Complex ecological models with simple dynamics: From individuals to populations

The aim of this work is to study complex ecological models exhibiting simple dynamics. We consider large scale systems which can be decomposed into weakly coupled subsystems. Perturbation Theory is used in order to get a reduced set of differential equations governing slow time varying global variables. As examples, we study the influence of the individual behaviour of animals in competition and predator-prey models. The animals are assumed to do many activities all day long such as searching for food of different types. The degree of competition as well as the predation pressure are dependent upon these activities. Preys are more vulnerable when doing some activities during which they are very exposed to predators attacks rather than for others during which they are hidden. We study the effect of a change in the average individual behaviour of the animals on interspecific relationships. Computer simulations of the whole sets of equations are compared to simulations of the reduced sets of equations.     

Characterization of an abscisic acid carrier in suspension-cultured barley cells

The uptake of [³H]-abscisic acid in barley (Hordeum vulgareL. cv. Heartland) cell cultures was found to be mediated throughboth non-saturable and saturable components. The kinetic parametersof the saturable component, determined at pH 4.5 and 21 °C,showed a K_m for natural or (+ )-ABA of 1.3±0.7µMand an apparent V_mex of 7.0 ± 2.8 nmol g^–1 cellsh^–1. The carrier showed a strong preference for the naturalenantiomer of ABA as compared to the unnatural one. Other substancestested, e.g. amino acids, organic acids, and other growth regulators,did not appear to interfere with the carrier-mediated uptakeof ABA. At low external concentrations of ABA (below 2.0 µM),the saturable component was greater than the diffusion component.Similarly, between pH 4.0 and 6.0, the saturable uptake wasresponsible for more than 50% of the total uptake. The carriermay be important in vivo for mediating uptake when endogenouslevels of ABA are low (c. 1 µM). The carrier specificity was evident in inhibition experimentsdone with ABA analogues. Our data showed that the carrier couldaccommodate small modifications in the ABA structure. Four analogueswere able to compete efficiently with ( + )-ABA for the bindingsite of the carrier. Three of these competitors were of the(+)-series. Only one ( –)-analogue, (–)-ABA, wasable to inhibit markedly the saturable uptake of ( + )-ABA.The induction of the ABA-respons-ive gene WCS120 (Houde et al.,1992) presented stricter requirements for the ABA molecule thanthe carrier, although with a similar preference for the ( +)-analogues. Besides ( + )-ABA itself, only two of the analoguestested, both ( + )-series, were able to induce the WCS120 geneafter a 24 h incubation period. The absence of correlation betweenthe activity of the analogues as ABA inhibitors in the carriersystem, and their capacity to induce the WCS120 gene tend tosuggest that the carrier is not directly involved in the signaltransduction pathway leading to the induction of this specificgene. Key words: Abscisic acid, barley, gene induction, Hordeum vulgare, uptake carrier     

Actin and spectrin-like (Mr= 260–240 000) proteins in gregarines

In Gregarina blaberae a M_r = 47 000 and a M_r = 260–240 000 doublet polypeptides reacted in immunoblotting: i) with a polyclonal monospecific rabbit antibody to frog muscular actin, a monoclonal anti-actin antibody against chicken gizzard; and ii) with polyclonal and monoclonal antibodies to human erythrocyte β-spectrin, respectively. The M_r = 47 000 actin-like protein is associated with the ghost and a contractille cytoplasmic extract. The presence of an actin-like protein in Gregarina and Lecudina and its cellular distribution in the cortex indicated that the gliding movement might involve an actin-myosin system in contrast to previous studies. Immunofluorescence showed clear differences between the anterior part of Gregarina and Lecudina which illustrated the high cell polarity of these protozoa. The M_r = 260–240 000 doublet was detected in SDS-PAGE from G. blaberae trophozoite ghosts but not in the cytoplasmic extracts or in extracts from sexual stages, indicating that the presence of these spectrin-like proteins is stage-dependent. Visualization of the M_r = 260–240 000 by immunofluorescence showed clear species differences, with rings arranged perpendicular to the longitudinal narrow folds of G. blaberae, with longitudinal lines underlying the folds of L. pellucida and with lines separating the large folds of Selenidium pendula. The cellular distribution is consistent with a stabilizer function of the spectrin-like proteins in the scaffolding of the cortex of gregarines according to the high diversity of the cell-shape and the cell motility systems in gregarines. The presence of spectrin-like proteins in protozoa and particularly in parasites from primitive arthropods indicated that ancestral spectrin genes could the M_r = 260–240 000 form.     

A spatial model of interspecific competition and selective predation: The case of the two Hippolais

Mutual exclusion between congeneric species has been observed such as the case of the grey and red squirrels in Great Britain and the case of the twoHippolais warbler speciesHippolais icterina andH. polyglotta in Europe. This process can lead to the formation of an extinction wave which propagates. Two main assumptions are tested, competition and selective predation. The aim of this work is to present spatial models of these two processes. The animals of two species are assumed to move on a two dimensional array of spatial patches with local interactions of competition or of selective predation between them. We focus on the case of mutual exclusion. Initially, the two competing species occupy complementary areas in an array of spatial patches with a small common zone. Numerical simulations show that under particular conditions, one species gets extinct and the other invades the whole set of spatial patches. These simulations show that with time, the length of the overlapping zone stabilizes and moves at a constant velocity. The limit length of the overlapping band and the velocity of the extinction wave are found to be functions of the parameters of the models. We relate this general model to the case of two sibling species of birds:H. icterina andH. polyglotta.