The responses of peripheral and central mechanosensory lateral line units of weakly electric fish to moving objects

Mechanosensory lateral line afferents of weakly electric fish (Eigenmannia) responded to an object which moved parallel to the long axis of the fish with phases of increased spike activity separated by phases of below spontaneous activity. Responses increased with object speed but finally may show saturation. At increasingly greater distances the responses decayed as a power function of distance. For different object velocities the exponents (mean±SD) describing this response falloff were -0.71±0.4 (20 cm/s object velocity) and-1.9±1.25 (10 cm/s). Opposite directions of object movement may cause an inversion of the main features of the response histograms. In terms of peak spike rate or total number of spikes elicited, however, primary lateral line afferents were not directionally sensitive.Central (midbrain) lateral line units of weakly electric fish (Apteronotus) showed a jittery response if an object moved by. In midbrain mechanosensory lateral line, ampullary, and tuberous units the response to a rostral-tocaudal object movement may be different from that elicited by a caudal-to-rostral object motion. Central units of Apteronotus may receive input from two or more sensory modalities. Units may be lateral line-tuberous or lateral line-ampullary. Multimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. The receptive fields of central units demonstrate a weak somatotopic organization of lateral line input: anterior body areas project to rostral midbrain, posterior body areas project to caudal midbrain.Abbreviation EOD electric organ discharge     

Integrating sample similarities into latent class analysis: a tree-structured shrinkage approach

This paper is concerned with using multivariate binary observations to estimate the probabilities of unobserved classes with scientific meanings. We focus on the setting where additional information about sample similarities is available and represented by a rooted weighted tree. Every leaf in the given tree contains multiple samples. Shorter distances over the tree between the leaves indicate a priori higher similarity in class probability vectors. We propose a novel data integrative extension to classical latent class models with tree-structured shrinkage. The proposed approach enables (1) borrowing of information across leaves, (2) estimating data-driven leaf groups with distinct vectors of class probabilities, and (3) individual-level probabilistic class assignment given the observed multivariate binary measurements. We derive and implement a scalable posterior inference algorithm in a variational Bayes framework. Extensive simulations show more accurate estimation of class probabilities than alternatives that suboptimally use the additional sample similarity information. A zoonotic infectious disease application is used to illustrate the proposed approach. The paper concludes by a brief discussion on model limitations and extensions.     

Between Xylem and Phloem: The Genetic Control of Cambial Activity in Plants

Abstract: Post-embryonic development is controlled by two types of meristems: apical and lateral. There has been considerable progress recently in understanding the function of root and shoot apical meristems at the molecular level. Knowledge of analogous processes in the lateral, or secondary, meristems, i.e. the vascular cambium or cork cambium, is, however, rudimentary. This is despite the fact that much of the diversity in the plant kingdom is based on the differential functions of these meristems, emphasizing the importance of lateral meristems in the development of different plant forms. The vascular cambium is particularly important for woody plants, but it also plays an important role during the development of various herbaceous species, such as Arabidopsis thaliana. In this review, we focus on the two basic functions of cambial activity: cell proliferation and pattern formation.     

Occurrence of hemoglobin in the nitrogen-fixing root nodules of Alnus glutinosa

A true hemoglobin (Hb) was shown to be present in the root nodules of Alnus glutinosa L. After purification by gel filtration and ion exchange, the Hb formed a stable complex with oxygen. This oxygen complex could then be converted to carboxyhemoglobin by treatment with CO. Optical absorption spectra typical of Hb were observed. The molecular weight was estimated to be 15 100 by gel filtration, and 18 300 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The Hb was largely insoluble when the initial homogenization was done in the absence of a detergent. Under these conditions much of the Hb appears to be associated with clusters of Frankia , the nitrogen-fixing actinomycete that infects plant cells within the nodules. The exact localization of the Hb in vivo is uncertain. The relatively low average concentration of Hb in Alnus nodules suggests that it is either confined to a relatively small fraction of total nodule volume, or has a function other than facilitation of O₂ transport.     

Mathematical simulation of the interactions among cyanobacteria, purple sulfur bacteria and chemotrophic sulfur bacteria in microbial mat communities

Abstract: A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stratification patterns and population dynamics of cyanobacteria, purple and colorless sulfur bacteria as found in marine microbial mats. The model involves the major biogeochemical processes of the sulfur cycle and includes growth metabolism and their kinetic parameters as described from laboratory experimentation. Hence, the metabolic production and consumption processes are coupled to population growth. The model is used to calculate benthic oxygen, sulfide and light profiles and to infer spatial relationships and interactions among the different populations. Furthermore, the model is used to explore the effect of different abiotic and biotic environmental parameters on the community structure. A strikingly clear pattern emerged of the interaction between purple and colorless sulfur bacteria: either colorless sulfur bacteria dominate or a coexistence is found of colorless and purple sulfur bacteria. The model predicts that purple sulfur bacteria only proliferate when the studied environmental parameters surpass well-defined threshold levels. However, once the appropriate conditions do occur, the purple sulfur bacteria are extremely successful as their biomass outweighs that of colorless sulfur bacteria by a factor of up to 17. The typical stratification pattern predicted closely resembles the often described bilayer communities which comprise a layer of purple sulfur bacteria below a cyanobacterial top-layer; colorless sulfur bacteria are predicted to sandwich in between both layers. The profiles of oxygen and sulfide shift on a diel basis similarly as observed in real systems.     

Effects of diffusion limitation on immobilized nitrifying microorganisms at low temperatures

Activation energies of suspended and immobilized nitrifying bacteria were determined and compared to determine if diffusion limitation results in decreased sensitivity for temperature. The activation energy for the respiration activity of suspended Nitrosomonas europaea and Nitrobacter agilis was found to be 86.4 and 58.4 kJ mol(-1), respectively. The activation energy for oxygen diffusion in the support material, kappa-carrageenan, determined from the effect of temperature on the effective diffusion coefficient (D), was 17.2 kJ mol(-1). Consequently, the apparent actvation energy of diffusion limited cells should be lower. It was indeed shown that due to the effect of diffusion limitation and to temperature effects on the Monod constant K(s), the immobilized-cell activity was less sensitive to temperature. The apparent activation energy for immobilized Ns. europaea was between 28.6 and 94.2 kJ mol(-1) and for immobilized Nb. agilis between 1.4 and 72.9 kJ mol(-1), depending on the oxygen concentration and temperature. (c) 1995 John Wiley & Sons, Inc.     

The effective diffusion coefficient and the distribution constant for small molecules in calcium-alginate gel beads

The effective diffusion coefficient, D(e), and the distribution constant, K(i), for selected mono- and disaccharides and organic acids were determined in homogeneous calcium-alginate gel with and without entrapped bacteria. Results were obtained from transient concentration changes in well-stirred solutions of limited volume, in which the gel beads were suspended. The effective diffusioncoefficients and the distribution constants were estimated by fitting mathematical model predictions to the experimental data using a nonlinear model fitting program (MODFIT). Both single solute diffusion and multiple solute diffusion were performed. A small positive effect was obtained onthe values of D(e) for the system of multiple solute diffusion; however, the values of K(i) were not significantly influenced. For the nine solutes tested, D(e) for 2% Ca-alginate gel beads was found to be approximately 85% of the diffusivity measured in water. The effects on D(e) and K(i), for lactose and lactic acid were determined for variations of alginate concentration, pH, temperature, and biomass content in the beads. D(e) decreased linearly for both lactose and lactic acid with increasing cell concentration in the Ca-alginate gel. K(i), was constant for both lactose and lactic acid with increasing cell concentration. D(e) was significantly lower at pH 4.5 than at pH 5.5 and 6.5 for both lactose and lactic acid. Furthermore, D(e) seemed to decrease with increased alginate concentration in the range of 1% to 4%. The diffusion rate increased with increasing temperature, and the activation energy for the diffusion process for both lactose and lactic acid was constant in the temperature range tested. (c) 1995 John Wiley & Sons Inc.     

Assessment of protein solution versus crystal structure determination using spin- diffusion-suppressed NOE and heteronuclear relaxation data

A spin-diffusion-suppressed NOE buildup series has been measured for E. coli thioredoxin.The extensive 13C and 15N relaxation data previously reported for this protein allow fordirect interpretation of dynamical contributions to the 1H-1H cross-relaxation rates for a largeproportion of the NOE cross peaks. Estimates of the average accuracy for these derived NOEdistances are bounded by 4% and 10%, based on a comparison to the corresponding X-raydistances. An independent fluctuation model is proposed for prediction of the dynamicalcorrections to 1H-1H cross-relaxation rates, based solely on experimental structural andheteronuclear relaxation data. This analysis is aided by the demonstration that heteronuclearorder parameters greater than 0.6 depend only on the variance of the H-X bond orientation,independent of the motional model in either one- or two-dimensional diffusion (i.e., 1– S2 = 3/4 sin2 2 ). The combination of spin-diffusion-suppressed NOEdata and analysis of dynamical corrections to 1H-1H cross-relaxation rates based onheteronuclear relaxation data has allowed for a detailed interpretation of various discrepanciesbetween the reported solution and crystal structures.