Phycobiliprotein synthesis in protoplasts of the unicellular cyanophyte, Anacystis nidulans.

Stable and metabolically active protoplasts were prepared from the unicellular cyanophyte, Anacystis nidulans, by enzymatic digestion of the cell wall with 0.1% lysozyme. The yield of protoplasts from intact algal cells was approx. 50%. Incorporation of L-[U-14C]leucine into cold trichloroacetic acid-insoluble material from protoplasts preparations was linear for 1.5 h and continued for an additional 2.5 h. Incorporation of radiolabeled leucine into hot trichloroacetic acid-insoluble material from protoplast preparations demonstrated protein synthesis in protoplasts in vitro. Phycocyanin is the principal phycobiliprotein and allophycocyanin is a minor phycobiliprotein in A. nidulans cells. The light-absorbing chromophore of both of these phycobiliproteins is the linear tetrapyrrole (bile pigment), phycocyanobilin. Radiolabeled phycocyanin and allophycocyanin were isolated from protoplast preparations which had been incubated with L-[U-14]leucine or delta-amino[4-14C] levulinic acid (a precursor of phycocyanobilin). The radio-labeled phycobiliproteins were purified by ammonium sulfate fractionation and ion-exchange chromatography on brushite columns. The specific radioactivity of phycocyanin and allophycocyanin in brushite column eluates (protoplasts incubated with radiolabeled leucine) was 106 000 and 82 000 dpm/mg, respectively. The specific radioactivity of phycocyanin and allophycocyanin in brushite column eluates (protoplasts incubated with radiolabeled delta-aminolevulinic acid) was 33 000 and 38 000 dpm/mg, respectively. Phycobiliproteins from protoplasts incubated with radiolabeled leucine were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. 25% of the incorporated radioactivity in protoplast lysates and approx. 60% of the incorporated radioactivity in protoplast lysates and approx. 60% of the incorporated ratioactivity in phycocyanin and allophycocyanin (in brushite column eluates) comigrated with the subunits of these phycobiliproteins on sodium dodecyl sulfate-polyacrylamide gels. Chromic acid degradation of phycobiliproteins from protoplast preparations incubated with delta-amino[4-14C] levulinic acid yielded radiolabeled imides which were derived from the phycocyanobilin chromophore. Imides from radiolabeled phycobiliproteins isolated from protoplast preparations incubated with L-[U-14C]leucine did not contain radioactivity. These results show that both the apoprotein and tetrapyrrolic moieties of phycocyanin and allophycocyanin were synthesized in A. nidulans protoplasts in vitro.     

Brucellosis, botflies, and brainworms: the impact of edge habitats on pathogen transmission and species extinction

Ecological interactions between species that prefer different habitat types but come into contact in edge regions at the interfaces between habitat types are modeled via reaction-diffusion systems. The primary sort of interaction described by the models is competition mediated by pathogen transmission. The models are somewhat novel because the spatial domains for the variables describing the population densities of the interacting species overlap but do not coincide. Conditions implying coexistence of the two species or the extinction of one species are derived. The conditions involve the principal eigenvalues of elliptic operators arising from linearizations of the model system around equilibria with only one species present. The conditions for persistence or extinction are made explicit in terms of the parameters of the system and the geometry of the underlying spatial domains via estimates of the principal eigenvalues. The implications of the models with respect to conservation and refuge design are discussed. Received: 10 June 1999 / Revised version: 7 July 2000 / Published online: 20 December 2000     

Leadership, social learning, and the maintenance (or collapse) of migratory populations

Long-distance animal migrations are complex, population-level phenomena that emerge in seasonal landscapes as a result of the interplay between environmental influences (e.g., resources, predators) and social interactions among conspecifics. When landscapes change with respect to phenology or connectivity, the dynamics of migratory species can abruptly shift, in many cases leading to a cessation of migration and dramatic decreases in population size. We develop a difference equation modeling framework to explore how the social transfer of knowledge from informed “leader” individuals enhances the performance of seasonally migratory versus resident populations. The model permits a wide range of population-level behaviors including alternative stable states, partial migration equilibria, and complex dynamics, but we focus our efforts on investigations of migration collapse mediated by a lack of informed leaders that can arise from changes in landscape structure, survivorship, reproduction, and/or social learning. Migration collapse is a hysteretic phenomenon in this model and results either in extinction of the population or purely resident behavior. The hysteretic nature of migration failure, which hinges on cultural transmission of knowledge, highlights a potentially critical role for behavior and social learning in aspects of spatial ecology and conservation biology.     

Evolutionary stability of ideal free nonlocal dispersal

We study the evolutionary stability of nonlocal dispersal strategies that can produce ideal free population distributions, that is, distributions where all individuals have equal fitness and there is no net movement of individuals at equilibrium. We find that the property of producing ideal free distributions is necessary and often sufficient for evolutionary stability. Our results extend those already developed for discrete diffusion models on finite patch networks to the case of nonlocal dispersal models based on integrodifferential equations. The analysis is based on the use of comparison methods and the construction of sub- and supersolutions.     

Habitat edges and predator-prey interactions: effects on critical patch size.

We use partial differential equation models to examine the effects of cross-edge incursions by a predator on the persistence or extinction of a patch-resident prey species. For each of two predator-incursion profiles (namely, a constant incursion distance and a constant loss rate for predators during incursions), we examine the conditions under which the predator can (and cannot) influence the critical patch size of a prey species.     

A comparison of foraging strategies in a patchy environment.

In this paper we compare foraging strategies that might be used by predators seeking prey in a patchy environment. The strategies differ in the extent to which predators aggregate in response to prey density. The approach to the comparison is suggested by the idea of evolutionarily stable strategies. A strategy is said to be evolutionarily stable if it cannot be invaded by another strategy. Thus we examine scenarios where a small number of individuals using one strategy are introduced into a situation where a large number of individuals using the other strategy are already present. However, our foraging models do not explicitly incorporate predator population dynamics, so we use net energy uptake as a surrogate for reproductive fitness. In cases where all of the patches visited by predators sustain prey populations, we find that for any pair of strategies one of them will have a higher net energy uptake than the other whether it is the resident or the introduced strain. However, which one is higher will typically depend on the total predator population, which is determined by the resident strain. If the predators leave prey densities high, the more aggregative strain will have the advantage. If the predators reduce prey densities to low levels the less aggregative strain will have the advantage. In cases where one strain of predators aggregates in response to prey density and the other does not, then there might be patches which do not contain prey but do contain (non-aggregating) predators. In those cases, there is the possibility that whichever strategy is used by the introduced strain will yield a higher energy uptake than that used by the resident strain. This suggests that if some patches are empty of prey then aggregative and non-aggregative strategies may be able to coexist.     

Effects of spatial grouping on the functional response of predators.

A unified mechanistic approach is given for the derivation of various forms of functional response in predator-prey models. The derivation is based on the principle of mass action but with the crucial refinement that the nature of the spatial distribution of predators and/or opportunities for predation are taken into account in an implicit way. If the predators are assumed to have a homogeneous spatial distribution, then the derived functional response is prey-dependent. If the predators are assumed to form a dense colony or school in a single (possibly moving) location, or if the region where predators can encounter prey is assumed to be of limited size, then the functional response depends on both predator and prey densities in a manner that reflects feeding interference between predators. Depending on the specific assumptions, the resulting functional response may be of Beddington-DeAngelis type, of Hassell-Varley type, or ratio-dependent.     

On the effects of spatial heterogeneity on the persistence of interacting species

 The dynamics of two interacting theoretical populations inhabiting a heterogeneous environment are modelled by a system of two weakly coupled reaction–diffusion equations having spatially dependent reaction terms. Longterm persistence of both populations is guaranteed by an invasibility condition, which is itself expressed via the signs of certain eigenvalues of related linear elliptic operators with spatially dependent lowest order coefficients. The effects of change in these coefficients upon the eigenvalues are here exploited to study the effects of spatial heterogeneity on the persistence of interacting species through two particular ecological topics of interest. The first concerns when the location of favorable hunting grounds within the overall environment does or does not affect the success of a predator in predator–prey models, while the second concerns cases of competition models in which the outcome of competition in a spatially varying environment differs from that which would be expected in a spatially homogeneous environment. Received: 9 June 1997