Bumblebee visits to Impatiens spp.: pattern and efficiency

Summary Three Japanese species of Impatiens, which secrete nectar continuously in long spurs, were visited by Bombus diversus workers consecutively throughout the day. B. diversus workers showed characteristic patterns of behavior in flower use, flower choice, and patch departure. (1) Bumblebees stayed longer on a flower which had been unvisited for a while than on a flower which had been visited recently. (2) Bumblebees preferred visiting flowers which had been unvisited for a while to visiting those which had been visited recently, and to visiting those which had been unvisited for a long period. (3) Bumblebees had a higher probability of leaving a patch after they had stayed on a flower for a short period than after they had stayed for a longer period. The bumblebees appeared to perceive both remotely and proximately chemical cues deposited by other foraging individuals, which indicated nectar rewards in a flower, and thus obtained a higher nectar intake than the mean amount of nectar left in a flower.     

Global asymptotic stability of density dependent integral population projection models

Many stage-structured density dependent populations with a continuum of stages can be naturally modeled using nonlinear integral projection models. In this paper, we study a trichotomy of global stability result for a class of density dependent systems which include a Platte thistle model. Specifically, we identify those systems parameters for which zero is globally asymptotically stable, parameters for which there is a positive asymptotically stable equilibrium, and parameters for which there is no asymptotically stable equilibrium.     

Logical Probability and Risk Assessment

A risk assessment is intended to provide a statement of current knowledge which is intended to inform a decision-maker of the current state of knowledge in response to a particular concern. Because answering the concerns of decision-makers often requires inferences to be drawn, doubt often arises over how the inference is to be drawn. In quantitative risk assessment, where a mathematical equation or model is used to draw the inference, this uncertainty is referred to as model uncertainty. A two-step process, which is referred to as logical probability, is proposed as a technique for representing model uncertainty in a risk assessment. The first step involves assigning model weights in which the degree of evidential support for each of the alternative models is considered. The second step involves assigning a unique interval in the range of 0 to 1 for each model which reflects the models' weight, to form a probability distribution. While the second step is straightforward, the first step is not. Assigning model weights requires consideration of any line of evidence that may reasonably impact the validity of the assertion of a model. While the development of a procedure for doing so may be expected to be a process which reflects the subjective preferences of whomever is involved in creating it, there are some historical precedents on which to build. Foremost among these are (1) the use of a correlation coefficient or other goodness-of-fit criteria to measure the degree of correspondence between a given model and a set of observations which are used as evidence to support it, and (2) preference given to models which are simpler, which may be ascertained as the number of adjustable parameters the model contains. Additional principles, which have little or no tradition to stand on, must be used to reflect the impact of other empirically supported beliefs on model preference. The procedure proposed is comparable to the procedure known as decision analysis, in which probabilities are assigned to alternative models based on expert or subjective input. The principal difference in the present case is that it is suggested that principles which transcend the decision at hand should be sought and articulated in order to generate a consistent measure of uncertainty arising from interpretation.     

Modelling nutrient responses in the field

Models of the yield responses of crops to applied nutrients are a recent addition to the methods available for making fertilizer recommendations. They have a place in integrating nutrient information with information on other factors which affect yield and its response to added nutrients. This review deals with nitrogen models classified into three groups: those which predict yield-response curves based on empirical factors; those which simulate the yield response from complex simulation models of many processes regulating crop growth and the soil environment; and those which aim to simulate yield and selected processes based on simplified functional relationships which apply to a target region or industry. Three case studies representing the three classes of model are drawn from research on dryland wheat in different parts of Australia. They show examples in which models provide information which is unobtainable from experimental procedures and which provide information useful to farmers in making decisions about fertilizers.Suggestions are made for future developments in crop-nutrient modelling including further comparisons of models, linkage of models with tissue tests, modelling co-limiting nutrients, deciding on the appropriate level of detail within a model and the need for methods for calibrating and testing models on attributes other than yield alone.     

Mapping of the plasmid (pLQ3) from Achromobacter and cloning of its cephalosporinase gene in Escherichia coli

We have constructed a physical map of the plasmid pLQ3 which was originally isolated from Achromobacter and which codes for a beta-lactamase. The enzyme specified by pLQ3 is expressed in Escherichia coli and is unusual in that it is a cephalosporinase, an enzyme usually coded for by chromosome. Plasmid pLQ3 is 12.4 kb in length and has a unique Bam HI site and two BglII sites. From a BamHI + BglII double digest of pLQ3, we have constructed a "shortened" plasmid, pLQ10, in which a 2.96-kb fragment is deleted. We have constructed a clone, pLQ22, in which a 3.27-kb fragment of pLQ3, carrying the beta-lactamase gene, is inserted into the BamHI site of pACYC184. By "comparative mapping" of single and multiple digests of each of these plasmids, we have been able to locate the cleavage sites for PstI, which makes seven cuts in pLQ3.     

Pair statistics clarify percolation properties of spatially explicit simulations

Dispersal is a fundamental control on the spatial structure of a population. We investigate the precise mechanism by which a mixed strategy of short- and long-distance dispersal affects spatial patterning. Using techniques from pair approximation and percolation theory, we demonstrate that dispersal controls the extent to which a population is completely connected by modulating the proportion of neighboring sites which are simultaneously occupied. We show that near the percolation threshold this pair statistic, rather than other metrics proposed earlier, best explains clustering, and we suggest more general circumstances under which this may hold.     

Electrofusion of myeloma cells on the single cell level. Fusion under sterile conditions without proteolytic enzyme treatment

A technique is presented which allows electrofusion of single cells under sterile conditions. The electrofusion chamber is placed in a Petri dish. Before a droplet of the fusion medium is pipetted between the electrodes, the chamber is completely covered with vaseline, which prevents the fusion medium evaporating. Additionally, the fusion chamber is treated with solutions containing poly(L)-lysine and pronase which results in a decreased movement of the cells on the glass between the electrodes and which allows electrofusion without any proteolytic pretreatment.     

VHS domain marks a group of proteins involved in endocytosis and vesicular trafficking

Endocytosis is driven by a mechanism which is characterized by an orderly congregation of a large number of proteins which effectuate, first, formation of a coated vesicles, second, pinching off the vesicle and, third, regulated transport. True to the nature of many other proteins involved in multimolecular complexes, also endocytosis-associated proteins, such as Eps15, clathrin and AP-2, are characterized by distinct domains which mediate the protein-protein interactions. We now report that a group of well-established endocytosis and/or vesicular trafficking proteins possess a VHS domain, a recently described domain with an unknown function. We suggest that in these proteins VHS serves as a membrane targeting domain which by its specific features together with FYVE, SH3 and/or TAM domains, which are also present in some VHS-containing proteins, is involved in the stage-specific assembly of the endocytic machinery.     

Isoelectric points of multi-domain proteins

Although the distribution of protein isoelectric points is multi-modal, large proteins show isoelectric points less variable than small proteins and their isoelectric points tend to converge to a unique value, close to the pH of the milieu in which the proteins are functional, as far as the protein dimension increases. This study demonstrates that large proteins, which contain more than a single domain, do have isoelectric points less variable than small proteins, which contains a single domain. However, the distribution of the isoelectric points of the single domains, contained in large proteins, resembles that of small proteins, which contain a single domain. Thus, large proteins can be soluble even if their pI is very close to the pH of the milieu, in which they perform their function, since they can contain several domains, the electrostatic properties of each of which mirror those of small proteins.     

Analysis of the proteins,glycoproteins and glycosaminoglycans of fibroblast adhesions to substratum

The focal adhesion preparations which remain attached to a glass substratum when fibroblast bodies are removed by a gentle stream of buffer have been analysed by gel electrophoresis coupled with other selective methods of analysis. The results are consistent with the presence of three classes of macromolecular components. (i) Muscle and associated proteins amongst which actin was abundant with significant amounts of tropomyosin, some myosin and traces of α-actinin. Some vimentin was present but no vinculin. We detected a major new protein component, as yet unidentified, with a molecular weight in the region of 50 000–55 000 which is not desmin or tubulin and could have an important function at the focal adhesion. (ii) Glycoproteins which are a specialised subset of those in the whole plasma membrane and included a family which bind ricin and therefore contain β-galactose end groups, together with a series having carbohydrate chains which bound neither ricin nor concanavalin A. The relative proportion of ricin-binding glycoproteins compared to concanavalin A-binding glycoproteins was higher than in whole plasma membranes. (iii) Glycosaminoglycans, with hyaluronate identified as the major component by column chromatography and its susceptibility to Streptomyces hyaluronidase.     

A modeling framework describing the enzyme regulation of membrane lipids underlying gradient perception in Dictyostelium cells

Gradient perception describes the process by which information about the chemoattractant concentration field surrounding a cell is transformed into an internal signal which is responsible for directed cell motion. Recently, many important biochemical details in immobilized and mobile Dictyostelium cells have been uncovered regarding the roles of enzymes regulating phosphoinositide lipids on the cell membrane which are responsible for gradient perception. We report on a modeling framework that describes the relationship between the membrane concentration of the primary 3'phosphoinositide lipids and the enzymes which regulate them. The model takes the form of partial differential equations describing the membrane concentration of these lipids. Working within this framework, we describe mechanisms which can be responsible for spatial amplification of these lipids and which do not employ lipid-enzyme feedback. An analysis of a basic module underlying this process is also performed.     

The expression of cecropin peptide in transgenic tobacco does not confer resistance to Pseudomonas syringae pv tabaci

Summary We used in vitro growth inhibition assays to demonstrate that synthetic cecropin protein has potent activity against a range of plant pathogenic bacteria. We then prepared transgenic tobacco plants which express cecropin mRNA and protein. We have used Pseudomonas syringae pv tabaci infection of these transgenic tobacco as a model system to evaluate whether the plants which express cecropin protein also have increased tolerance to infection. We found no dramatic difference in disease response between plants which are expressing cecropin protein and control plants which were derived from the transformation with a binary vector which did not carry the gene encoding cecropin protein.     

Gregariousness versus solitude: another look at parasite faunal richness in Canadian freshwater fishes

Summary The expectation of the ideal free distribution is a functional response in which intake is constant in all sites and, if interference is low, an aggregative response in which all individuals cluster in the high prey density sites. Incorporating individual differences in competitive ability results in a functional response in which intake increases with prey density and an aggregative response in which individuals are more evenly distributed.     

Hamilton's missing link

Hamilton's famous rule was presented in 1964 in a paper called "The genetical theory of social behaviour (I and II)", Journal of Theoretical Biology 7, 1-16, 17-32. The paper contains a mathematical genetical model from which the rule supposedly follows, but it does not provide a link between the paper's central result, which states that selection dynamics take the population to a state where mean inclusive fitness is maximized, and the rule, which states that selection will lead to maximization of individual inclusive fitness. This note provides a condition under which Hamilton's rule does follow from his central result.     

Heterodimerization of the yeast MATa1 and MAT alpha 2 proteins is mediated by two leucine zipper-like coiled-coil motifs.

The yeast Saccharomyces cerevisiae has three cell types distinguished by the proteins encoded in their mating type (MAT) loci: the a and alpha haploids, which express the DNA binding proteins a1 and alpha 1 and alpha 2, respectively, and the a/alpha diploid which expresses both a1 and alpha 2 proteins. In a/alpha cells, a1-alpha 2 heterodimers repress haploid-specific genes, while alpha 2 homodimers repress a-specific genes, indicating a dual regulatory function for alpha 2 in mating type control. a1 does not form homodimers. We have identified two sequences in the alpha 2 N-terminal domain which contain the 3,4-hydrophobic heptad repeat pattern characteristic of coiled-coils. Mutational analyses show that both sequences are important to a1-alpha 2 heterodimerization. We propose that these two sequences associate in a coiled-coil-like manner with a sequence within a1 which bears two adjacent, overlapping 3,4-hydrophobic heptad repeats. This model, which describes a novel dimerization motif for homeodomain proteins, also provides a mechanism by which a1-a1 homodimerization is prevented.     

A new baculovirus of cultured shrimps

By means of ultrathin section, negative staining and sucrose gradient ultra-centrifugation, a new baculovirus has been discovered and purified in lymphoid organs and such tissues as muscles of the shrimps which have been spontaneously attacked by diseases and artificially infected. With a diameter of 96-112 nm, this is the thickest baculovirus of shrimps ever known. In the center is the high-density nucleus. Between the capsid and the envelope is a broad space, which is not found in any of the baculoviruses of the prawns ever reported. On the surface of the puri-fied nucleocapsid, there is a subunit of the spiral arrangement, which is also characteristic of this virus. It has not been observed and found in the epithelial cells of the livers, intestines and cheeks, which is quite different from the fact that prawn baculoviruses infect a certain epithepilial cell of the above-mentioned ones without exception. The viruses only multiplicate inside the core of target cells, which will not form occluded bodie     

A preputial odor imprinted on female mice

Female mice (Mus musculus) which were reared by a preputialectomized female and exposed to an intact male from 14 to 18 days of age preferred the odor of an intact male over that of a preputialectomized male when tested at 15 weeks of age. However, those females which were exposed to an intact male from 28 to 32 days of age preferred the odor of a preputialectomized male over that of an intact male, and those females which were exposed to an intact male from 0 to 4 days of age showed no reliable preferences. Females which were reared by an intact female and exposed a preputialectomized male for 4 days tended to reverse preferences. Females which were exposed to a preputialectomized male from 14 to 18 days of age preferred the odor of a preputialectomized male. However, females which were exposed to a preputialectomized male from 28 to 32 days of age preferred the odor of an intact male, as did females which were exposed to a preputialectomized male from 0 to 4 days.     

Crystal structure of a thermostable lipase from Bacillus stearothermophilus P1

We describe the first lipase structure from a thermophilic organism. It shares less than 20% amino acid sequence identity with other lipases for which there are crystal structures, and shows significant insertions compared with the typical alpha/beta hydrolase canonical fold. The structure contains a zinc-binding site which is unique among all lipases with known structures, and which may play a role in enhancing thermal stability. Zinc binding is mediated by two histidine and two aspartic acid residues. These residues are present in comparable positions in the sequences of certain lipases for which there is as yet no crystal structural information, such as those from Staphylococcal species and Arabidopsis thaliana. The structure of Bacillus stearothermophilus P1 lipase provides a template for other thermostable lipases, and offers insight into mechanisms used to enhance thermal stability which may be of commercial value in engineering lipases for industrial uses.     

Cervical screening performance in general practice. An evaluation in a single health district

This paper describes a study of cervical screening in 50 general practices in the East Berkshire Health District over a period of 2 1/2 years. Six practices organized their own practice based screening scheme. Thirty-nine participated in a district organized scheme and five did not actively participate in any scheme. The proportion of women screened was highest among the practices which organized their own scheme. These practices were large, in non-urban locations and employed a practice nurse. In contrast, practices which had a poor record of screening were small, single handed, in an urban location and were unwilling to participate in a district call scheme or organize their own scheme. It is recommended that the practices which are prepared to organize their own screening programmes should be encouraged to do so. The resources which are saved could then be more usefully spent on providing assistance to the practices which do not offer a cervical screening service to women on their practice lists.     

Phytotropins: Conformational requirements for the abolition of the root geotropic response

Compounds of the phytotropin class have been assessed for possible conformational requirements with respect to their ability to aftect the root geotropic response. It is shown that part of the molecule may need to adopt a planar configuration, and evidence is adduced which indicates that the remainder of the molecule may also have conformational requirements which need further definition. It is suggested that molecules which favour a conformation such that the aromatic ring which bears the carboxyl function is out of plane with the remainder of the molecule may have activity. Coplanarity of the carboxyl group with the ring to which it is attached is not a prerequisite for activity.