Escherichia coli and studied their in vitro assembly. BtubB by itself formed rings with an outer diameter of 35–36 nm in the presence of GTP or GDP. Mixtures of BtubB and BtubA formed long protofilament bundles, 4–7 protofilaments wide (20–30 protofilaments in the three-dimensional bundle). Regardless of the starting stoichiometry, the polymers always contained equal concentrations of BtubA and BtubB, suggesting that BtubA and B alternate along the protofilament. BtubA showed negligible GTP hydrolysis, whereas BtubB hydrolyzed 0.40 mol GTP per min per mol BtubB. This GTPase activity increased to 1.37 per min when mixed 1:1 with BtubA. A critical concentration of 0.4–1.0 μM was indicated by light scattering experiments and extrapolation of GTPase versus concentration, thus suggesting a cooperative assembly mechanism

A canonical FtsZ protein in Verrucomicrobium spinosum, a member of the Bacterial phylum Verrucomicrobia that also includes tubulin-producing Prosthecobacter species

BACKGROUND: The origin and evolution of the homologous GTP-binding cytoskeletal proteins FtsZ typical of Bacteria and tubulin characteristic of eukaryotes is a major question in molecular evolutionary biology. Both FtsZ and tubulin are central to key cell biology processes – bacterial septation and cell division in the case of FtsZ and in the case of tubulins the function of microtubules necessary for mitosis and other key cytoskeleton-dependent processes in eukaryotes. The origin of tubulin in particular is of significance to models for eukaryote origins. Most members of domain Bacteria possess FtsZ, but bacteria in genus Prosthecobacter of the phylum Verrucomicrobia form a key exception, possessing tubulin homologs BtubA and BtubB. It is therefore of interest to know whether other members of phylum Verrucomicrobia possess FtsZ or tubulin as their FtsZ-tubulin gene family representative. RESULTS: Verrucomicrobium spinosum, a member of Phylum Verrucomicrobia of domain Bacteria, has been found to possess a gene for a protein homologous to the cytoskeletal protein FtsZ. The deduced amino acid sequence has sequence signatures and predicted secondary structure characteristic for FtsZ rather than tubulin, but phylogenetic trees and sequence analysis indicate that it is divergent from all other known FtsZ sequences in members of domain Bacteria. The FtsZ gene of V. spinosum is located within a dcw gene cluster exhibiting gene order conservation known to contribute to the divisome in other Bacteria and comparable to these clusters in other Bacteria, suggesting a similar functional role. CONCLUSION: Verrucomicrobium spinosum has been found to possess a gene for a protein homologous to the cytoskeletal protein FtsZ. The results suggest the functional as well as structural homology of the V. spinosum FtsZ to the FtsZs of other Bacteria implying its involvement in cell septum formation during division. Thus, both bacteria-like FtsZ and eukaryote-like tubulin cytoskeletal homologs occur in different species of the phylum Verrucomicrobia of domain Bacteria, a result with potential major implications for understanding evolution of tubulin-like cytoskeletal proteins and the origin of eukaryote tubulins

Stability Analysis of Asynchronous States in Neuronal Networks with Conductance-Based Inhibition

Oscillations in networks of inhibitory interneurons have been reported at various sites of the brain and are thought to play a fundamental role in neuronal processing. This Letter provides a self-contained analytical framework that allows numerically efficient calculations of the population activity of a network of conductance-based integrate-and-fire neurons that are coupled through inhibitory synapses. Based on a normalization equation this Letter introduces a novel stability criterion for a network state of asynchronous activity and discusses its perturbations. The analysis shows that, although often neglected, the reversal potential of synaptic inhibition has a strong influence on the stability as well as the frequency of network oscillations