Growth versus molting time of caterpillars as a function of temperature,nutrient concentration and the phenolic rutin

Summary A factorial experiment tested the effects of varying nutrient concentration (normal versus diluted), presence or absence of the phenolic allelochemical rutin and daytime temperature (20, 25 and 30° C) on growth, molting and food utilization efficiencies of tobacco hornworms (Manduca sexta). Two of the utilization efficiencies (approximate digestibility and efficiency of conversion of ingested food) were unaffected by temperature; the third one, efficiency of conversion of digested food, was affected by temperature but there was no consistent effect. Lower temperatures significantly increased the proportion of the stadium spent molting, with larvae at a daytime temperature of 20° C spending 9% more of the stadium in molting than larvae at 30° C. Growth time was not influenced by nutrient concentration. When temperature was low and rutin absent, molt time and the proportion of the stadium spent molting were affected by nutrient concentration. Addition of the phenolic rutin did not have an appreciable effect on growth time or digestive processes. However, it increased molting time by 7 to 14% and thus increased the duration of the stadium and reduced relative growth rate. These results indicate that the effect of food quality on growth rate is a function of the thermal conditions of insect herbivores. At cooler temperatures, a disproportionate increase in time spent molting, rather than altered food utilization efficiencies, contributed to lower growth rates. The consequences for larval growth of fluctuating temperatures due to diurnal cycles and the presence of predators forcing larvae into thermally suboptimal microhabitats are discussed.     

THE EFFECTS OF PREDATION ON THE AGE AND SIZE OF MATURITY OF PREY

The effects of nonselective predation on the optimal age and size of maturity of their prey are investigated using mathematical models of a simple life history with juvenile and adult stages. Fitness is measured by the product of survival to the adult stage and expected adult reproduction, which is usually an increasing function of size at maturity. Size is determined by both age at maturity and the value of costly traits that increase mean growth rate (growth effort). The analysis includes cases with fixed size but flexible time to maturity, fixed time but flexible size, and adaptively flexible values of both variables. In these analyses, growth effort is flexible. For comparison with previous theory, models with a fixed growth effort are analyzed. In each case, there may be indirect effects of predation on the prey's food supply. The effect of increased predation depends on (1) which variables are flexible; (2) whether increased growth effort requires increased exposure to predators; and (3) how increased predator density affects the abundance of food for juvenile prey. If there is no indirect effect of predators on prey food supply, size at maturity will generally decrease in response to increased predation. However, the indirect effect from increased food has the opposite effect, and the net result of predation is often increased size. Age at maturity may either increase or decrease, depending on functional forms and parameter values; this is true regardless of the presence of indirect effects. The results are compared with those of previous theoretical analyses. Observed shifts in life history in response to predation are reviewed, and the role of size-selective predation is reassessed.     

A medium optimization strategy for improvement of growth and methane production by Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum, strain Hveragerdi, has been cultivated in a completely defined mineral salts medium, under strictly anaerobic conditions with CO₂ and H₂ as sole carbon and energy source, respectively. During optimization of the medium an iron limitation was identified that could not be overcome by the simple addition of iron—or iron complexed with nitrilotriacetate (NTA)—to the medium, due to the formation of insoluble FeS complexes. In order to define a medium optimization strategy, and to avoid laborious empirical optimization procedures, a theoretical model has been developed in order to describe the solubility of iron and other mineral species in the medium as a function of the concentration of sulfur and NTA. This model may be applied for the optimization of any medium component. With this information sulfide has been replaced by a combination of cysteine and thiosulphate in conjunction with a non-toxic reducing agent (titanium (III) citrate). Using this defined medium precipitation was avoided and an iron limitation was overcome resulting in a 5-fold improvement of the final biomass concentration from 2–3 g l⁻¹ to 11.2 g l⁻¹ together with a 2-fold increase (from 45 to almost 100%) in the conversion of CO₂ and H₂ to CH₄, even at gas flow rates as high as 6 l min⁻¹.     

Effects of various environmental conditions on growth and reproduction of the sea hareAplysia oculifera (Adams and Reeve, 1850)

We studied the influences of food type, food quantity, water currents, starvation and light on growth and reproduction of the sea hareaplysia oculifera (Adams and Reeve, 1850) under laboratory conditions. Out of five species of algae served as food,Enteromorpha intestinalis promoted the fastest growth ofA. oculifera, Ulva spp. slower growth,Cladophora sp. allowed maintenance spp. slower growth,Cladophora sp. allowed maintenance of steady body mass, and the brown algaeColpomenia sp. andPadina pavonia were rejected by the sea hares. When sea hares were exposed to four levels of water currents, growth rates decreased as water currents increased. Sea hares fed on 50% ration grew slower than those fed on 100% ration (ad libitum). During 10 days of starvation sea hares lost weight, but when subsequently fed 100% ration they recovered and grew at a rate similar to those fed continuously with 100% ration. Under shade and under natural sunlight sea hares grew at the same rates. Whenever growth rates decreased, sea hares began to spawn at a smaller body size.A. oculifera demonstrated physiological plasticity that adapted them to varied and unpredictable environmental conditions. At different conditions of food availability they applied different tactics of resource allocation between growth and reproduction.     

Heuristic optimization of the general life history problem: A novel approach

Summary The general life history problem concerns the optimal allocation of resources to growth, survival and reproduction. We analysed this problem for a perennial model organism that decides once each year to switch from growth to reproduction. As a fitness measure we used the Malthusian parameterr, which we calculated from the Euler-Lotka equation. Trade-offs were incorporated by assuming that fecundity is size dependent, so that increased fecundity could only be gained by devoting more time to growth and less time to reproduction. To calculate numerically the optimalr for different growth dynamics and mortality regimes, we used a simplified version of the simulated annealing method. The major differences among optimal life histories resulted from different accumulation patterns of intrinsic mortalities resulting from reproductive costs. If these mortalities were accumulated throughout life, i.e. if they were senescent, a bangbang strategy was optimal, in which there was a single switch from growth to reproduction: after the age at maturity all resources were allocated to reproduction. If reproductive costs did not carry over from year to year, i.e. if they were not senescent, the optimal resource allocation resulted in a graded switch strategy and growth became indeterminate. Our numerical approach brings two major advantages for solving optimization problems in life history theory. First, its implementation is very simple, even for complex models that are analytically intractable. Such intractability emerged in our model when we introduced reproductive costs representing an intrinsic mortality. Second, it is not a backward algorithm. This means that lifespan does not have to be fixed at the begining of the computation. Instead, lifespan itself is a trait that can evolve. We suggest that heuristic algorithms are good tools for solving complex optimality problems in life history theory, in particular questions concerning the evolution of lifespan and senescence.     

Modeling approach to control of carbohydrate metabolism during citric acid accumulation by Aspergillus niger: II. Sensitivity analysis

Steady state sensitivity analysis of a model of carbohydrate metabolism and anaplerotic synthesis of oxalacetate were, in Aspergillus niger under conditions of citric acid accumulation, carried out. The flux and metabolite concentration control structure of the system obtained shows that the hexokinase/substrate transport step is the main controlling step of the pathway. The quantitative contribution of the other enzyme catalyzed or transport steps are also discussed. These results allow the design of a proper strategy of biotechnological manipulation aimed at improvement of the process. (c) 1994 John Wiley & Sons, Inc.     

Predictions for oxygen supply control to enhance population stability of engineered production strains

The simultaneous growth and product formation in a microbial culture is an important feature of several laboratory, industrial, and environmental bioprocesses. Metabolic burden associated with product formation in these bioprocesses may lead to growth advantage of a nonproducing mutant leading to a loss of the producing population over time. A simple population dynamics model demonstrates the extreme sensitivity of population stability to the engineered productivity of a strain. Here we use flux balance analysis to estimate the effects of the metabolic burden associated with product secretion on optimal growth rates. Comparing the optimal growth rates of the producing and nonproducing strains under a given processing condition allows us to predict the population stability. In order to increase stability of an engineered strain, we determine processing conditions that simultaneously maximize the growth rate of the producing population while minimizing the growth rate of a nonproducing population. Using valine, tryptophan, and lysine production as specific examples, we demonstrate that although an appropriate choice of oxygenation may increase culture longevity more than twofold, total production as governed by economic criterion can be increased by several orders of magnitude. Choice of optimal nutrient and oxygen supply rates to enhance stability is important both for strain screening as well as for culture of engineered strains. Appropriate design of the culture environment can thus be used to enhance the productivity of bioprocesses that use engineered production strains. (c) 1994 John Wiley & Sons, Inc.     

Sulfate reduction in methanogenic bioreactors

Abstract: In the anaerobic treatment of sulfate-containing wastewater, sulfate reduction interferes with methanogenesis. Both mutualistic and competitive interactions between sulfate-reducing bacteria and methanogenic bacteria have been observed. Sulfate reducers will compete with methanogens for the common substrates hydrogen, formate and acetate. In general, sulfate reducers have better growth kinetic properties than methanogens, but additional factors which may be of importance in the competition are adherence properties, mixed substrate utilization, affinity for sulfate of sulfate reducers, relative numbers of bacteria, and reactor conditions such as pH, temperature and sulfide concentration. Sulfate reducers also compete with syntrophic methanogenic consortia involved in the degradation of substrates like propionate and butyrate. In the absence of sulfate these methanogenic consortia are very important, but in the presence of sulfate they are thought to be easily outcompeted by sulfate reducers. However, at relatively low sulfate concentrations, syntrophic degradation of propionate and butyrate coupled to HZ removal via sulfate reduction rather than via methanogenesis may become important. A remarkable feature of some sulfate reducers is their ability to grow fermentatively or to grow in syntrophic association with methanogens in the absence of sulfate.     

Reproductive ecology of a cleistogamous annual,Impatiens noli-tangere L., occurring under different environmental conditions

Demographic and reproductive schedules were compared among five populations of a cleistogamous annual,Impatients noli-tangere L., occurring in habitats with contrasting moisture and/or light environments. In all populations, flowering extended for 2–3 months during which light environment and mortality changed. Seasonal patterns of growth and mortality were significantly different among the five populations studied. The beginning and duration of flowering and the ratio of chasmogamous flowers to cleistogamous ones were also significantly different among populations. An experiment was conducted under different light conditions (open and closed) to separate the genetic and environmental components of the variation in reproductive traits observed among populations ofI. noli-tangere. Transplanted plants showed significant among-population variation in flowering time, as is observed in natural populations, suggesting genetic differentiation among populations of this species. On the other hand, the ratio of chasmogamous flowers did not differ among plants transplanted from three populations. Based on these results, the authors suggest that facultative cleistogamy is a conditional strategy under seasonally changing environments.     

玉米胚性愈伤组织的长期继代及其染色体分析

对５种基因型幼胚诱导的愈伤组织继代培养表明,玉米胚性愈伤组织的长期继代受基因型,培养基成分,激素,培养条件的影响。适时继代,逐代筛选对胚性保持起重要作用。适当降低培养温度（１２±２℃）有利于愈伤组织的保存和胚性保持,可以减少愈伤组织长期继代所需的物质和工作量。长期继代培养的胚性愈伤组织,胚状体发生能力和植株再生率无显著变化,但正常苗的再生频率显著下降。观察愈伤组织细胞染色体发现：（１）基因型对不同倍性细胞的比例有明显影响。（２）随着继代时间的延长,二倍体细胞下降,四倍体和非二倍体细胞增多。（３）愈伤组织中出现多种染色体结构变异,这些结构变异有可能导致非整倍体细胞的形成。