Competition between Different Scrapie Agents in Mice

STRAINS of scrapie agent differ in a number of their biological properties, one of which is average incubation period^1,2. Incubation period is inversely proportional to dose for all known strains of scrapie and some differ so markedly that there is no overlap in their dose-incubation period curves. A large dose of one agent takes longer to produce the disease than the LD₅₀ dose of another agent³. Two such scrapie agents are 22A and 22C when compared by intracerebral injection of VM mice. Agent 22A has a much shorter incubation period than 22C in VM mice because they carry the p7 allele of the sine gene that controls scrapie incubation. It should therefore be possible to produce mixed infections in VM mice, but to ensure that the clinical disease is produced by the 22A agent. It can be checked that 22A causes the disease, rather than 22C, by examining the distribution of brain lesions². Interaction during pathogenesis between the two agents—competition or synergism—would be seen as a difference in the incubation period of 22A. We achieved mixed infection by injecting 22C first and 22A on a later occasion.     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

Modification of the Pollen-Stigma Interaction in Brassica oleracea by Water

The presence of a film of distilled water on the stigma surfaceof freshly opened flowers results in complete inhibition ofpollen following both incompatible and compatible pollinationsin self-incompatible (SI) genotypes of Brassica oleracea, SIgenotypes of B. campestris and one self-compatible (SC) genotypeof B. campestris. The application of water to the stigmas afterpollination also resulted in a marked reduction in pollen germinationand tube penetration. An increase in the time intervals betweenthe application of pollen onto the stigma and the water treatmentprogressively reduced this inhibition. Pollen germination wasalso completely inhibited when stigmas from freshly-opened flowersof SI B. campestris and B. oleracea genotypes were washed inwater, dried and pollinated with pollen grains of either compatibility.The ability of stigmas to induce pollen germination and tubegrowth was restored over a period, the length of which was dependenton the incompatibility (S) genotype. Stigmas of B. napus (SC)and SC mutants of SI B. campestris were found to be affectedby washing, but stigmas of a SC variety of B. campestris andthe immature stigmas from buds of B. oleracea were found tobe considerably less affected. Microscopic examination of pollenplaced on washed stigmas reveals that grains, irrespective oftheir compatibility, fail to hydrate normally. When inducedto hydrate by raising atmospheric humidity, pollen grains onwashed stigmas did germinate, but most of the tubes failed topenetrate the papillar wall and very few entered the style.It is proposed that the water treatment mobilises componentsof the pellicle which reorganize to block the activity of molecules,present in both SC and SI individuals, responsible for establishingfull contact between the pellicle and pollen grain coating. Brassica, pellicle, pollen, recognition, self-incompatibility     

Acclimation of Trees to Pollution Stress: Cellular Metal Tolerance Traits

Cell suspension cultures were established from shoot explantsof mature trees of Acer pseudoplatanus L. (sycamore) at a sitecontaminated by aerial deposition of copper and cadmium frommetal processing industry, and from the same species at uncontaminatedsites. The responses of cell cultures to elevated metal concentrationsin growth media differed markedly according to site of origin.Both Cu and Cd, applied singly at concentrations of 10–15mg l^–1, inhibited growth and were toxic to cultures originatingfrom the uncontaminated sites, but not to cultures from thecontaminated site. This metal tolerance trait in the culturesfrom the contaminated site was stable through repeated sub-culturing.It could also be induced in one culture originating from thereference uncontaminated site, by gradually exposing the cultureto increasing concentrations of Cu. A reduced level of metalremoval from the media was found in tolerant cultures, comparedto non-tolerant cultures. The results of these experiments demonstratethe occurrence of an alteration of gene expression in responseto pollution stress, suggesting that metal tolerance may beinduced within shoot meristems in vivo. It also represents thefirst example of non-mycorrhizal adaptation to metal toxicityidentified in woody plants. Trees, pollution, metal tolerance, acclimation, plant tissue culture, Acer pseudoplaianus L., sycamore     

Does microhabitat structure affect foliar mite assemblages?

1. Habitat structure is an important factor influencing population dynamics and trophic organisation of terrestrial invertebrates. The phylloplane zone on vascular plant leaves is topographically complex, containing a multitude of microhabitats such as leaf hairs, lesions, and structural refugia such as domatia, which may modify interactions between resident invertebrate communities, colonisers, and subsequent trophic relationships. Leaf domatia are small indentations on the underside of leaves and are often inhabited by potentially beneficial mites and other arthropods. 2. This study investigated the relationship between domatia availability and foliar mite assemblages in contrasting habitats (native forest, plantation forest, and pasture) using a standard test plant (the endemic New Zealand shrub Coprosma lucida, J.R. & G. Forst.). 3. Diverse woody native vegetation types supported higher numbers of mite species than either plantation forest or pastoral grasses. The highest number of mite species occurred in the native forest (63%), plantation forest (38%), and pastoral grasses (25%). In the native vegetation type, experimental C. lucida leaves with domatia supported higher mite densities, greater colonisation success, and more diverse mite assemblages than those without domatia. Mite assemblages within the pastoral site were significantly different from the other two vegetation types. Only one fungivorous mite species, Orthotydeus californicus, occurred compared to five mite species in native and plantation forests. 4. This study indicated that foliar mite assemblages in native vegetation on experimental C. lucida shrubs are influenced by domatia availability, resident foliar mites, and local mite assemblages.     

The Control of Mechanical Power in Insect Flight

SYNOPSIS. The cost of locomotion is rarely constant, but rathervaries as an animal changes speed and direction. Ultimately,the locomotory muscles of an animal must compensate for thesechanging requirements by varying the amount of mechanical powerthat they produce. In this paper, we consider the mechanismsby which the mechanical power generated by the asynchronousflight muscles of the fruit fly, Drosophila melanogaster, isregulated to match the changing requirements during flight controlbehaviors. Our data come from individual flies flown in a flightarena under conditions in which stroke kinematics, total metaboliccost, and flight force are simultaneously measured. In orderto increase force production, flies must increase wing beatfrequency and wing stroke amplitude. Theory predicts that thesekinematics changes should result in a roughly cubic increasein the mechanical power requirements for flight. However, themechanical energy generated by muscle should increase only linearlywith stroke amplitude and frequency. This discrepancy impliesthat flight muscles must either recruit myofibrils or increaseactivation in order to generate sufficient mechanical powerto sustain elevated force production. By comparing respirometricallymeasured total metabolic power with kinematically estimatedmechanical power, we have calculated that the stress in theflight muscles of Drosophila must increase by 50% to accommodatea doubling of flight force. Electrophysiological evidence suggeststhat this change in stress may be accomplished by an increasedneural drive to the asynchronous muscles, which in turn mayact to recruit additional cross bridges through an increasein cytosolic calcium.