The effect of caffeine,different fixation regimes and low temperature on microtubules in the cells of higher plants

Caffeine, (1:3:7-tri-methyl-xanthine), either as a prefixation treatment or included with glutaralde-hyde as the primary fixative, destroys or disorganises the microtubules associated with the formation of secondary walls in fibres from the flowering stem of the grass Lolium temulentum L. There is no observable effect of caffeine treatment on the microtubules associated with primary wall formation in collenchyma and young fibres from L. temulentum or in root cap cells of Zea mays L. and Phaseolus vulgaris L. The microtubules associated with primary wall formation are destroyed by cold treatment but not those associated with secondary wall formation. Tannic acid included in the fixative shows the microtubules associated with secondary wall formation in fibres of L. temulentum to be composed of 13 subunits. Treatment with lanthanum hydroxide does not stain the core or the halo of the microtubules.Abbreviation PIPES Piperazine N-N- bis 2 ethanol sulphonic acid The Grassland Research Institute is financed through the Agricultural Research Council     

Clinical Problems: Progressive Neurological Deficits in Primary Polycythaemia

Patients with primary polycythaemia may present clinical features which lead to an erroneous diagnosis of intracerebral neoplasm. Three such cases are described in detail in this report.     

Invertebrate predator-prey body size relationships: an explanation for upper triangular food webs and patterns in food web structure?

Summary It has been suggested by Cohen and Newman (1985) that many of the patterns in published food webs can be derived from a stochastic model in which the species are arranged in a trophic hierarchy (the cascade model). We suggest that, if predators are larger than their prey, a trophic hierarchy can be generated on the basis of body size Empirical evidence from the literature shows that there is a positive relationship between predator and prey size for a range of invertebrates and that predators are usually larger than their prey. Using experimental data on an aquatic food web we show that body size can lead to the type of trophic hierarchy used in the cascade model, suggesting that many food web patterns may be a product of body size. This conclusion is discussed with respect to the limitations of the food web data and the relationship between static and dynamic models of web structure.     

Food-web models that generate constant predator-prey ratios

Summary An approximately constant ratio of number of predator species/number of prey species is observed in several natural communities, although the exact value of the ratio may vary with habitat and the types of organisms in the food web. We test the hypothesis that a constant predator/prey ratio can be generated by what Holt (1977) terms apparent competition and what Jeffries and Lawton (1984) call competition for enemy-free space. We create simple, two trophic-level communities by drawing species of predators and prey at random from a species pool, simulating their interactions using Lotka-Volterra models. The simulated food webs converge over successive periods of invasion and extinction to locally stable systems with approximately constant ratios of number of predator species/number of prey species, despite varying initial conditions. As expected, predator/prey ratios take different values depending upon the biology of the simulated species. We conclude that apparent competition between prey species via shared enemies may be one mechanism whereby approximately constant predator/prey ratios are generated in natural communities.     

Physical property improvements of a pellicular biocatalyst

The thermal stability of an immobilization technique using a pellicular latex matrix was examined in a packed-bed column reactor. The stability was found to vary with liquid flow rate, the type of latex, temperature of operation, and the amount of yeast cells. Adjusting these parameters and introducing particulate inorganic fillers strengthened the latex matrix and improved the thermal stability. Optimization of this immobilization technique resulted in a procedure that allowed latex polymers to be mechanically stable at temperatures up to 50°C. The biological viability of this improved immobilization scheme was demonstrated through the production of L-aspartic acid by immobilized cells of E. coli.     

Identification and characterization of three genes and two pseudogenes on chromosome 13

A study was conducted on the feasibility of isolating genes and pseudogenes that map to chromosome 13 by a hybridization-based approach using a 13-specific library and pools of repeat-free cDNA clones. Five pairs of cDNA and chromosome 13 genomic clones were identified and characterized. Partial or full-length sequence was derived from all cDNAs, and database searches were performed for putative gene identification. Partial sequence was also obtained from the chromosome 13 genomic clones for comparison with those of the hybridizing cDNAs. As a result of these analyses we identified three genes, a putative homologue of a porcine mRNA encoding an unidentified hepatic protein, a putative homologue of a yeast integral membrane protein, and a gene for a translationally controlled tumor protein, and two processed pseudogenes, ribosomal proteins L23a and S3a. The latter was formerly identified as the v-fos transformation effector gene, Fte-1, and recently cited as a possible candidate for the BRCA2 gene on chromosome 13. All genes and pseudogenes were localized to cytogenetic bands by in situ hybridization of metaphase chromosomes with probes derived from the chromosome 13 genomic clones.     

Range, population abundance and conservation

Several patterns in the distribution and abundance of organisms have now been documented. They include broad (but not universal) positive correlations between range sizes and population abundances; a decline in the proportion of sites occupied and in average population densities from the centre to the edge of a species' range, with either unimodal or multimodal peaks of abundance and occupancy in the core of the range; and intriguing, but still poorly documented phylogenetic effects on both range size and abundance. All these patterns require further work to establish their generality, and all of them lack generally agreed explanations. They are important, however, not only theoretically but also practically, because of the constraints and opportunities they appear to provide for the management and conservation of species.     

Acquired Resistance Signal Transduction in Arabidopsis Is Ethylene Independent

To clarify the role of ethylene in systemic acquired resistance (SAR), we conducted experiments using Arabidopsis ethylene response mutants. Plants that are nonresponsive to ethylene (i.e., [theta]tr1 and [theta]in2) showed normal sensitivity to the SAR-inducing chemicals salicylic acid (SA) and 2,6-dichloroisonicotinic acid with respect to SAR gene induction and pathogen resistance. This indicated that chemically induced SAR is not an ethylene-dependent process in Arabidopsis. Ethephon, an ethylene-releasing chemical, induced SAR gene expression in both the wild type and ethylene mutants, whereas ethylene alone did not, suggesting that induction of these genes by ethephon is not due to the action of ethylene. Furthermore, transgenic plants expressing salicylate hydroxylase, a bacterial enzyme that degrades SA to catechol, did not accumulate SAR mRNAs in response to ethephon. Thus, SAR gene induction by ethephon appears to be mediated through SA. Other experiments suggested that ethylene may play a role in SAR by enhancing tissue sensitivity to the action of SA.