A UV resonance Raman investigation of poly(rI): Evidence for cation-dependent structural perturbations

Poly(rI) stabilized by either Na⁺ or K⁺ was investigated using uv resonance Raman (UVRR) spectroscopy. Raman excitation profiles of inosine 5′-monophosphate demonstrated the 250 nm excitation selectively enhances base stacking interactions, while ribose and carbonyl stretching vibrations are preferentially enhanced with 210 nm excitation. These wavelengths were used to examine the structure of poly(rI) in the presence of either K⁺ or Na⁺ as a function of temperature. UVRR studies revealed that the K⁺ stabilized form is more thermally stable, yielding a T_m of ∼ 47°C compared to a T_m of ∼ 30°C for the Na⁺ stabilized form. We observed that both the ribosyl conformation and the coordination of the carbonyl groups depend on the nature of the cation. The C₆O stretching frequency indicates that Na⁺ coordinates much more strongly to the carbonyl groups than K⁺ (1672 cm⁻¹ Na⁺ vs 1684 cm⁻¹ K⁺ at 4°C). Conformationally sensitive modes of the phosphate backbone and the ribosyl ring indicate that Na⁺ stabilized poly(rI) predominantly exists in a C_3′-endo ribose conformation, whereas K⁺ stabilized poly(rI) adopts a C_2′-endo conformation possibly as a consequence of the larger ionic radius of the K⁺ ion. © 1998 John Wiley & Sons, Inc. Biopoly 46: 475–487, 1998     

The geometry of coexistence

Understanding the processes that maintain diversity has been the focus of extensive study, yet there is much that has not been integrated into a cohesive framework. First, there is a separation of perspective. Ecological and evolutionary approaches to diversity have progressed in largely parallel directions. Second, there is a separation of emphasis. In both ecology and population genetics, classical theories favour local explanations with emphasis on population dynamics and selection within populations, while contemporary theories favour spatial explanations, with emphasis on population structure and interactions among populations. What is lacking is a comparative approach that evaluates the relative importance of local and spatial processes in maintaining genetic and ecological diversity. I present a framework for diversity maintenance that emphasizes the comparative approach. I use a well-known but little-used mathematical approach, the perturbation theorem for dynamical systems, to identify key points of contact between ecological and population genetic theories of coexistence. These connections provide for a synthesis of several important concepts: population structure (source-sink versus extinction-colonization), spatial heterogeneity (intrinsic versus extrinsic) in fitness and competitive ability, and temporal scales over which local and spatial processes influence diversity. This framework ties together a large and diverse body of theory and data from ecology and population genetics. It yields comparative predictions that can serve as guidelines in biodiversity management.     

Spatial pattern of size distribution of four fish species on Réunion coral reef flats

A comparative study of the size distribution of four fish species was performed on three geomorphological zones (back reef, inner reef flat and outer reef flat) on two comparable fringing reef areas (disturbed and non-disturbed) of Reunion Island, S. W. Indian Ocean. The species chosen Chaetodon trifasciatus, Dascyllus aruanus, Stethojulis albovittata and Ctenochaetus striatus were selected both for their abundance and their diverse diets. The method used is based on the estimation of fish size directly underwater by comparison to a graduated table, i.e. photographs of the species for which different size-classes were previously determined. For the two areas, the size of fishes increases from the back reef zone to the outer reef flat, except for D. aruanus which is not present in sufficient numbers on the outer reef flat to allow a statistical analysis. This suggests the existence of a nursery zone in the back reef from which these species might colonize the reef flat. On the back reef zone and the inner reef flat, there is a significant difference in the size distribution of fishes between the disturbed and non-disturbed areas, except for S. albovittata. The number of juveniles is significantly lower in the disturbed area, probably due to environmental perturbations.

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Résumé Une étude de la distribution des tailles de quatre espéces de poissons coralliens a été réalisée sur trois zones géomorphologiques (arriére récif, platier interne et platier externe) de deux secteurs récifaux comparables (dégradé et non dégradé) de l'île de la Réunion, océan Indien occidental. Les espéces étudiées, Chaetodon trisfaciatus, Dascyllus aruanus, Stethojulis albovittata et Ctenochaetus striatus ont été choisies pour leur abondance et leur diversité de régime alimentaire. La technique utilisée consiste á estimer la longueur du poisson directement sous l'eau á l'aide d'une tablette graduée. Des photographies de poissons de différentes tailles, préalablement définies, y ont été fixées, et ceci pour les quatre espéces. Pour les deux secteurs, une augmentation réguliére de la taille des poissons depuis l'arriére récif jusqu' au platier externe est apparue, sauf pour D. Aruanus ui n'est apas suffisamment abondante sur le platier externe pour permettre une analyse statistique. Ces observations suggérent l'existence d'une nurserie dans l'arriére récif á partir duquel pourrait s' opérer la colonisation du platier récifal. Dans les zones d' arriére récif et de platier interne, il y a par ailleurs une différence significative dans la distribution des tailles des poissons entre le secteur dégradé et le secteur non dégradé, sauf pour S. albovittata. Le nombre des juvéniles est significativement plus faible dans le secteur dégradé, ce qui pourrait être dû aux dégradations de l'environment corallien.

     

Discrete and reversible vacuole-like dilations induced by osmomechanical perturbation of neurons

In cultured Lymnaea stagnalis neurons, osmolarity increases (upshocks) rapidly elicited large membranous dilations that could be dislodged and pushed around inside the cell with a microprobe. Subsequent osmolarity decreases (downshocks) caused these vacuole-like dilations (VLDs) to disappear. Additional upshock/downshock perturbations resulted in repeated appearance/disappearance (formation/reversal) of VLDs at discrete sites. Confocal microscopy indicated that VLDs formed as invaginations of the substrate-adherent surface of the neuron: extracellular rhodamine-dextran entered VLDs as they formed and was expelled during reversal. Our standard VLD-inducing perturbation was: 2–4 min downshock to distilled water, upshock to normal saline. However, a wide range of other osmotic perturbations (involving osmolarities up to 3.5x normal, perturbations with or without Ca²⁺, replacement of ions by sucrose) were also used. We concluded that mechanical, not chemical, aspects of the osmo-mechanical shocks drove the VLD formation and reversal dynamics and that extracellular Ca²⁺ was not required.Following a standard perturbation, VLDs grew from invisible to their full diameter (>10 m) in just over a minute. Over the next 0.5–3 hr in normal saline, neurons recovered. Recovery eliminated any visible VLDs and was accompanied by cytoplasmic turmoil around the VLDs. Recovery was prevented by cytochalasin B, brefeldin A and N-ethylmaleimide but not by nocodazole. In striking contrast, these drugs did not prevent repeated VLD formation and reversal in response to standard osmo-mechanical perturbations; VLD disappearance during reversal and during recovery are different.The osmo-mechanical changes that elicited VLDs may, in an exaggerated fashion, mimic tension changes in extending and retracting neuntes. In this context we postulate: (a) the trafficking or disposition of membrane between internal stores and plasma membrane is mechano sensitive, (b) normally, this mechanosensitivity provides an on demand system by which neurons can accommodate stretch/release perturbations and control cell shape but, (c) given sudden extreme mechanical stimuli, it yields VLDs. Present address: Biology Department, Washington University, St Louis, MO 63130-4899This work was supported by NSERC of Canada research grants to CEM and to LRM. CR was the recipient of a postdoctoral fellowship from the Ministere francais de la Recherche et de l'Espace. We thank J-M. Trifaro for the use of his image processing equipment.     

Linear discrete models with different time scales

Aggregation of variables allows to approximate a large scale dynamical system (the micro-system) involving many variables into a reduced system (the macro-system) described by a few number of global variables. Approximate aggregation can be performed when different time scales are involved in the dynamics of the micro-system. Perturbation methods enable to approximate the large micro-system by a macro-system going on at a slow time scale. Aggregation has been performed for systems of ordinary differential equations in which time is a continuous variable. In this contribution, we extend aggregation methods to time-discrete models of population dynamics. Time discrete micro-models with two time scales are presented. We use perturbation methods to obtain a slow macro-model. The asymptotic behaviours of the micro and macro-systems are characterized by the main eigenvalues and the associated eigenvectors. We compare the asymptotic behaviours of both systems which are shown to be similar to a certain order.     

Lotka-Volterra equations: Decomposition,stability, and structure

Summary The major objective of this paper is to propose a new decomposition-aggregation framework for stability analysis of Lotka-Volterra equations employing the concept of vector Liapunov functions. Both the disjoint and the overlapping decompositions are introduced to increase flexibility in constructing Liapunov functions for the overall system. Our second objective is to consider the Lotka-Volterra equations under structural perturbations, and derive conditions under which a positive equilibrium is connectively stable. Both objectives of this paper are directed towards a better understanding of the intricate interplay between stability and complexity in the context of robustness of model ecosystems represented by Lotka-Volterra equations. Only stability of equilibria in models with constant parameters is considered here. Nonequilibrium analysis of models with nonlinear time-varying parameters is the subject of a companion paper.Research supported by U.S. Department of Energy under the Contract EC-77-S-03-1493.On leave from Kobe University, Kobe, Japan.     

Patterns of fish species richness in the Seine River basin,France

Variation in fish assemblage structure was examined in different rivers of the Seine Basin. Factor analysis was used to identify similarities among samples and species, and to show ichtyological changes along an upstream-downstream gradient. Fish species richness was correlated with catchment area, distance from the source and number of individuals. Relationships between species richness and number of individuals in a sample were largely an artifact of sampling.Species richness increased with river size, reached a maximum in midsize rivers, then decreased in large rivers. This species richness pattern was consistent with the model of the River Continuum Concept. However, possible causes could include greater pollution effects in large rivers. Distribution of feeding guilds was related to river size. Species richness and proportion of omnivores and piscivores increased with river size, whereas species richness and proportion of invertivores declined downstream. These patterns suggest that trophic diversity of fish assemblages may be related to food availability.     

A relaxation wave solution of the FitzHugh-Nagumo equations

For the system of FitzHugh-Nagumo equations and certain values of parameters, there exists a relaxation wave solution: a moving spatially periodic structure for which one of the dependent variables has intervals of slow and fast changes within its period. An asymptotic expansion for the solution is constructed (as a power series in the small parameter ) consisting of regular terms and transition layers. Along with the asymptotic expansion, the velocity of the structure, its period, and the distance between jumps of one of the functions are determined.Visiting from the Moscow State University, Russia.     

The carbon budget of the Spanish forests

A model for the calculation of anthropogenic CO₂ emissions from perturbed forests (extraordinary emissions) is described. Timber production as well as wildfire statistics are used, and relevant physicochemical parameters are derived from the literature, to calculate the annual amounts of perturbed biomass and extraordinary emissions to the atmosphere from the Spanish forests — including soils — in the years 1960–1990; these emissions increased from 5.3 10⁶ t (metric tons) of carbon in 1970 to 10.6 10⁶ t C in 1990. A sensitivity analysis of the results has been performed to identify the most critical parameters. Contributions of observed timber growth and natural vegetal detritus and soil organic matter to the net forest carbon flux have been estimated to calculate the carbon budget in the Spanish forests between 1966 and 1974, which represents a net gain of carbon, ranging from 9.2 to 18 millions t. Finally, the methods used to calculate biospheric carbon balance and their results are compared and discussed.     

Small perturbations in nonlinear age-structured population equations

A non-linear age-structured population dynamic model described by partial integro-differential equations is considered, where the non-linear term reflects the interaction among individuals. This non-linear model is interpreted as a perturbed linear one. An approximation method based on the ideas of averaging, of a slow time transformation and a power series expansion is suggested. The asymptotic properties of the approximate solution are analyzed. It is shown that the approximate solution remains close to the true solution ast .