A model for gramicidin A′-phospholipid interactions in bilayers

A model is proposed for the effect of gramicidin A on the order and structure of phospholipid dispersions. According to this model, the addition of gramicidin A influences the surrounding lipids via two independent mechanisms. The first arises from a drop in surface pressure for those lipids substantially bounded by gramicidin A. The second mechanism arises from the increase in the phospholipid headgroup spacing due to the small polar region of the polypeptide. The model provides an explanation for the currently available NMR, X-ray diffraction and Langmuir monolayer results. The model also suggests mechanisms for the ability of gramicidin A to trigger a transition of the lipid from the lamellar to hexagonal II phase, the dependence of this transition on the lipid chain length and the formation of a lamellar phase with lysophosphatidylcholine.Abbreviations NMR nuclear magnetic resonance - DMPC dimyristoylphosphatidylcholine - S molecular order parameter - CSA chemical shift anisotropy - DPPC dipalmitoylphosphati-dylcholine - LPC lysophosphatidylcholine     

On a simplified model for pattern formation in honey bee colonies

We present a simplified version of a previously presented model (Camazine et al. (1990)) that generates the characteristic pattern of honey, pollen and brood which develops on combs in honey bee colonies. We demonstrate that the formation of a band of pollen surrounding the brood area is dependent on the assumed form of the honey and pollen removal terms, and that a significant pollen band arises as the parameter controlling the rate of pollen input passes through a bifurcation value. The persistence of the pollen band after a temporary increase in pollen input can be predicted from the model. We also determine conditions on the parameters which ensure the accumulation of honey in the periphery and demonstrate that, although there is an important qualitative difference between the simplified and complete models, an analysis of the simplified version helps us understand many biological aspects of the more complex complete model. Corresponding author     

Structure,development and function of the branchial sieve of the common bream,Abramis brama,white bream,Blicca bjoerkna and roach,Rutilus rutilus

Synopsis The filter feeding organ of cyprinid fishes is the branchial sieve, which consists of a mesh formed by gill rakers and tiny channels on the gill arches. In order to establish its possible role during growth we measured the following morphological gill raker parameters over a range of sizes in three cyprinid fishes, bream, white bream and roach: inter raker distance, bony raker length, raker width, cushion length and channel width. At any given standard length common bream has the largest inter raker distance, roach the lowest and white bream is intermediate. In the comb model of filter feeding the inter raker distance is considered to be a direct measure of the mesh size and retention ability (= minimal size of prey that can be retained) of a filter. For the three species under study there is a conflict between the comb model and experimental data on particle retention. Lammens et al. (1987) found that common bream has a large retention ability whereas roach and white bream have a much smaller one. A new model, the channel model (Hoogenboezem et al. 1991) has been developed for common bream; in this model the lateral gill rakers can regulate the mesh size of the medial channels on the other side of the gill slit. The present data indicate that this model is not appropriate for white bream and roach. At any given standard length white bream and roach only reach 70% of the raker length of common bream, which means that in this model the gill slits should to be very narrow during filter feeding. The gill rakers consist of a bony raker and a fleshy cushion. The bony rakers have a rather long needle-like part outside the cushion in bream, but not in white bream and roach which have blunt gill rakers. Blunt gill rakers are not suited to reduce the diameter of the medial channels. The comb model seems more appropriate for white bream and roach, but doubts about the validity of this simple model remain. The sum of the areas of the medial channels is an approximation of the area through which water flows in the filter. This channel area therefore gives an impression of the capacity or flow rate of the filter. With this capacity estimation and an estimation of energy consumption we calculated an energy ratio of filter feeding. The energy ratio decreases with increasing standard length with an exponent close to the expected exponent of -0.40. The energy ratio is highest in bream, intermediate in white bream and lowest in roach.     

The mechanism of extrachromosomal homologous DNA recombination in plant cells

Summary By cotransfecting plasmids carrying particular mutations in the -glucuronidase (GUS) gene into Nicotiana plumbaginifolia protoplasts and by monitoring the recombination rates using a recently developed transient assay, we were able to obtain insights into the mechanism of extrachromosomal recombination operating in plant cells. An exchange of flanking markers takes place in over 90% of the recombination events. In most of the remaining cases two consecutive, independent single crossover events occur. These events involve the same DNA substrate and lead to two successive exchanges of flanking markers, thus mimicking a presumed double crossover intermediate. A comparison of the outcome of our experiments with the predictions of two recombination models originally proposed for mammalian cells indicates that extrachromosomal recombination in plant cells is best described by the single strand annealing model. According to this model all recombination events result in an exchange of flanking markers. Our results rule out the double strand break repair model which predicts that flanking markers are exchanged in only half of all events.     

Outgrowth dependency of forelimb regeneration on nerves in adult African clawed frog,Xenopus laevis

Summary The relationship between the size and shape of regenerative outgrowth and the quantity of innervation was studied in adult Xenopus laevis. The forelimbs, of which the nerve supply was artificially altered, were amputated midway through the stylopodium and were kept for 1 year. The regenerative outgrowths that formed in normal limbs with an intact nerve supply were mainly spike-shaped and occasionally rod-shaped. However, when the nerve supply to the distal part of the forelimb was augmented by surgically diverting ipsilateral sciatic nerve bundles, the quantity of innervation was increased to about two and a half times that of the normal limb. These hyperinnervated outgrowths were somewhat larger than those of the normally innervated outgrowths and the majority of them were oar-shaped, a type hardly ever encountered in normal regeneration. In contrast, when partial denervation was performed concomitantly with limb amputation, by ablation of the N. radialis at the shoulder joint, the quantity of innervation decreased to about one half that of the normal limb. The outgrowths obtained were spike-shaped in all cases, with their size being about half that of the normally innervated outgrowths. Furthermore, when both the N. radialis and N. ulnaris were ablated in the same way, the amputated limbs were mostly non-regenerative, but some of them regenerated small conical outgrowths. Based on these results, a discussion is presented concerning the relationship between a regenerative outgrowth and the innervation of the forelimb in Xenopus.     

Use of the Golden Section search method to estimate the parameters of the Monod model employing spread-sheets

An alternative procedure to obtain the parameters of Monod's growth model in batch culture is presented. It is based on the integral kinetic analysis methodology, employs a one-dimensional Golden Section search optimization method and is implemented on a spread-sheet programme. The procedure is discussed in detail and is illustrated by analysis of batch substrate consumption data by an aerobic bacterial consortium.     

Use of disease progress curves to study the effects of the biocontrol agent Sporidesmium sclerotivorum on lettuce drop

At the end of the spring 1987 growing season, the mycoparasite Sporidesmium sclerotivorum was applied at 0, 0.2, 2 or 20 kg ha^‐1 to lettuce plants infected with Sclerotinia minor. Disease incidence was monitored in the same plots for five subsequent crops (three fall and two spring crops) without additional application of either pathogen or mycoparasite. Logistic growth curves were fitted to the data to describe disease progression over time for each inoculum level within each of the five crops. Within each crop, increasing the quantity of mycoparasite inoculum resulted in positive horizontal displacement of the curve with respect to time. As quantities of inoculum of S. sclerotivorum increased, inflection points of the disease progress curves increased at a decreasing rate. Thus, additional mycoparasite inoculum resulted in ever‐smaller increases in inflection point, and after a certain threshold level of mycoparasite inoculum (< 0.2 kg ha^‐1), increases in inflection point did not result in meaningful increases in harvestable lettuce. Maximum rates of disease increase were not different among the treatments within each crop, but were different between crops. Maximum rates of disease increase averaged 3.4, 3.4, 2.1, 3.6 and 1.5% day^‐1 for the fall 1987, spring 1988, fall 1988, spring 1989, and fall 1989, respectively. At all inoculum levels, the fall epidemics began later after planting than the spring epidemics.     

Some epidemiological models with nonlinear incidence

Epidemiological models with nonlinear incidence rates can have very different dynamic behaviors than those with the usual bilinear incidence rate. The first model considered here includes vital dynamics and a disease process where susceptibles become exposed, then infectious, then removed with temporary immunity and then susceptible again. When the equilibria and stability are investigated, it is found that multiple equilibria exist for some parameter values and periodic solutions can arise by Hopf bifurcation from the larger endemic equilibrium. Many results analogous to those in the first model are obtained for the second model which has a delay in the removed class but no exposed class.Research supported in part by Centers for Disease Control Contract 200-87-0515. Support services provided at University House Research Center at the University of IowaResearch supported in part by NSERC A-8965 and the University of Victoria President's Committee on Faculty Research and Travel     

Aspects of the life history of Cercopithifilaria johnstoni (Nematoda:Filarioidea)

and 1988. Aspects of the life history of Cercopithifilaria johnstoni (Nematoda:Filarioidea). International Journal for Parasitology 18: 1087–1092. Cercopithifilaria johnstoni (Nematoda:Filarioidea) occurs in the subcutaneous connective tissues of a spectrum of native murid and marsupial hosts in Eastern Australia. Life cycle studies revealed that: (i) microfilaria occur in lymphatic capillaries and extravascular connective tissue of the dermis (= ‘skin-inhabiting’), (ii) ixodid ticks, particularly Ixodes trichosuri, are intermediate hosts in nature, (iii) development from microfilariae to infective third-stage larva occurs only while the tick is off the host, that is, during ecdysis from larva to nymph or from nymph to adult. Transmission of C. johnstoni in a wild population of bush rats (Rattus fuscipes) occurred in summer and winter, and was associated with peaks in the number of larval and/or nymphal stages of ticks on rats. C. johnstoni was transmitted experimentally to bandicoots (Isoodon macrourus, Perameles nasuta), bush rats and laboratory rats (R. norvegicus), indirectly by subcutaneous inoculation of third-stage larvae and directly by tick feeding. The prepatent period was approximately 3 months and the longest duration of microfilariae in the ‘ skin’ was more than 25 months. Dermal and ocular lesions were observed in R. fuscipes. The host-parasite relationship has the potential for development as an inexpensive and practical model for human onchocerciasis.     

Respiration Rate of Steinernema feltiae Infective Juveniles at Several Constant Temperatures

Respiration was measured in dauer stages of the insect-parasitic nematode Steinernema feltiae (= Neoaplectana carpocapsae) at 7, 17, and 27 C. Respiration, Q₁₀, and nematode viability were temperature dependent. Mean O₂ consumption for 5 × 10⁵ nematodes the first 24 hours was 0.27 ml at 7 C, 0.83 ml at 17 C, and 2.68 ml at 27 C. The Q₁₀ was 3.10 for 7-17 C and 3.24 for 17-27 C. Some nematodes died during 2, 14, and 21 days at 27, 17, and 7 C, respectively. The respiratory quotient was below 1 at all temperatures tested. A standard asymptotic model is expressed as oxygen consumed = 2.77 * {1 - exponent[-time * exponent(-B + C * temperature)]}; where 2.77 is the maximum response at 27 C. This model estimates nematode O₂ consumption and viability at storage temperatures between 7 and 27 C. The nematodes died when the O₂ concentration reached 0.5 ml/5 × 10⁵ nematodes. This model may be used to predict O₂ requirements of S. feltiae infective juveniles when stored as a waterless concentrate.