Limb Regeneration in the Fiddler Crab, Uca pugilator: Hormonal and Growth Factor Control

This paper summarizes recent work on various aspects of hormonalcontrol of regeneration in the crustacean, Uca pugilator. Hormonalcontrol in this crab is effected by means of the crustacteansteroid hormones, the ecdysteroids. New evidence is presentedsupporting a role for the retinoid hormones, all-trans retinoicacid and 9-cis retinoic acid, in the control of regenerationin these animals. The possible role of fibroblast growth factorsin organization of the limb blastema is explored and the similaritiesbetween vertebrate and invertebrate control of regenerationare discussed.     

A Model for the Simulation of the Growth of Beta vulgaris L.

A simple explanatory model for the simulation of the growthof Beta is described. It takes account of photosynthesis, respiration,and the light-intercepting properties of the foliage, but usesthe observed distribution pattern of dry matter. A crop of Beta was grown and, by periodic sampling of plants,growth curves were obtained.These are compared with simulatedgrowth curves and the reasons for the discrepancies are discussed. From simulations with different values of the constants of themodel, the effects on yield of variations in these constantsare predicted     

Limb Regeneration in the Fiddler Crab, Uca pugilator: Histological, Physiological and Molecular Considerations

This paper summarizes our recent work on the histological, physiologicaland molecular aspects of limb regeneration in the fiddler crabUca pugilator and new information is presented on mitotic activityin the blastema during the first days of blastemal organization.We also report for the first time the localization of vertebrategrowth factor immunoreactivity (FGF 2 and FGF 4) in the regeneratingblastema. In the first part of this paper we review recent histologicalfindings concerning the physical events that accompany autotomyof limbs and propose a new function for the autotomy membrane—thetethering of the regenerating pedal nerve to the walls of thecoxa. In the second part of the paper we review our recent findingson the identification and characterization of the Uca ecdysteroidreceptor (UpEcR, and its potential dimer partner, the retinoid-X-receptor,UpRXR). Using Uca-specific antibody probes raised in our lab,we have been able to identify specific cells in the early blastemathat express receptor proteins. The regenerating limb of thefiddler crab is responsive to both steroids and retinoids andmRNA for steroid and retinoid receptors are expressed in theregenerating limb buds during all stages of regeneration. TheDNA and deduced amino acid sequences of the ecdysteroid receptoris very similar to the sequences of insect EcRs, while the retinoidreceptor is similar to insect protein (ultraspiracle) in theDNA-binding domain, but closer to vertebrate RXRs in the ligandbinding domain     

Limb Autotomy as an Investigatory Tool: Host Molt-Stage Affects the Success Rate of Infective Larvae of a Rhizocephalan Barnacle

D. M. Skinner was the first biologist to systematically investigatethe effect of multiple limb autotomy on ecdysis in decapod crustaceans.She proposed the existence of (1) limb autotomy factor anecdysis(LAF_an) which initiates precocious molting, and (2) limb autotomyfactor proecdysis (LAF^pro) which postpones proecdysis. Meantime to ecdysis did not differ significantly among groups ofsmall juvenile Callinectes sapidus with zero, two, four, andsix limbs removed. Variance was significantly less for the groupmissing six limbs; i.e., autotomy of six limbs synchronizedthe molt-cycle. These patterns were consistent with the hypothesesthat LAF^an exerts an additive effect, i.e., more anecdysialcrabs enter proecdysis as more limbs are autotomized and thata delay of proecdysis occurs above a threshold (>4) of autotomizedlimbs. Multiple limb autotomy provides investigators with easyaccess to crabs in metecdysial molt-stages which is useful instudying interactions between rhizocephalan barnacles and theirhosts. While 71% of metecdysial C. sapidus exposed to infectivelarvae of the sacculinid rhizocephalan, Loxothylacus texanus,developed the external stage of the parasite, no similarlysizedanecdysial crab was parasitized, suggesting that these crabsare not susceptible to infection during anecdysis. Size of C.sapidus at infection was inversely proportional to the numberof ecdyses between infection and emergence of the parasite,but not correlated to final host size. These data suggest thatthere is a minimum threshold for host size; smaller hosts undergomore ecdyses before attaining the threshold. Rhizocephalansare being considered as biological control agents and thereis a need to understand how they find and infect hosts. Becauselimb autotomy is such a useful research tool, D. M. Skinner'scontributions to our understanding of how limb autotomy influencesthe crustacean molt cycle will continue to pay significant dividendsin crustacean biology.     

Two Perspectives on the Evolution of the Tetrapod Limb

SYNOPSIS. The evolution of the tetrapod limb is examined fromtwo perspectives: structural and functional. Rosen et al. (1981)argued that lungfishes are the sister group of tetrapods, withlimb characteristics comprising an important subset of theirevidence. A re-analysis of the limb characters advocated byRosen et al. does not support their contention, but insteadsuggests that rhipidistian fishes of the family Osteolepidaeare the closest relatives of the tetrapods. In order to understandthe probable selective pressures leading to evolution of thetetrapod limb, a functional analysis of the fins of antennariidanglerfishes was performed. Antennariids use their limb-likefins to traverse underwater substrates. The analysis revealsa large number of functional and morphological convergencesbetween antennariid fins and tetrapod limbs. It is suggestedthat tetrapod limbs were evolved for underwater transport ratherthan for locomotion on dry land.     

Predicting the Growth Interactions between Plants in Mixed Species Stands using a Simple Mechanistic Model

The Conductance model is a simple mechanistic model used topredict the growth of species in monoculture or mixtures fromparameter values derived from plants grown in isolation. Incontrast to many mechanistic models that require extensive parameterization,the Conductance model is able to capture the growth of a broadrange of species using a few simplified assumptions regardingplant growth and easily derived species-specific parameter values.We examine the assumptions within the Conductance model thattotal leaf area per plant is proportional to total plant weight,and that an isolated plant has a projected crown zone area thatis proportional to the 2/3 power of its weight. Power ratherthan linear relations were found between weight and leaf areafor Brassica oleracea, Daucus carota, Matricaria inodora, Solanumnigrum,Stellaria media , Trifolium repens and Veronica persica.For all seven species, the value of the power was less thanunity. All species also exhibited a power relation between crownzone area and weight, with the slope of this relation beingless than 2/3 for B. oleracea, D. carota and S. media. Althoughmorphology type accounted for some of the variation in the parametervalues relating to light interception, there were considerabledifferences between species within upright or prostrate foliagespecies groups. The Conductance model was used to predict yieldsof B. oleracea, S. nigrum and V. persica grown in both monocultureand binary weed-crop mixtures over a range of temporal and spatialscales. After calibrating the model to non-competing plants,the model was used to predict growth of the weed and crop speciesin contrasting densities and stand types. In some crop-weedcombinations, predicted crop and weed weights were within 17%of observed values, with no systematic deviations. In others,systematic and large deviations occurred.Copyright 2001 Annalsof Botany Company Brassica oleracea L., Daucus carota L., Matricaria inodora L., Solanum nigrum L.,Stellaria media L., Trifolium repens L., Veronica persica L., competition, growth, leaf area, crown zone area, light, shoot morphology, canopy architecture     

Quantitative Analysis of Cold Hardening and Dehardening in Lolium

The change in cold hardiness of three Lolium multiflorum Lam.varieties was followed in plants exposed to hardening or dehardeningconditions at a range of temperatures. Hardening and dehardeningwere analysed as quantitative processes dependent upon temperatureand time. Their time courses changed exponentially to an asymptotewhich was logistically related to temperature. Both componentsof the model were fitted simultaneously. Parameters of biologicalinterest, such as the initial rates of hardening and dehardeningfor a given temperature and the percentage of the process completedin a given time, were derived and compared for varieties ofcontrasting hardiness. The analysis demonstrated the importanceof dehardening in determining hardiness. Similar results wereobtained when the model was applied to hardening and dehardeningdata for Lolium perenne L. The potential of this quantitative analysis for distinguishingbetween possible mechanisms of cold hardiness is discussed andfurther experiments required to characterise the kinetics ofhardening and dehardening more fully are identified. Lolium multiflorum, Lolium perenne, cold hardening, cold dehardening, processes, model, varieties     

Effects of Acetylene and External Oxygen Concentration on the Respiratory Quotient (RQ) of Nodulated Roots of Soyabean and White Clover

Carbon dioxide output and oxygen uptake by intact nodulatedroots of soyabean and white clover were determined over a rangeof external oxygen concentrations (10 to 60%) in the presenceand absence of 10% acetylene. The responses of both sets ofroot systems were in agreement with the known characteristicsof the variable oxygen diffusion resistance of the nodules.These measurements were used to calculate the respiratory quotient(RQ, CO2/O2) for both total root respiration and nitrogenase-linkedrespiration values for each external oxygen concentration. Inthe absence of acetylene, the RQ values of soyabean total rootrespiration showed no significant response to external oxygen,with a mean of 1.06, but differences in values for white cloverroots were significant at P=0.05, with a mean of 1.09. The sametrend was shown for RQ values of nitrogenase-linked respiration,with mean values of 1.10 and 1.12 for soyabean and clover, respectively.The addition of acetylene caused a small but significant initialreduction in RQ of total root respiration with both soyabeanand white clover, as did the subsequent reduction of externaloxygen to 10%. For white clover, increases in external oxygenconcentration to 50% and 60% also produced significant increasesin the RQ of total root respiration compared to that in air.These variations were more pronounced for RQ values of nitrogenaselinkedrespiration. Results are discussed in relation to the use ofRQ values of unity in calculations of nodular oxygen diffusionresistance and the involvement of physical and biochemical processesin the regulation of oxygen supply to bacteroids. Key words: Respiratory quotient, nodules, oxygen, diffusion resistance     

Modelling Light Absorption and Canopy Net Photosynthesis of Glasshouse Row Crops and Application to Cucumber

A model of light absorption and photosynthesis applicable toglasshouse row crops is constructed and applied to cucumber.Light absorption is calculated using a method suggested fordiscontinuous canopies; photosynthesis is modelled with a non-rectangularhyperbola. The predictions of this model are compared with experimentaldata in the preceding paper. Here the model is used to simulateresponses to light and CO₂ concentration and especially to examinethe effects of varying the parameters of the crop that can becontrolled by the grower. These include the number of plantsin each row, the number and width of the rows, the gap betweenrows, and the height of the crop. For example, it is shown that,for high values of crop net photosynthesis, the number of rowsis more important at high light than at low light, whereas cropheight is more important at low light than at high light. Theimplications of these and other findings are discussed. Key words: Cucumis sativus L., glasshouse crops, cucumber, model, light absorption, photosynthesis, CO₂, row crops, simulation     

A DEMOGRAPHIC ANALYSIS OF y-NEOTRICULA APERTA (GASTROPODA: POMATIOPSIDAE) POPULATIONS IN THAILAND AND SOUTHERN LAOS, IN RELATION TO THE TRANSMISSION OF SCHISTOSOMIASIS

The results of field work undertaken along the tropical reachesof the Mekong river in Thailand and Laos are presented. Demographicdata concerning the schistosome-transmitting snail y-Neotriculaaperta are examined and related to the life-cycle of this species.On the basis of these data, and a review of the literature,the only available, empirically substantiated, model for thelife-cycle of y-N. aperta (see Upatham et al., 1980) is shownto be equivocal. In this model snails survive the annual spatein the Mekong river as eggs, which remain dormant below theflood waters, whilst the adults die soon after the waters beginto rise. Here, the above described model of ‘egg survival’is shown to fail to balance the demographic equation for y-N.aperta. An alternative model of delayed oviposition with post-spatesurvival of mature snails is proposed and shown to be a betterfit to the available data. The implications of this alternativemodel for the evolutionary biology and ecology of N. apertaare discussed. The possible effects of the Pak-Mul dam project(Northeast Thailand) on local populations of schistosome-transmittingsnails are assessed and the hydrodynamic conditions conduciveto vector snail proliferation are considered. Attenuation ofthe normal flood-cycle of the Mekong river anc alteration ofthe river bed topography will probably lead to increased numbersof N. aperta at Pak-Mul. These changes, together with the influxof casual labour from endemic areas in Laos, may result in anepidemic of human schistosomiasis in Northeast Thailand. (Received 18 April 1994; accepted 23 June 1994)     

Population Responses to Environmental Change: Life History Variation, Individual-based Models, and the Population Dynamics of Short-lived Organisms

SYNOPSIS. We review two potentially important approaches topredicting the consequences of environmental change for populationsof short-lived organisms. First, we examine the concepts of"feasible life histories" and "feasible demographies" and presentthe results of a set of simulations in which the effects onpopulation growth rate of varying one of the demographic variables(average nest survival, average juvenile survival rates, averageannual adult survival rates, or age-specific fecundity) overa broad range of values while the others are maintained at long-termpopulation average values for the Grapevine Hills, Texas populationof the short-lived lizard Sceloporus merriami. The results ofthese simulations are compared to an analogous set of simulationsfor a Michigan population of the relatively long-lived snappingturtle (Chelydra serpentina, Congdon et al., 1994). The implicationsof differences in feasible demographies and life histories suchas described for these two species are discussed. We also discuss the approach of using individual-based, physiologicallystructured models to predict population response to environmentalvariation and present the results of simulations using a modeldeveloped for predicting population-level effects of operativeenvironmental variation in the lizard S. merriami under twodifferent climate change scenarios. This individual-based, physiologicallystructured model incorporates population-specific data on ecologicalenergetics, thermal and size dependence of digestive physiologyand metabolic rates, energetics of individual growth, allometricrelationships, social structure and mating system, and the dependenceof mortality rates on age, size, and social status of individuals.The data necessary to such models of population response toenvironmental variation can come only from detailed long-termstudies of individual populations.     

The Rate of Growth and Partitioning of Assimilates in Young Grass Plants: A Mathematical Model

A model describing the increase in weight with time of younggrass plants is formulated. The parameters are the relativegrowth rates of the root and shoot systems; k, the ratio ofthe relative growth rate of the root system relative to thatof the shoot system; b, the weight of the root system when thatof the shoot system is unity, and u the rate of increase inweight of the whole plant per unit of shoot system per unitof time, k and b are the constants in the allometric formula,r = bs^k where r and are the weights of the root and shoot systems.The model enables the effect of changes in the distributionof assimilates between the root and shoot systems upon the rateof growth of the plant to be assessed. Data from a number ofexperiments are analysed in this manner and the significanceof the results discussed.     

Neurotrophic Contribution to a Proposed Tripartite Control of the Mitotic Cycle in the Regeneration Blastema of the Newt, Notophthalmus (Triturus) viridescens

My work has shown that the neural dependence of the limb regenerate(blastema) for growth, is retained in vitro and it follows thenormal in vivo pattern. Implanted dorsal root ganglia promotelocalized growth in cultured blastemata whereas in the absenceof nerves little or no development ensues. Our transfilter studiesshow that the neurotrophic effect, which appears to be mediatedby a factor capable of transmission through thin filters oflow porosity, is manifested in a proximo-distal gradient ofmitotic activity which diminishes with distance from the nervesource. This mitogenic effect appears to require at least 48hr for expression, which corresponds to one revolution of thecell cycle. In the absence of nerves, both DNA synthesis andmitotic index in the blastema decline significantly; however,a basal level is maintained which may be nerve independent.In this context and in the light of pertinent recent literature,we present a model in which nerves, the apical epidermis andinsulin (tripartite control) all play essential roles in themitotic cycle.     

Evolution of Hoxa-11 Expression in Amphibians: Is the Urodele Autopodium an Innovation?

In this paper we present preliminary results on the expressionof Hoxa-11 in the frog Xenopus laevis and the newt Notophthalmusviridescens. In amniote limbs, Hoxa-11 is expressed exclusivelyin the prospective zeugopodium and is involved in the developmentof zeugopodial character identity. While the Hoxa-11 expressionpattern in Xenopus is similar to those described in mice andchick, the newt Notophthalmus has a phase of autopodial Hoxa-11expression starting with the development of digit III. Expressionis particularly strong in the buds of the postaxial digits.This Hoxa-11 expression pattern is unique among tetrapods andadds to the list of developmental peculiarities of urodele limbdevelopment, which also include an anterior to posterior polarityof digit development. We propose a scenario to explain the evolutionaryorigin of urodele limb development. We assume that recent urodelesare derived from a lineage of amphibians which underwent partialdigit reduction, leaving only two digits intact, most likelydigits III and IV according to Morse's law. This implies thaturodele digits I and II are homologous to amniote digits IIIand IV, and that the postaxial digits of urodeles are an evolutionaryinnovation (re-invention). Supporting evidence for this homologyhypothesis is found in the developmental connections betweenthe basale commune and the postaxial zeugopodial element.     

Two Geometrical Models of Branching of Botanical Trees

A botanical tree may be regarded as a system of axes which developby repeated bifurcation. A complicated tree can be describedby a few parameters of bifurcation which determine the geometryof the bifurcation process. A bifurcation model (H-model) haspreviously been proposed and shown to be useful for comparisonwith trees which have branch complexes which approach a horizontalplane (e.g. Terminalia). Another bifurcation model (P-model)is now proposed which is appropriate to branching systems inwhich successive branch planes are perpendicular to each other(e.g. Cameraria and Tabernaemontana of the Apocynaceae). Bymodification of the P-model to take into account a geotropiceffect, a more realistic branching model for one kind of treespecies is produced. The relationship among these geometricalmodels of branching is discussed and illustrated with computersimulations. computer simulations, tree crown geometry, branching patterns, bifurcation models, Terminalia, Cameraria latifolia, Tabernaemontana, sp     

考考赖沟流域植被生产力模拟

 该文应用气象数据、土壤物理属性实测数据、土壤水分分布式动态模型和植被表面净辐射模型的模拟结果,利用多元线性回归分析方法,建立 了植被净第一性生产力模型,实现了鄂尔多斯高原东部砂质荒漠化地区考考赖沟流域尺度上30 m×30 m空间分辨率的植被生产力精确模拟,并 且用植被生产力的野外实测数据对模拟结果进行了验证表明: 实测值与模拟值在固定沙丘、半固定沙丘和样线2上都达到0.05显著性相关水平; 不同位置的实测植被生 产力数据多分布在1∶1直线附近,模拟值与实测值吻合较好;植被生产力实测值与模拟值的相对误差范围为3.22%～ 6.27%,偏斜度范围在-12.84%～4.43%。该文的研究方法可以为流域尺度上植被生产力的精确模拟提供借鉴和参考。