How do cytokinins affect the cell?

The lecture presents modern knowledge of the mechanisms of cytokinin perception and signal transduction to the genes of primary and secondary responses. It also demonstrates the relations between the rapid cytokinin-induced processes and cytokinin-induced physiological effects. The characteristics of the cytokinin regulatory system and its role in the control of plant growth and development are discussed.     

How do synonymous mutations affect fitness?

While it has often been assumed that, in humans, synonymous mutations would have no effect on fitness, let alone cause disease, this position has been questioned over the last decade. There is now considerable evidence that such mutations can, for example, disrupt splicing and interfere with miRNA binding. Two recent publications suggest involvement of additional mechanisms: modification of protein abundance most probably mediated by alteration in mRNA stability and modification of protein structure and activity, probably mediated by induction of translational pausing. These case histories put a further nail into the coffin of the assumption that synonymous mutations must be neutral.     

How do toxicants affect epidemiological dynamics?

Populations are formed of their constituent interacting individuals, each with their own respective within‐host biological processes. Infection not only spreads within the host organism but also spreads between individuals. Here we propose and study a multilevel model which links the within‐host statuses of immunity and parasite density to population epidemiology under sublethal and lethal toxicant exposure. We analyse this nested model in order to better understand how toxicants impact the spread of disease within populations. We demonstrate that outbreak of infection within a population is completely determined by the level of toxicant exposure, and that it is maximised by intermediate toxicant dosage. We classify the population epidemiology into five phases of increasing toxicant exposure and calculate the conditions under which disease will spread, showing that there exists a threshold toxicant level under which epidemics will not occur. In general, higher toxicant load results in either extinction of the population or outbreak of infection. The within‐host statuses of the individual host also determine the outcome of the epidemic at the population level. We discuss applications of our model in the context of environmental epidemiology, predicting that increased exposure to toxicants could result in greater risk of epidemics within ecological systems. We predict that reducing sublethal toxicant exposure below our predicted safe threshold could contribute to controlling population level disease and infection.     

How do African game animals control trypanosome infections?

African trypanosomiasis is endemic over much of sub-saharan Africa. But whereas domestic animals - especially cattle - often succumb to the infection, wild mammals generally show a high degree o f resistance. Many species o f wildlife living in tsetse-infested areas carry trypanosome infections, and so act as important reservoir hosts, but generally show no obvious ill-effects. How they survive the infection is an important question in understanding mechanisms o f trypanotolerance, yet relevant data are sparse and scattered. Here, Ayub Mulla and Roy Rickman review some of the previous studies to illustrate how complex the question is.     

How do macrophyte distribution patterns affect hydraulic resistances?

In eutrophic river systems, macrophytes attain high biomass with reduced drainage and increased flooding risk. To avoid these problems, water managers remove vegetation. Total removal, however, increases wash out of macro-invertebrate communities reducing the ecological value of rivers. Partial vegetation removal reduces this washout and prevents an increase in hydraulic resistance. In this, study the hydraulic performance of three partial vegetation removal patterns was tested. From the results it was seen that hydraulic resistance, expressed as Manning's n, was varying between 0.025 m^−1/3 s and 0.050 m^−1/3 s. Compared with the empty situation, the different distribution patterns increased resistance between 14 and 23%. Hydraulic resistance of these patterns was also significantly influenced by the species present in the vegetation patches. Three groups of macrophyte plants (emerged, floating leaved and submerged) with significantly different hydraulic resistances were determined. The emerged species Sparganium erectum generated the least resistance with an average friction of 0.03 m^−1/3 s. Stuckenia pectinata and Potamogeton natans had slightly higher friction values around 0.4 m^−1/3 s. Ranunculus penicillatus and Callitriche platycarpa had average friction values around 0.05 m^−1/3 s.The proposed vegetation removal patterns are good alternatives to create a management system, which minimally increases hydraulic resistance but still guarantees the ecological functions.     

How do species interactions affect species distribution models?

One of the most promising recent advances in biogeography has been the increased interest and understanding of species distribution models – estimates of the probability that a species is present given environmental data. Unfortunately, such analyses ignore many aspects of ecology, and so are difficult to interpret. In particular, we know that species interactions have a profound influence on distributions, but it is not usually possible to incorporate this knowledge into species distribution models. What is needed is a rigorous understanding of how unmeasured biotic interactions affect the inferences generated by species distribution models. To fill this gap, we develop a general mathematical approach that uses probability theory to determine how unmeasured biotic interactions affect inferences from species distribution models. Using this approach, we reanalyze one of the most important classes of mechanistic models of competition: models of consumer resource dynamics. We determine how measurements of one aspect of the environment – a single environmental variable – can be used to estimate the probability that an environment is suitable with species distribution models. We show that species distribution models, which ignore numerous facets of consumer resource dynamics such as the presence of a competitor or the dynamics of depletable resources, can furnish useful predictions for the probability that an environment is suitable in some circumstances. These results provide a rigorous link between complex mechanistic models of species interactions and species distribution models. In so doing they demonstrate that unmeasured biotic interactions can have strong and counterintuitive consequences on species distribution models.     

How do grazers affect periphyton heterogeneity in streams?

The effects of grazing by stream invertebrates on algal biomass and spatial heterogeneity were tested experimentally in flow-through microcosms with natural substrates (rocks). One experiment tested the effects of fixed densities of three species of grazers (the caddisfly Allomyia sp. and two mayflies, Epeorus deceptivus and Baetis bicaudatus) on periphyton. Baetis was tested with and without chemical cues from fish predators, which reduced grazer foraging activity to levels similar to the less mobile mayfly (Epeorus). Mean algal biomass (chlorophyll a; chl a) was reduced in grazer treatments compared to ungrazed controls, but there were no differences among grazer treatments. Algal heterogeneity (Morisita index) increased with grazer mobility, with the highest heterogeneity occurring in the Baetis-no fish treatment (most mobile grazer) and the lowest in the caddisfly treatment (most sedentary grazer). A second experiment used a three factorial design, and tested whether initial resource distribution (homogeneous vs. heterogeneous), Baetis density (high vs. low) and fish odor (present vs. absent) affected grazer impact on algal resources. Abundances of Baetis and chl a on individual rocks were recorded to explore the mechanisms responsible for the observed distributions of algae. Initial resource heterogeneity was maintained despite being subjected to grazing. Mean chl a was highest in controls, as in experiment I, and effects of Baetis on algal biomass increased with grazer density. There were no fish effects on algal biomass and no effects of grazer density or fish on algal heterogeneity. At the scale of individual rocks Baetis was unselective when food was homogeneously distributed, but chose high-food rocks when it was heterogeneously distributed. Results of these mechanistic experiments showed that Baetis can track resources at the scale of single rocks; and at moderate densities mobile grazers could potentially maintain periphyton distributions observed in natural streams.     

How do host immune responses affect nematode infections?

Host immune responses limit, and in some instances eliminate, nematode infections. There is considerable interest in enhancing these natural processes by the use of antinematode vaccines to achieve control of infection or disease. How nematodes are damaged is unclear. Worms might be damaged directly by effector cells and molecules of the immune system. Alternatively, they might be damaged by the physiological stress of their efforts to resist attack. Separating these possibilities could have important implications for approaches to the control of nematode infections and the disease that they cause.     

How do earthworms affect microfloral and faunal community diversity?

Much of the work regarding earthworm effects on other organisms has focused on the functional significance of microbial-earthworm interactions, and little is known on the effects of earthworms on microfloral and faunal diversity. Earthworms can affect soil microflora and fauna populations directly and indirectly by three main mechanisms: (1) comminution, burrowing and casting; (2) grazing; (3) dispersal. These activities change the soil's physico-chemical and biological status and may cause drastic shifts in the density, diversity, structure and activity of microbial and faunal communities within the drilosphere. Certain organisms and species may be enhanced, reduced or not be affected at all depending on their ability to adapt to the particular conditions of different earthworm drilospheres. A large host of factors (including CaCO₃, enzymes, mucus and antimicrobial substances) influence the ability of preferentially or randomly ingested organisms to survive (or not) passage through the earthworm gut, and their resultant capacity to recover and proliferate (or not) in earthworm casts. Small organisms, particularly microflora and microfauna, with limited ability to move within the soil, may benefit from the (comparatively) long ranging movements of earthworms. Microflora and smaller fauna appear to be particularly sensitive to earthworm activities, and priming effects enhancing nutrient release, particularly in casts, are common. Larger fauna such as microarthropods, enchytraeids and Isopods may be enhanced under some conditions (e.g., in earthworm middens), but in other cases earthworm activity may lead to a decrease in their populations due to competition for food (microbes and organic materials), and spatial and temporal changes in food abundance. Nevertheless, considering the presently available data, the beneficial interactions of earthworms and microflora and fauna appear to far outweigh the potential negative effects. However, much is still unknown regarding the interactions of earthworms of different ecological categories on the diversity and function of microfloral and faunal communities, and much more interdisciplinary research is needed to assess the potential role of earthworms in regulating the diversity of microflora and fauna in soil systems and the potentially beneficial or harmful effects this regulation may have on ecosystem function and plant growth in different ecosystems.     

How do organic solvents affect peroxidase structure and function?

The effect of organic solvents on horseradish peroxidase structure and function has been studied. Some, but not complete, enzyme denaturation occurs even in low volumes of water-miscible organic solvents (e.g., greater than 30% v/v dioxane, greater than 50% v/v methanol, and greater than 20% v/v acetonitrile) as determined by the decreased difference between the fluorescence of peroxidase's sole tryptophan residue and free L-tryptophan in solution. Absorbance and electron paramagnetic resonance spectroscopies indicate exposure of peroxidase's active site to the organic solvent. This reduces the local polarity in the enzyme's active site and results in stronger hydrogen bonding of phenolic substrates to the enzyme. In extreme cases (e.g., 95% v/v dioxane, 90% v/v acetonitrile, and ethyl and butyl acetate containing 2 and 1% v/v aqueous buffer, respectively), the transition state of the enzymic reaction is sufficiently perturbed so as to alter the magnitude of the Hammett rho value. This is most likely the result of the increased strength of hydrogen bonding between electron-donating alkoxyphenols (negative sigma values) and an electrophilic group in the enzyme's active site, thereby reducing catalytic efficiencies for such substrates relative to alkyl- and chlorophenols. Perhaps the most important effect of the organic solvent, however, is the significant ground-state stabilization of phenolic substrates in organic media as opposed to aqueous buffer. This stabilization can account for nearly 4 orders of magnitude in reduction of catalytic efficiency and is manifested in increased Km's. This study indicates that enzymes can maintain much of their native active-site structure in organic media and that the effect of solvent on substrate thermodynamics must be considered.     

How do childhood ADHD symptoms affect labour market outcomes?

Inattention, hyperactivity and impulsivity are the main symptoms of ADHD, which affects up to one in ten European and North American children. Existing research shows these symptoms are associated with school exclusion and poor academic performance during childhood and adolescence. Using the British Cohort Study (n = 17,196 people born in April 1970), this is the first study of relationships between ADHD symptoms measured during childhood (age 10) and labour market outcomes measured beyond early adulthood (ages 26–46); and the first to explore the role of childhood circumstances (at birth) and academic performance (ages 10 and 26) in explaining those relationships. A one standard deviation increase in childhood symptoms reduced employment by up to two percentage points and pay by up to four percentage points. Differences in academic performance at age 10 accounted for almost half the observed variation in employment outcomes, indicating a possible role for educational interventions in schools.     

How complex is GTPase signaling in trypanosomes?

Many signaling pathways in higher eukaryotes use Ras-like small GTPases. Here, we ask how complex are these small GTPase signaling pathways in trypanosomes? We seek to address this issue by comparisons with the representation of both the GTPase molecules and their accessory factors in several genomes.     

How do point amino acid substitutions affect the protein structure?

In order to estimate the influence of point mutations on the protein structure, 552 pairs of structurally aligned proteins with pairwise sequence identities greater than 60% were analyzed. We selected all isolated point mismatches from these alignments. The statistics of local conformational changes corresponding to these mismatches was studied. This study revealed two surprising aspects: (a) only an extremely minor fraction (< 3%) of all observed mutations leads to significant changes in local conformations; (b) observed substitutions, which affect local conformation are more frequently those of similar (high scoring) amino acid pairs rather than the substitutions thought of as "hazardous" (low scoring), although the overall average score of the changing substitutions is still lower than the corresponding average score for the substitutions that leave the structure unchanged.     

How do homozygous parents affect TDT as a test for association?

The traditional transmission disequilibrium test (TDT) (Spielman et al., 1993) is a powerful test for association only in the presence of linkage. Since allele transmissions from homozygous parents do not carry any information on linkage, the TDT statistic uses data only on heterozygous parents. However, homozygous parents carry information on association between alleles at a marker locus and a disease locus. In this article, we explore whether inclusion of homozygous parents increases the power to detect association. The resultant test statistic follows a chi(2) distribution with 2 degrees of freedom. Monte-Carlo simulations are included to compare the performance of this test with the traditional TDT under different disease models.     

How do weather conditions affect the huddling behaviour of emperor penguins?

Huddling allows emperor penguins to conserve energy and survive their long winter fast while facing harsh climatic conditions. Here we report the first investigation into the effects of changes in wind speed and ambient temperature on different components of penguin huddling behaviour. We attached light and temperature recorders to male emperor penguins at the Pointe Géologie colony, Antarctica, which recorded huddling events. We then compared the frequency, duration, occurrence and intensity of huddling bouts, with ambient air temperatures and wind speeds. Huddling occurrence increased with lower ambient temperatures and higher wind speeds, whereas huddling intensity was mainly enhanced by lower ambient temperatures. Moreover, huddling group movements were linked to wind direction and its global density to lower ambient temperatures. Hence, emperor penguins complex huddling behaviour was modulated differently depending on these two parameters. Weather conditions may then affect emperor penguins ability to save energy and survive their winter fast.     

How do trabeculae affect the calculation of structural properties of bone?

One difficulty that arises in an analysis of the cross-sectional properties of bone is whether to include cancellous bone in the analysis. The purpose of this paper is to determine how different amounts of cancellous bone affect the measurement of structural properties of bone cross-sections. Thirty-two tibial and femoral cross-sections were chosen at random from a series of cross-sectioned nonhuman primate bones. Geometrical properties were calculated for the cross-sections, and torsional and bending stress analyses were performed. The results suggest that the effect of including cancellous bone in the analysis is closely related to the amount of bone, where it lies within the cross-section, and the type of analysis performed. Including cancellous bone in calculations of structural properties of bone cross-sections may cause the strength and stiffness of the bone to be exaggerated.     

How do additives affect enzyme activity and stability in nonaqueous media?

The catalytic activities of lyophilized powders of alpha-chymotrypsin and Candida antarctica lipase were found to increase 4- to 8-fold with increasing amounts of either buffer salts or potassium chloride in the enzyme preparation. Increasing amounts of sorbitol in the chymotrypsin preparation produced a modest increase in activity. The additives are basically thought to serve as immobilization matrices, the sorbitol being inferior because of its poor mechanical properties.Besides their role as supports, the buffer species were indispensable for the transesterification activity of chymotrypsin because they prevented perturbations of the pH during the course of the reaction. Hence, increasing amounts of buffer species yielded a 100-fold increase in transesterification activity. Effects of pH changes were not as predominant in the peptide synthesis and the lipase-catalyzed reactions.Immobilization of the protease on celite resulted in a remarkable improvement of transesterification activity as compared to the suspended protease, even in the absence of buffer species. Immobilization of the lipase caused a small improvement of activity. The activity of the immobilized enzymes was further enhanced 3-4 times by including increasing amounts of buffer salts in the preparation.The inclusion of increasing amounts of sodium phosphate or sorbitol to chymotrypsin rendered the catalyst more labile against thermal inactivation. The denaturation temperature decreased with 7 degrees C at the highest content of sodium phosphate, as compared to the temperature obtained for the denaturation of the pure protein. The apparent enthalpy of denaturation increased with increasing contents of the additives. The enhancement of hydration level and flexibility of the macromolecule upon addition of the compounds partly provides the explanation for the observed results. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 67-76, 1997.