Hippocampal rhythm generation: Gamma-related theta-frequency resonance in CA3 interneurons

 During different behavioral states different population activities are present in the hippocampal formation. These activities are not independent: sharp waves often occur together with high-frequency ripples, and gamma-frequency activity is usually superimposed on theta oscillations. There is both experimental and theoretical evidence supporting the notion that gamma oscillation is generated intrahippocampally, but there is no generally accepted view about the origin of theta waves. Precise timing of population bursts of pyramidal cells may be due to a synchronized external drive. Membrane potential oscillations recorded in the septum are unlikely to fulfill this purpose because they are not coherent enough. We investigated the prospects of an intrahippocampal mechanism supplying pyramidal cells with theta frequency periodic inhibition, by studying a model of a network of hippocampal inhibitory interneurons. As shown previously, interneurons are capable of generating synchronized gamma-frequency action potential oscillations. Exciting the neurons by periodic current injection, the system could either be entrained in an oscillation with the frequency of the inducing current or exhibit in-phase periodic changes at the frequency of single cell (and network) activity. Simulations that used spatially inhomogeneous stimulus currents showed anti-phase frequency changes across cells, which resulted in a periodic decrease in the synchrony of the network. As this periodic change in synchrony occurred in the theta frequency range, our network should be able to exhibit the theta-frequency weakening of inhibition of pyramidal cells, thus offering a possible mechanism for intrahippocampal theta generation. Received: 23 February 2000 / Accepted in revised form: 30 June 2000     

Descriptions and phylogenetic systematics of Myxobolus spp. from cyprinids in Algonquin Park, Ontario

Eight species of Myxobolus were collected from four species of cyprinids in Algonquin Park, Ontario. On the basis of spore morphology, five of these species are described as new and two are redescribed. The evolutionary relationships among these eight species were studied using partial small subunit ribosomal DNA (ssu-rDNA) sequence data. The resulting cladograms, which were highly resolved and with strongly supported relationships, allowed for the evaluation of spore morphology, host specificity, and tissue tropism, criteria traditionally used in species identification. These criteria, recently criticized for creating artificial rather than natural taxonomic groupings, were evaluated for their reliability in the systematics of the species examined. The data showed that distantly related species often infect the same host and tissue, and that closely related species often occur in different hosts. Morphologically similar species are more closely related to each other and the taxonomy based on spore morphology is consistent with the relationships depicted in the phylogenies. These results suggest that spore morphology is better than host specificity and tissue tropism as a species character, as well as for determining evolutionary relationships among the species of Myxobolus examined.     

Two simple and rapid methods for the detection of polymer-degrading enzymes on high-resolution, alkaline, cold, in situ-native (HiRACIN)-PAGE and high-resolution, in situ-inhibited native (HiRISIN)-PAGE

Two sensitive, high-resolution and exceedingly versatile methods for the detection of isoenzymes of polymer-degrading enzymes on high-resolution, alkaline, cold, in situ-native (HiRACIN)-PAGE and high-resolution in situ-inhibited, native (HiRISIN)-PAGE are described. Extracellular crude extracts containing xylanases and carboxymethylcellulases from Scopulariopsis sp. and glucoamylases from Aspergillus niger were subjected to non-denaturing PAGE containing substrates in the resolving gel. In case of HiRACIN-PAGE, the enzymes were prevented from degrading their respective substrates during run by carrying out electrophoresis at 4°C and the pH of running and resolving gel buffer systems were increased from 8.5 to 10.6. In case of HiRISIN-PAGE, adding competitive inhibitor of the enzyme, cellobiose, in the resolving gel prevents the degradation of polymer during the run. These techniques were successfully applied, for the first time, to visualize four, three and four sharp and distinct bands of xylanases, glucoamylases and CMCases, respectively.     

Protein phosphatase 2A holoenzyme and its subunits from Medicago sativa

We detected an about 200 kDa holoenzyme of protein phosphatase 2A (PP2A) in the crude extract of Medicago sativa microcallus cells by gel permeation chromatography. By polymerase chain reaction (PCR) we isolated two M. sativa cDNA fragments corresponding to the catalytic (C) subunit, and one each coding for the A and the B regulatory subunits of PP2A. The C subunit sequences were different from that published previously, indicating the existence of at least three different isoforms in M. sativa. Using the PCR fragments as probes, we obtained two distinct full-length clones for both the A and B subunits from an alfalfa cDNA library. Our results demonstrate that the components of the PP2A holoenzyme, namely the catalytic and regulatory subunits, are present in alfalfa in several isoforms and that their sequences are highly similar to their plant, yeast and animal counterparts. The distinct regulatory subunit genes are constitutively expressed during the cell cycle. Interestingly, two A-B subunit pairs had parallel mRNA steady-state levels in different plant tissues suggesting that not all of the possible isoform combinations are present in all tissues. The expression of the MsPP2A B subunit form was induced by abscisic acid indicating a specific function for this protein in the stress response.     

The design and evaluation of a virtual distributed computing environment

Current advances in high-speed networks such as ATM and fiber-optics, and software technologies such as the JAVA programming language and WWW tools, have made network-based computing a cost-effective, high-performance distributed computing environment. Metacomputing, a special subset of network-based computing, is a well-integrated execution environment derived by combining diverse and distributed resources such as MPPs, workstations, mass storage, and databases that show a heterogeneous nature in terms of hardware, software, and organization. In this paper we present the Virtual Distributed Computing Environment (VDCE), a metacomputing environment currently being developed at Syracuse University. VDCE provides an efficient web-based approach for developing, evaluating, and visualizing large-scale distributed applications that are based on predefined task libraries on diverse platforms. The VDCE task libraries relieve end-users of tedious task implementations and also support reusability. The VDCE software architecture is described in terms of three modules: (a) the Application Editor, a user-friendly application development environment that generates the Application Flow Graph (AFG) of an application; (b) the Application Scheduler, which provides an efficient task-to-resource mapping of AFG; and (c) the VDCE Runtime System, which is responsible for running and managing application execution and for monitoring the VDCE resources. We present experimental results of an application execution on the VDCE prototype for evaluating the performance of different machine and network configurations. We also show how the VDCE can be used as a problem-solving environment on which large-scale, network-centric applications can be developed by a novice programmer rather than by an expert in low-level details of parallel programming languages. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tooth replacement rates in early chondrichthyans: a qualitative approach

The continuous replacement of teeth throughout their lifetime is a common characteristic of most chondrichthyans. This process was already present in the earliest representatives of the group. It has been well established that different species of extant sharks show rapid tooth replacement rates; however, some authors have suggested that in early chondrichthyans this rate might have been much slower. Here we present a qualitative approach to analyse tooth replacement rates in the Early Devonian shark Leonodus carlsi , the earliest tooth-bearing shark known to date. For this, we have examined 1,103 isolated teeth from Celtiberia, Spain. Our study provides strong evidences of an extremely slow dental replacement in this primitive chondrichthyan based on three independents analyses: (1) statistical analysis of the wear degree, demonstrating that teeth remain functional for a long period of time; (2) analysis of both the histological and the morphological features of the teeth cusps suggests that this chondrichthyan used a maturation process that optimizes its function, thus worn teeth show an efficient working shape that implies their teeth remained functional for a long time after being modelled by use; and (3) estimations of size increments between teeth (Δs) of the same dental family for some recent sharks whose rates of replacement were known prove that Δs is inversely proportional to the rate of replacement ( R ² = 0.8327). The estimated values of tooth replacement rates obtained from Δs for L. carlsi and for some Late Devonian cladoselachian sharks are significatively slower than those observed in current sharks.     

Mitochondrial DNA variation and the evolutionary history of the Mediterranean species of Cicada L. (Hemiptera,Cicadoidea)

Cicadas are widely distributed in the Mediterranean area and are mainly distinguished by male acoustic signals, which act as specific mate recognition systems. Within the genus Cicada L. several species have diverged in their calling songs without showing external morphological differences, so acoustic recordings and genetic studies are particularly useful for systematic, biogeography and evolutionary studies. This study assesses sequence variation in closely related species of Cicada from the Mediterranean area, using domain III of the 12S rRNA mitochondrial gene in order to determine the phylogenetic relationships and the evolutionary history of this group, as well as the population structure of the two most common species, C. orni and C. barbara. Five distinct haplogroups were identified, C. orni, C. barbara, C. mordoganensis, C. cretensis and C. lodosi, each corresponding to a distinct evolutionary group. C. barbara was the most divergent species within this group, while C. orni and C. mordoganensis were the most similar. The population structure and demographic parameters of the species were not completely resolved. However, there is evidence for the separation of the C. orni Greek populations from the rest of Europe and also for demographic expansions probably related to Pleistocene climate changes. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 266–288.     

Patch time allocation in male parasitoids

Abstract 1. Patch time allocation has been mostly studied in female parasitoids exploiting patches of hosts. Different parameters such as oviposition, host encounters, patch quality, etc. have been repeatedly shown to modify the time females invest on hosts. 2. Male parasitoids are expected to maximise their lifetime fitness by maximising the number of females inseminated during their life. Because they can be sperm and/or time limited, they should optimise their time allocation on emergence patches. 3. Patch time allocation thus appears to be an important question for both male and female parasitoids. 4. In this study, we determined the parameters used by males of the egg parasitoid Trichogramma turkestanica to decide when to leave the emergence patch. Among the different patch‐leaving parameters tested, only contacts with parasitised hosts and presence of virgin females significantly influenced the patch‐leaving tendency. 5. Our results suggest that males express behaviours that could enable them to optimise their patch exploitation time, as females do, but using different strategies.