Pioneer growth cone behavior at a differentiating limb segment boundary in the grasshopper embryo

The first neurons to extend axons through embryonic grasshopper limbs are a pair of sibling pioneer neurons. After migrating proximally along the limb axis, the pioneer growth cones normally make an abrupt ventral turn. In some cases (less than 20%) this turn is directly toward the proximo-ventrally located Cx1 guidepost neurons. However, in the majority of cases (greater than 80%) the pioneer growth cones make a more acute ventral turn along a single circumferential line which lies distal to the Cx1 neurons. Growth cones from other afferent neurons orient along the same line. Growth cones can extend along this line around more than half of the circumference of the limb and can grow in either direction along it. The circumferential line appears to be the prospective trochanter-coxa segment boundary. Afferent axons on the segment boundary leave it and contact the proximo-ventrally located Cx1 neurons. The site at which pioneer growth cones leave the boundary is variable and appears to be the point from which filopodial contact with Cx1 cells is first established. In addition to the trochanter-coxa segment boundary, the pioneer growth cones and axons also respond to the tibia-femur and femur-trochanter segment boundaries. The role of segment boundaries as barriers to growth cone movement and the effect of such barriers on the timing and placement of differentiation of pioneer neurons are discussed.     

The importance of habitat temperatures for growth in the tropical cichlid Tilapia rendalli Boulenger

Like many cichlids living in shallow tropical freshwater systems of Africa, Tilapia rendalli exhibit a marked daily movement into warm shallow water during the day and return to cooler, deeper water at night. Such movements have an important impact on the growth of juvenile fish. Growth is enhanced by this movement and is predicted to be optimal if the fish occupy water with a temperature of 30°C during the day but retire to nocturnal habitats with temperatures of 18°C. The reasons for this phenomenon are discussed using a bio-energetic model incorporating measurements of feeding intensity, metabolic energy demands and assimilation efficiencies at temperatures ranging from 18°C—3°C and the ecological implications discussed using examples from natural systems.     

Cell-Cell Interactions and Distal Outgrowth in Amphibian Limbs

SYNOPSIS. A current model concerning the process of limb regenerationin vertebrates is examined. According to this model (Bryantet al, 1981), new positional values in the proximal-distal limbaxis are laid down as a result of local interactions betweencells in the limb circumference. Cells with disparate circumferentialpositional values come together at the site of future outgrowthand intercalation between them generates more distal levelsof the pattern. The results of a number of experiments on surgicallycreated symmetrical limb stumps are discussed in relation tothis model. In addition, an extension of this model to accountfor digit formation is presented, and the implications of thisformulation for limb evolution are discussed.     

Quantitative trait loci for growth trajectories in Populus

Growth trajectories are a biological process important to plant and animal breeding, and to evolutionary genetic studies. In this article, we report the detection of quantitative trait loci (QTLs) responsible for growth trajectories in poplars that are used as a model system for the study of forest biology. These QTLs were localized on a genetic linkage map of polymorphic markers using a statistical mapping method incorporating growth-curve models. The effects of the QTLs on growth are described as a function of age, so that age-specific changes in QTL effects can be readily projected throughout the entire growth process. The QTLs identified display increased effects on growth when trees age, yet the timing of QTL activation is earlier for stem height than diameter, which is consistent with the ecological viewpoint of canopy competition. The implications of the results for breeding and silviculture are discussed.     

Enzymes of ammonium metabolism in ectendomycorrhizal and ectomycorrhizal symbionts of pine

Ammonium assimilation enzymes from several strains of ectendo- and ectomycorrhizal fungi were assayed after three weeks culture on a buffered synthetic medium containing ammonium as sole nitrogen source. Activity of NADP-dependent glutamate dehydrogenase (GDH, EC 1.4.1.4) of ectomycorrhizal strains was very low despite excellent mycelial growth. Only ectendomycorrhizal fungus MrgX isolated from roots of Pinus sylvestris showed high GDH activity. Similar results were obtained when the enzyme extracts were subjected to starch gel electrophoresis. Growth of the fungi, except ectendomycorrhizal MrgX, was arrested when inhibitors of glutamine synthetase (GS, EC 6.3.1.2) or glutamate synthase (GOGAT. EC 1.4.7.1) (methionine sulphoximine or albizine, respectively) were included in the culture medium. Glutamine synthetase activity was found in all fungi tested. The results suggest that the GS pathway for ammonium assimilation is potentially operative in ectomycorrhizal fungi and imply only a minor role for GDH in ammonium assimilation by the studied ectomycorrhizal symbionts of pine. Some physiological and ecological implications of these results are discussed.     

Modeling and analysis of stoichiometric two-patch consumer-resource systems

Ecological stoichiometry studies the balance of energy and multiple chemical elements in ecological interactions to establish how the laws of thermodynamics affect food-web dynamics and nutrient cycling in ecosystems. In this paper, we incorporate stoichiometric principles in a model with habitat heterogeneity and dispersal in order to better understand population growth dynamics. This model describes a situation where a resource is separated into two patches by a barrier. Growth of the resource in each patch is limited by soil fertility and self-crowding. The consumer disperses between the two patches and is not affected by the barrier. The consumer's growth is potentially limited by the phosphorus content of the acquired resource. Mathematical analysis of this model and simulations are performed. Several biological implications, including an observed "stoichiometric extinction effect," are demonstrated with simulation where the stoichiometric mechanism appears to promote extinction in a patchy environment. This is in sharp contrast to the notion that stoichiometry mechanism promotes diversity in spatially homogeneous settings. Another important result is the rediscovery of a simple and plausible biological mechanism that generates local and global extinction. In this setting, which can be considered a spatially mediated form of apparent competition, the dispersal of the consumer from the rich patch can des the growth of the resource in the poor patch and in some situations can lead to the extinction of the resource in the poor patch.     

Stochastic population dynamics of clonal plants: Numerical experiments with ramet and genet models

Dynamics of ramer and genet populations were analyzed by use of stochastic matrix models. Based on field data, population development and extinction rates during 50 simulated years were estimated for ramet populations of three speciesPotentilla anserina, Rubus saxatilis andLinnaea borealis. Only small initial populations (below 125–250 ramets), experienced a detectable risk of extinction within this time interval. ForP. anserina andR. saxatilis, population increase occurred in some simulations despite negative average growth rates. A model for stochastic genet dynamics was constructed by combining field data and hypothesized parameter values. Growth rate and population structure were insensitive to variation in disturbance intensity and frequency, whereas variation in recruitment affected population structure but only to a minor extent growth rate. Decreasing recruitment causes extinction of genet populations, but the time-scale for the decline is in the magnitude of centuries for initial genet populations of about 1000 individuals. Dynamics of genets in clonal plants thus incorporate processes occurring on widely different scales. Some implications of the results for models of population dynamics in long-lived clonal plants are discussed.     

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.     

Dynamics of microbial propagation: Models considering inhibitors and variable cell composition

Mathematical models for microbial growth in batch and continuous cultures are formulated. The models have been referred to as distributed models since the microbial population in a culture is looked upon as protoplasmic mass distributed uniformly throughout the culture. Growth is regarded as the increase in this mass by conversion of medium components into biological mass and metabolic products. Two sets of models have been presented. The first arise from introducing additional considerations into the model proposed by Monod to account for the stationary phase and the phase of decline in a batch culture. These have been referred to as unstructured, distributed models since they do not recognize any form of structure in the protoplasmic mass. The models in the second set are referred to as structured, distributed models. Structure is introduced by considering the protoplasmic mass to be composed of two groups of substances which interact with each other and with substances in the environment to produce growth. The structured models account for the dependence of growth on the past, history of the cells; thus they predict all growth phases observed in batch cultures, whereas the unstructured models do not predict a lag phase. The full implications of the models for continuous propagation, as determined by the method of stability analysis and transient calculations, are discussed. The models prediet a number of new results and should be confronted with experiments.     

Selected aspects of microbial osmoregulation

Abstract Some salient characteristics of microbial osmoregulation are reviewed, with specific examples drawn from eukaryotes. As well as the need for an osmoregulatory solute to be 'compatible' with cellular processes under all conditions, the importance of the physiological method of regulating the content of the solute as a factor determining xerotolerance is emphasized. The significance of turgor/volume homeostasis is discussed and examples are cited in which, during exponential growth, there is apparently no homeostatic control of the cellular content of the major osmoregulatory solute. Some implications of this for the overall mechanism of osmoregulation are considered.
A recent experiment is described which raises questions about the timing of an osmoregulatory 'signal' in Saccharomyces cerevisiae . Other experiments are summarized which distinguish between osmoregulatory and compatible solutes in yeast. These experiments implicate trehalose as a non-osmoregulatory compatible solute in certain circumstances.     

A naturally-occurring structural analogue of the phytotoxin coronatine

Chlorosis-inducing compounds in liquid cultures of the phytopathogenic bacterium Pseudomonas syringae pv. atropurpurea have been investigated. In addition to coronatine as previously reported, a new compound was discovered. This gave a mass spectral fragmentation pattern which indicated that it was, like coronatine, an amide of coronafacic acid. Acid-hydrolysis of the new toxin liberated the amino acid valine. This observation, together with mass spectral and NMR data, established the structure of the new toxin as N-coronafacoylvaline. Some implications to biosynthesis are discussed. Along with the two chlorosis-inducing compounds, the biologically inactive coronafacic acid was also isolated from the growth medium.