Relationships among recent models for insect population dynamics with variable rates of development

A classification scheme for those population models which allow variation in development rates is proposed, based on two ways of modifying standard age-structured models. The resulting classes of models are termed development index models and sojourn time models. General formulations for the two classes of models are developed from two basic balance equations, and numerous specific models from the literature are shown to fit into the scheme. Concepts from competing risks theory are shown to be important in understanding the interplay between mortality and maturation. Relationships among the classes are investigated both for the most general forms of the models and for the simpler forms often used. The scheme can provide guidance in developing appropriate insect population models for specific modelling situations.Contribution 3878871     

Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic

The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species showed that different species respond in individualistic ways to different combinations of these environmental variables.     

Nitrogen dynamics in two antarctic streams

The many glacier meltwater streams of southern Victoria Land flow through catchments where life forms are almost entirely microbial. Allochthonous inputs of nitrogen from two study streams near McMurdo Sound were derived mostly from the melting glaciers (ca. 100–200 mg N m^–3) with some originating from N₂-fixation by heterocystous cyanobacteria (max. 939 mg N m^–2 year^–1). Thirty to fifty per cent of the glacier derived N was dissolved organic N and a major proportion of this was identified as urea N which was utilised by the rich algal and cyanobacterial mats in the streams. A nutrient budget for Fryxell Stream was estimated, quantifying uptake of urea-N and dissolved inorganic N and the release of dissolved organic (non urea) and particulate N by the stream communities. An index of in-stream nitrogen processing, the Net Uptake Length Constant in these streams was compared with that from temperate climates and was found to be similar. Despite the influence of low temperatures on microbial activity (mean daily water temperature = 5 °C) nutrient removal rates from these antarctic streams are high because of the large standing stock of microbial biomass there.     

Effect of Moniliformis moniliformis density on distribution within the definitive host population (Rattus norvegicus)

The population dynamics of Moniliformis moniliformis was studied in ‘free-ranging’ laboratory rats, Rattus norvegicus, presented with different relative density levels of M. moniliformis in cockroaches, Periplaneta americana. Changes in selected population parameters of the negative binomial distribution were evaluated as indicators of changes in aggregation. A significant increase in the degree of aggregation of parasites occurred as a result of the increase in relative density of infective stages available to the rats. This increase in aggregation was due to the increase in over-dispersion that occurred in female rats only. The degree of aggregation in females was found to be significantly higher than that in males at both treatment levels. The best indicators of the degree of aggregation were found to be the ratio of the variance to the relative density and the ratio of the log-variance to log-relative density. Changes in k were not correlated with changes in over-dispersion or the relative density.     

How does l-glutamate transport relate to selection of mixed nitrogen sources in Rhizobium leguminosarum biovar trifolii MNF1000 and cowpea Rhizobium MNF2030?

When growing on a mixture of ammonia and l-glutamate as nitrogen sources, Rhizobium leguminosarum biovar trifolii MNF1000 utilizes ammonia exclusively, while cowpea Rhizobium MNF2030 utilizes both compounds at similar rates. l-Glutamate transport in both strain MNF1000 and MNF2030 is active, giving rise to a 60-fold concentration gradient across the membrane of cells of strain MNF2030. Both strains produce two kinetically distinguishable glutamate transport systems under all conditions of growth — a high affinity system with an apparent K _m of 0.06–0.17 M but of relatively low V _max, and a low affinity system with a K _m of 1.2–6.7\ M, but of higher overall capacity. l-Glutamate transport activity in cells of MNF2030 was relatively insensitive to the presence of ammonia in the growth medium. By contrast, ammonia in the growth medium resulted in low activities of glutamate transport in cells of MNF1000 which were provided with a carbon source, offering one explanation for the failure of this strain to use glutamate in the presence of ammonia. However, in cells of MNF1000 growing on glutamate as sole source of carbon and nitrogen, the glutamate transport system is synthesized, even in the presence of accumulated or added ammonia. This suggests that the regulation of the glutamate permease also depends on availability of carbon source.Abbreviations CCCP carbonyl cyanide m-chlorophenyl hydrazone - HEPES N-hydroxyethylpiperazine-N-2-ethanesulphonic acid     

The abundance of the blue shark,Prionace glauca,in the western English Channel

Synopsis Since 1952, a sport fishery for the blue shark,Prionace glauca, has existed off the south coast of Cornwall in England. Annual catches from this fishery have ranged from < 200 to>6000 sharks. The fishery was based on a previously unexploited stock in the 1950s. The abundance of the species in the English Channel declined in the early 1960s and again in the mid-1970s. The declining abundance was investigated in relation to sea surface temperature (SST), prey abundance, and fishing pressure. Short-term fluctuations in SST were found to be responsible for changes in the distribution of the population, but not for changes in abundance. The abundance of prey species in the Channel was observed to be inversely related to the abundance of blue sharks. The reduced abundance of blue sharks lowered the level of effective predation on the prey populations, allowing their abundance to increase. It was concluded that the nature of the fishing practice off the Cornish coast was responsible for a significant part of the decrease in shark abundance. By killing large numbers of sub-adult females, the reproductive capacity of the population was lowered. Continued fishing pressure prevented the population from recovering; and as of 1987, the abundance of the species in the Channel is still declining. Various conservation measures have been proposed.     

Medicago truncatula,a model plant for studying the molecular genetics of theRhizobium-legume symbiosis

Medicago truncatula has all the characteristics required for a concerted analysis of nitrogen-fixing symbiosis withRhizobium using the tools of molecular biology, cellular biology and genetics.M. truncatula is a diploid and autogamous plant has a relatively small genome, and preliminary molecular analysis suggests that allelic heterozygosity is minimal compared with the cross-fertilising tetraploid alfalfa (Medicago sativa). TheM. truncatula cultivar Jemalong is nodulated by theRhizobium meliloti strain 2011, which has already served to define many of the bacterial genes involved in symbiosis with alfalfa. A genotype of Jemalong has been identified which can be regenerated after transformation byAgrobacterium, thus allowing the analysis ofin-vitro-modified genes in an homologous transgenic system. Finally, by virtue of the diploid, self-fertilising and genetically homogeneous character ofM. truncatula, it should be relatively straightforward to screen for recessive mutations in symbiotic genes, to carry out genetic analysis, and to construct an RFLP map for this plant.     

Conformational and helicoidal analysis of the molecular dynamics of proteins: "curves," dials and windows for a 50 psec dynamic trajectory of BPTI

A new procedure for the graphic analysis of molecular dynamics (MD) simulations on proteins is introduced, in which comprehensive visualization of results and pattern recognition is greatly facilitated. The method involves determining the conformational and helicoidal parameters for each structure entering the analysis via the method "Curves," developed for proteins by Sklenar, Etchebest, and Lavery (Proteins: Structure, Function Genet. 6:46-60, 1989) followed by a novel computer graphic display of the results. The graphic display is organized systematically using conformation wheels ("dials") for each torsional parameter and "windows" on the range values assumed by the linear and angular helicoidal parameters, and is present in a form isomorphous with the primary structure per se. The complete time evolution of dynamic structure can then be depicted in a set of four composite figures. Dynamic aspects of secondary and tertiary structure are also provided. The procedure is illustrated with an analysis of a 50 psec in vacuo simulation on the 58 residue protein, bovine pancreatic trypsin inhibitor (BPTI), in the vicinity of the local minimum on the energy surface corresponding to a high resolution crystal structure. The time evolution of 272 conformational and 788 helicoidal parameters for BPTI is analyzed. A number of interesting features can be discerned in the analysis, including the dynamic range of conformational and helicoidal motions, the dynamic extent of 2 degrees structure motifs, and the calculated fluctuations in the helix axis. This approach is expected to be useful for a critical analysis of the effects of various assumptions about force field parameters, truncation of potentials, solvation, and electrostatic effects, and can thus contribute to the development of more reliable simulation protocols for proteins. Extensions of the analysis to present differential changes in conformational and helicoidal parameters is expected to be valuable in MD studies of protein complexes with substrates, inhibitors, and effectors and in determining the nature of structural changes in protein-protein interactions.     

Influence of temperature and nitrogen concentration on photosynthesis ofDunaliella viridis Teodoresco

The photosynthetic behaviour ofDunaliella viridis has been studied under a combination of three variables: irradiance (0–900 mol m^–2 s^–1), temperature (15, 23, 31, 38, 42 °C) and nitrogen concentration (0.05, 0.5, 1.5, 5, 10 mM NO ₃ ^- ) at a salinity of 2 M NaCl.The highest rates of photosynthesis have been found at 31 °C and a nitrate concentration of 10 mM. There exists a synergistic effect between temperature and nitrogen availability on the photosynthesis ofD. viridis; under nitrogen deficiency oxygen evolution is low, even null at high temperature. The interaction between these two variables of control occurs in a multiplicative way. There is also a general increase in photosynthetic pigments following the increase in nitrogen concentration in the culture medium. The normalization of net photosynthesis data in relation to chlorophylla shows that nitrogen concentration makes an indirect control of the photosynthetic rate ofD. viridis through the variation of pigment concentration.     

A role for haemoglobin in all plant roots?

Abstract. We have found haemoglobin in plant roots whereas previously it has been recorded only in nitrogen fixing nodules of plants. Haemoglobin occurs not only in the roots of those plants that are capable of nodulation but also in the roots of species that are not known to nodulate. We suggest that a haemoglobin gene may be a component of the genome of all plants. The gene structure and sequence in two unrelated families of plants suggests that the plant haemoglobins have had a single origin and that this origin relates to the haemoglobin gene of the animal kingdom. At present we cannot completely rule out the possibility of a horizontal transfer of the gene from the animal kingdom to a progenitor of the dicotyledonous angiosperms but we favour a single origin of the gene from a progenitor organism to both the plant and animal kingdoms. We speculate about the possible functions of haemoglobin in plant roots and put the case that it is unlikely to have a function in facilitating oxygen diffusion. We suggest that haemoglobin may act as a signal molecule indicating oxygen deficit and the consequent need to shift plant metabolism from an oxidative to a fermentative pathway of energy generation.