Cycles in cannibalistic egg-larval interactions

A model of a cannibalistic larval-egg interaction such as occurs in Tribolium is developed which leads to a system of nonlinear Volterra integral equations. I determine the local stability properties of the unique equilibrium point of the model. A Hopf bifurcation analysis shows that the model always undergoes a subcritical bifurcation when stability is lost. Numerical solutions confirm the presence of multiple attractors over a range of parameter values. The form of the cycles observed in the numerical solutions is analogous to that observed in laboratory populations of Tribolium.     

Diagnosis of genetic disease using recombinant DNA. Supplement

Summary Recombinant DNA methodology has greatly increased our knowledge of the molecular pathology of the human genome at the same time as providing the means to diagnose inherited disease as the DNA level. We present here a list of recent reports of both direct and indirect analysis of human inherited disease which is intended to serve as a guide to current molecular genetic approaches to diagnostic medicine.     

Population biology of two land snails (Mesomphix spp.): variation among six southern appalachian sites with differing disturbance histories

Summary Ecological and genetic properties of two North American terrestrial gastropods (Mesomphix spp.) were characterized in paired control and previously logged watersheds in two North Carolina forests (Coweeta and the Great Smoky Mountains National Park) of the Southern Appalachian Biosphere Reserve Cluster. Shell growth was greater in the control sites, but density and mortality were largely independent of prior logging history and forest reserve. Based on starch gel electrophoresis data, both species showed their highest levels of genetic diversity in the Coweeta forest, the component of the reserve cluster which had the most extensive and variable history of logging disturbance. M. subplanus also exhibited higher levels of heterozygosity in logged than in control watersheds, and M. andrewsae showed over twice as many rare alleles in disturbed sites as in control sites. F-statistic analysis depicted both excess levels of homozygosity and moderate genetic differentiation among the populations, reflecting the effects of small population size and perhaps drift and inbreeding. Estimated gene flow was relatively low. These results correspond to the recent finding by Bryant et al. (1987) and others on the effects of bottlenecks, and to the contrasting history of habitat instability of the two major study forests.     

The effects of ultraviolet-B radiation on loblolly pine. I. Growth, photosynthesis and pigment production in greenhouse-grown seedlings

One-year old loblolly pine ( Pinus taeda L.) seedlings were grown in an unshaded greenhouse for 7 months under 4 levels of ultraviolet-B (UV-B) radiation simulating stratospheric ozone reductions of 16, 25 and 40% and included a control with no UV-B radiation. Periodic measurements were made of growth and gas exchange characteristics and needle chlorophyll and UV-B-absorbing-compound concentrations. The effectiveness of UV-B radiation on seedling growth and physiology varied with the UV-B irradiance level. Seedlings receiving the lowest supplemental UV-B irradiance showed reductions in growth and photosynthetic capacity after only 1 month of irradiation. These reductions persisted and resulted in lower biomass production, while no increases in UV-B-absorbing compounds in needles were observed. Seedlings receiving UV-B radiation which simulated a 25% stratospheric ozone reduction showed an increase in UV-B-absorbing-compound concentrations after 6 months, which paralleled a recovery in photosynthesis and growth after an initial decrease in these characteristics. The seedlings grown at the highest UV-B irradiance (40% stratospheric ozone reduction) showed a more rapid increase in the concentration of UV-B-absorbing compounds and no effects of UV-B radiation on growth or photosynthetic capacity until after 4 months at this irradiance. Changes in photosynthetic capacity were probably the result of direct effects on light-dependent processes, since no effects were observed on either needle chlorophyll concentrations or stomatal conductance. Further studies are necessary to determine whether these responses persist and accumulate over subsequent years.     

How important is seed predation to recruitment in stable populations of long-lived perennials?

Summary The many evolutionary modifications to seed biology in response to seed predation do not necessarily imply that seed predators have an important impact on population recruitment. This is because competition between individual plants for rare safe sites can cause an oversupply of seeds so far as a population is concerned. The importance of seed losses to population recruitment at any point in time is related to the abundance of safe sites (Fig. 1): it is zero when safe sites are absent, negligible when safe sites are rare, and greatest when safe sites are numerous enough for recruitment to be limited by seed supply. Here I interpret the impact of severe seed losses on population recruitment in four species of long-lived perennials (Eucalyptus baxteri, Leptospermum juniperinum, Casuarina pusilla and L. myrsinoides) by considering these losses in terms of the overall seed dynamics of the populations. I focus on seed supply and seedling survival, as a measure of the current abundance of safe sites, and the maintenance of seed banks, as a measure of the ability of populations to exploit any future changes in safe site abundance. Insect seed predators destroyed about 95% of total seeds in each case. However, these losses do not necessarily have an important impact on population recruitment, because: (i) in most years recruitment appears to be limited by a rarity of safe sites, and not by seed supply (which was as high as 43 germinable seeds/m²/yr); and (ii) the losses did not prevent the establishment of large seed banks (ranging from >30 to >1000 viable seeds/m²) potentially capable of exploiting temporary conditions favourable for recruitment. In contrast to the situation with many annual plants, patterns of recruitment in stable populations of long-lived perennials are often extremely complex, and the significance of seed losses therefore difficult to determine.     

A pump/leak/buffer model for plant nitrate uptake

A simple simulation model is described to account for the rates at which plants take up nitrate and reduce it to protein. It is based on the pump and leak principle, with the pump working at a constant rate per unit sap volume provided that there is an adequate concentration of nitrate at the root surface. The rate of leakage is assumed to be proportional to the concentration difference between the inside and the outside of the plant. Nitrogen is removed from the plant nitrate pool (the buffer) at a constant fraction of the photosynthesis rate. When applied to data for the diurnal variation in nitrate uptake by ryegrass, the model predicts an uptake pattern similar to that actually observed, with a time lag of about 5 hours between photosynthesis and uptake.     

A novel vitronectin receptor integrin (alpha v beta x) is responsible for distinct adhesive properties of carcinoma cells

Carcinoma cells express a novel integrin involved in cell adhesion to vitronectin, but not to fibrinogen or von Willebrand factor, whereas melanoma and endothelial cells express a vitronectin receptor (alpha v beta 3) that promotes cell attachment to all of these matrix components. The integrin responsible for this adhesive phenotype of carcinoma cells is composed of an alpha subunit that is indistinguishable from the alpha v of the vitronectin receptor and a beta subunit (beta x) that is distinct from any known integrin beta subunit. Accordingly, Northern blot analysis identifies an mRNA for alpha v, but not for beta 3 in carcinoma cells. This receptor appears to mediate cell adhesion to vitronectin as well as fibronectin since an antibody directed to its alpha subunit blocked carcinoma cell adhesion to both of these matrix proteins. These results suggest that homologous integrins with identical alpha subunits and structurally distinct beta subunits can account for the functional recognition of different matrixes by two cell types.