Evolutionary plasticity in prokaryotes: A panglossian view

Enzyme directed genetic mechanisms causing random DNA sequence alterations are ubiquitous in both eukaryotes and prokaryotes. A number of molecular geneticist have invoked adaptation through natural selection to account for this fact, however, alternative explanations have also flourished. The population geneticist G.C. Williams has dismissed the possibility of selection for mutator activity on a priori grounds. In this paper, I attempt a refutation of Williams' argument. In addition, I discuss some conceptual problems related to recent claims made by microbiologists on the adaptiveness of molecular variety generators in the evolution of prokaryotes. A distinction is proposed between selection for mutations caused by a mutator activity and selection for the mutator activity proper. The latter requires a concept of fitness different from the one commonly used in microbiology.     

A comprehensive linkage map of the pig based on a wild pig - Large White intercross

A comprehensive linkage map, including 236 linked markers with a total sex-average map length of about 2300 cM, covering nearly all parts of the pig genome has been established. Linkage groups were assigned to all 18 autosomes, the X chromosome and the X/Y pseudoautosomal region. Several new gene assignments were made including the assignment of linkage group U1 (EAK-HPX) to chromosome 9. The linkage map includes 77 type I loci informative for comparative mapping and 72 in situ mapped markers physically anchoring the linkage groups on chromosomes. A highly significant heterogeneity in recombination rates between sexes was observed with a general tendency towards an excess of female recombination. The average ratio of female to male recombination was estimated at 1–4:1 but this parameter varied between chromosomes as well as between regions within chromosomes. An intriguing finding was that blood group loci were overrepresented at the distal ends of linkage groups.     

Root,shoot and soil parameters required for process-oriented models of crop growth limited by water or nutrients

A review is given of the prospects for using process-oriented models of water and nutrient uptake in improving integrated agriculture. Government-imposed restrictions on the use of external inputs will increase the likelihood of (temporary) nutrient or water stress in crop production in NW Europe and thus a better understanding is required of shoot-root-soil interactions than presently available. In modelling nutrient and water uptake, three approaches are possible: 1) models-without-roots, based on empirically derived efficiency ratios for uptake of available resources, 2) models evaluating the uptake potential of root systems as actually found in the field and 3) models which also aim at a prediction of root development as influenced by interactions with environmental factors. For the second type of models the major underlying processes are known and research can concentrate on model refinement on the one hand and practical application on the other. The main parameters required for such models are discussed and examples are given of practical applications. For the third type of models quantification of processes known only qualitatively is urgently needed.     

Critical evaluation of organic acid mediated iron dissolution in the rhizosphere and its potential role in root iron uptake

Both experimental extractions and theoretical calculations were undertaken to assess whether organic acid-mediated Fe dissolution could play a significant role in elevating the concentration of Fe-complexes in the rhizosphere, and further, whether this could satisfy the Fe demands of a plant utilizing ferric reduction to acquire Fe. Using a mathematical computer model, it was predicted that organic acids released from and diffusing away from the root would result in a solution organic acid concentration at the root surface of between 1 to 50 M. Over 99% of the organic acids lost by the root were predicted to remain within 1 mm of the root surface. The experimental results indicated that citrate-mediated Fe dissolution of amorphous Fe(OH)₃, was rapid in comparison with citrate dissolution of the Fe-oxides, Fe₂O₃ and Fe₃O₄. The rate of citrate and malate mediated Fe-dissolution was dependent on many factors such as pH, metal cations and phosphate saturation of the Fe(OH)₃ surface. At pH values 6.8, citrate formed stable complexes with Fe and dissolution proceeded rapidly. Under optimal growth conditions for a plant utilizing a reductive-bound mechanism of Fe acquisition (dicots and non-grass monocots), it can be expected that citrate and malate may be able to satisfy a significant proportion of the plant's Fe demand through the formation of plant-available organic-Fe³⁺ complexes in the rhizosphere. In high pH soils (pH7.0), the plant must rely on other sources of Fe, as citrate-mediated Fe dissolution is slow and Fe-citrate complexes are unstable. Alternatively, the root acidification of the rhizosphere could allow the formation of stable Fe-organic complexes. ei]H Marschner     

The analysis of the NSW wheat variety database. I. Modelling trial error variance

The retrospective analysis of a large database on wheat variety testing in New South Wales (NSW) is considered. This analysis involved three key steps. Initially error variance heterogeneity is modelled, indicating significant differences in error variance due to trial location, year of trialling, sowing date and trial mean yield. The implication of this modelling for the estimaion of variance components is discussed.     

Analysis of woody vegetation of Corbett National Park,India

The heterogeneous vegetation mosaic of the South Turkana region of north Kenya is associated with diversity in the region's physical environment. The abundance and distribution of the dominant species are related to gradients in those abiotic factors that influence water availability, including precipitation, soil texture, and topographic relief. Research focused on three Acacia species that are a major component of the Turkana vegetation; A. tortilis, A. senegal, and A. reficiens. These species each exhibit a different response to variations in abiotic factors. Consequently, species abundance varies independently across the landscape, creating a continuum of intergrading populations. Community types can be identified within the mosaic of intergrading populations. Although community borders are not discrete due to continual change in species abundance, types are identifiable and are repeated in areas with similar environmental conditions. The landscape patterns are representative of Whittaker's (1953) climaxas-pattern, with communities created by individual patterns of populations responding to environmental gradients, creating a continuum of community change across the landscape.     

The hemoglobin concentration ofChironomus cf.Plumosus l. (Diptera: Chironomidae) larvae from two lentic habitats

Hemoglobin concentrations ofChironomus cf.plumosus larvae were measured in two different habitats of the same pond. Larger larvae have higher hemoglobin concentrations than small larvae. There is strong indication that the animals of poorly oxygenated deep water, have higher hemoglobin concentrations than the animals from the well-oxygenated littoral zone.     

再生稻二化螟幼虫空间分布型和抽样技术的研究

再生稻二化螟幼虫的空间布型为负二项分布和聚集分布,其lwao的线性回归方程为m^*=0.0099 2.4753m,属负二项分布。根据聚集均数（λ）测定结果,分析了聚集原因,在分布型研究的基础上,探讨了资料代换模式,Kuno‘s种群序贯抽样模型和最适抽样数模型。     

The advantage of long-distance clonal spreading in highly disturbed habitats

Summary Classical theory states that cover of annual plants should increase relative to perennials as disturbance frequency increases. However, it has been suggested that long-distance clonal spreading can allow some perennial plants to survive in highly disturbed areas by quickly spreading into disturbed patches. To evaluate these hypotheses, we analysed data of plant distributions in two different ecosystems, a barrier island and a short-grass steppe. The disturbances studied were sand deposition during storms (overwash) on the barrier island and grazing by cattle in the short-grass steppe. In each case the disturbance frequency varied over the ecosystem; we categorized different areas in terms of their disturbance frequencies. All plant species in each area were categorized as one of four plant life forms (1) annual or biennial, (2) herbaceous perennial without long-distance clonal spreading (3) herbaceous perennial with long-distance clonal spreading (i.e guerilla form) and (4) woody plant. Percentage cover of each plant life form in each disturbance frequency category was calculated. In both ecosystems, (1) there was an increase in the relative cover of annuals as one moved from areas of low to moderate disturbance frequencies, but then a decrease in cover of annuals as one moved into the areas of highest disturbance frequency and (2) the guerilla forms showed the greatest relative increase in cover from moderately to highly disturbed areas. The combination of two factors can explain this pattern: (1) long-distance clonal spreading effectively reduces the time to colonization of recently disturbed sites and (2) effects of the disturbances in these two systems are probably more severe for seeds than for stems. We illustrate these effects using a spatially explicit simulation model of the population dynamics of plants in a disturbed landscape.     

Oscillatory reaction-diffusion equations on rings

We study the behavior of traveling waves in - systems on both homogeneous and inhomogeneous rings. The stability regions in parameter space of - waves were previously known [15, 19]; the results are extended here. We show the existence of Hopf bifurcations of traveling waves and the stability of the limit cycles born at the Hopf bifurcation for some parameter ranges. Using a Lindstedt-type perturbation scheme, we formally construct periodic solutions of the - system near a Hopf bifurcation and show that the periodic solutions superimposed on the original traveling wave have the effect of altering its overall frequency and amplitude. We also study the - system on an annulus ofvariable width, which does not possess reflection symmetry about any axis. We formally construct traveling waves on this variable-width annulus by a perturbation scheme, and find that perturbing the width of the annulus alters the amplitude and frequency of traveling waves on the domain by a small (order ²) amount. For typical parameter values, we find that the speed, frequency, and stability are unaffected by the direction of travel of the wave on the annulus, despite the rotationally asymmetric inhomogeneity. This indicates that the - system on a variable-width domain cannot account for directional preferences of traveling waves in biological systems.