Influence of environmental variables on algal associations from a floating vegetation mat (Schwingmoor Lake Lunzer Obersee,Austria)

In 2002, periphyton samples were collected 14 times from six stations within a floating vegetation mat in Lake Lunzer Obersee, a small mountain lake in Austria. Water temperature, conductivity, pH, alkalinity, Ca²⁺- and total-hardness were measured and quantitative samples were taken for algal biomass and chlorophyll-a analysis. Algal biomass results clearly indicated the dominance of green algae. Cluster analyses were carried out, based on both algal data and environmental parameters. The algal data separated sites, whereas the abiotic clustering revealed a temporal aspect. An indicator species analysis determined the characteristic taxa for each species group. Indicator species were then related to environmental factors using a redundancy analysis (RDA). Both pH and conductivity were significant factors, whereas water temperature, alkalinity, Ca²⁺- and total-hardness were not. The results of the RDA show that three species groups could be identified along a gradient of pH. One group was positively correlated with high alkalinity and another group occurred at higher values of conductivity.     

Fibroblast growth factor receptor signaling is essential for lens fiber cell differentiation

The vertebrate lens provides an excellent model to study the mechanisms that regulate terminal differentiation. Although fibroblast growth factors (FGFs) are thought to be important for lens cell differentiation, it is unclear which FGF receptors mediate these processes during different stages of lens development. Deletion of three FGF receptors (Fgfr1-3) early in lens development demonstrated that expression of only a single allele of Fgfr2 or Fgfr3 was sufficient for grossly normal lens development, while mice possessing only a single Fgfr1 allele developed cataracts and microphthalmia. Profound defects were observed in lenses lacking all three Fgfrs. These included lack of fiber cell elongation, abnormal proliferation in prospective lens fiber cells, reduced expression of the cell cycle inhibitors p27^kip1 and p57^kip2, increased apoptosis and aberrant or reduced expression of Prox1, Pax6, c-Maf, E-cadherin and α-, β- and γ-crystallins. Therefore, while signaling by FGF receptors is essential for lens fiber differentiation, different FGF receptors function redundantly.     

Common bacterial toxins and physiological vulnerability to sudden infant death: the role of deleterious genetic mutations

The common bacterial toxin hypothesis of sudden infant death syndrome (SIDS) is consistent with the epidemiological features of the condition including the age distribution, seasonal incidence, association with prone sleeping and with exposure to tobacco smoke. The hypothesis is supported by experimental evidence but there are two barriers to its acceptance: the speed of onset does not fit with conventional concepts of an infective process; furthermore, the hypothesis appears to offer a single explanation for what is regarded as a multifactorial disease. Concepts from information theory are used to explore these objections. Complex physiological systems process information and need a high level of redundancy to minimise error. Models show that deleterious mutations in such a system will interact synergistically. Environmental perturbations are most likely to cause failure (sudden death) in systems with several mutations. Models also indicate that mutation rates will pose a limit to the size of the functioning genome and, therefore, increased complexity in evolution depends on using old genes in new combinations rather than the chance appearance of new genes. The idea that we share our genes with the rest of creation (same genes but different combinations) leads to the following conjecture: for every receptor controlling the flow of information across a cell membrane there will be a bacterially coded molecule that can switch it off or on. Based on this premise, bacterial toxaemia could cause sudden death, merely the time it takes for a molecule to associate with or dissociate from its receptor. Regardless of the number of physiological systems involved in SIDS, the age distribution will have a unimodal peak corresponding to the age range during which infant serum IgG reaches its nadir. In this way, the two barriers to the common bacterial toxin hypothesis can be overcome: one explanation but multiple bacteria and toxins acting with variable speed on multiple target systems.     

achaete, but not scute, is dispensable for the peripheral nervous system of Drosophila

The achaete-scute complex of Drosophila has been the focus of extensive genetic and developmental analysis. Of the four genes at this locus, achaete and scute appear to act redundantly to specify the peripheral nervous system. They share cis-regulatory elements and are co-expressed at the same locations. A mutation removing scute activity has been previously described; it causes a loss of some sensory bristles. Thus, when Scute is absent, the activity of achaete allows formation of the remaining bristles. However, all existing achaete mutants are rearrangements affecting regulatory sequences common to both achaete and scute. To determine the level of redundancy between the two genes, we have used a P element approach to generate a null allele of achaete, which leaves scute and all cis-regulatory elements intact. We find that the peripheral nervous system of achaete null mutant larvae and imagos lacks any detectable phenotype. However, when the levels of Scute are limiting, then some sensory organs are missing in achaete mutant flies. achaete and scute are thought to have arisen from a duplication event about 100 Myr ago. The difference between achaete and scute null flies is surprising and raises the question of the retention of both genes during the course of evolution.     

Environmental influences on the structure of sedge meadows in the Canadian High Arctic

Wet sedge-dominated communities (sedge meadows) were sampled in five lowland oases in the Queen Elizabeth Islands of the Canadian High Arctic to assess species-environment relationships. The sites spanned 4° of latitude, and varied in lithology and intensity of grazing by muskoxen (Ovibos moschatus). A suite of 8 vascular species were common in all meadow stands, with an additional 4–6 species found in most stands. The position of these species in dominance-diversity curves was not significantly different between grazed and ungrazed meadows however, the grazed sites appeared to follow a log-normal distribution, while the ungrazed sites were more geometric. Redundancy analysis indicated that grazing intensity is important in determining structure in arctic sedge meadows, largely through increasing the cover of bryophytes and the availability of nitrogen. Greatest species richness was found in the more southerly sites which were moderately grazed and had diversity in microtopography. Abbreviations: AF – Alexandra Fiord, PBP – Polar Bear Pass, PMB – Princess Marie Bay, TL – Truelove Lowland, SP – Sverdrup Pass Nomenclature: Porsild, A.E. & Cody, W.J. 1980. Vascular plants of continental Northwest Territories. National Museums of Canada, Ottawa.     

The influence of soil depth on plant species response to grazing within a semi-arid savanna

Grassland patches within a semi-arid savanna were evaluated over 45-years for (1) local temporal dynamics of basal area for five dominant grass species within long-term heavily grazed and ungrazed treatments, (2) the influence of soil depth (resource availability) on vegetation dynamics, and (3) the applicability of community-level grazing response groups over fine-scale patterns of soil heterogeneity. Temporal patterns in species composition and basal area were dependent upon soil depth. In the heavy grazed treatment, Hilaria belangeri dominated deep soils while Erioneuron pilosum and Bouteloua trifida were restricted to shallow soils. In the ungrazed treatment, removal of grazing resulted in successional changes that were significantly different across soil depths. After 45 years without grazing, Eriochloa sericea was most abundant on deep soils while Bouteloua curtipendula was more abundant on intermediate and shallow soils. Community-level functional groups that are based on grazing were not appropriate when multiple pattern-driving variables were considered across multiple scales indicating that functional groups should only be applied to certain processes at specific scales. Within the ungrazed treatments, variable soil depths have resulted in a shifting mosaic in time and space where early- and late-successional species co-exist continuously but spatially separated within the community. In the heavily grazed treatment, species are somewhat spatially arranged by soil depths, but much of the inherent heterogeneity is eliminated and species composition is dominated by the three grazing-resistant short-grasses. Broad scale successional changes may appear linear and predictable while at finer scales, the same changes may be described as non-linear and dependent upon soil depth resulting in thresholds that are partially explained by weather patterns, seed bank limitations and competitive inhibitions.     

Numerical analysis of modern and fossil pollen spectra as a tool for elucidating the nature of fine-scale human activities in boreal areas

Modern pollen assemblages from the major vegetation units (both natural and anthropogenic) on the island of Hailuoto, Finland are studied from 29 surface moss samples. A total of 59 pollen and spore taxa are recorded. The pollen data-set is related by redundancy analysis (RDA) to six external synthetic variables that characterize in general terms the different major ecological situations relevant to the land-use on the island (sand, humus, forest, field, deforested, treeless). Weighted average (WA) optima are calculated to identify which pollen taxa are most indicative of four of these external variables. Two fossil pollen diagrams from Hailuoto are re-interpreted by positioning their samples on the RDA plot of the modern data-set and by classifying the modern and fossil spectra together in a minimum-variance cluster analysis. The RDA plot shows that the strongest features in the modern pollen assemblages are the contrast between spectra from dry sandy soils and those from damper soils with more organic content, and between assemblages from forested and unforested areas. For anthropogenic situations the numerical analyses detect a distinction in the pollen assemblages characterizing fields from those representing farms and trackways but is less successful in separating the latter two. The WA optima indicate that most pollen taxa considered as indicator types primarily represent a mixed field/farm category of land-use. The RDA and minimum-variance cluster analysis confirm that the two fossil sites have had different vegetational histories. In neither is there any phase equivalent to the modern farm situation on Hailuoto. The initial phase in each diagram represents shore meadows and fields but because Poaceae pollen is identifiable only to family level it is not possible to separate these two habitat types.     

Vegetation ecology of flatwoods on the Illinoian till plain

Abstract. Midwestern flatwoods are open woodlands of level uplands and terraces that occur on impervious subsoil horizons and have seasonally wet and dry soils. Vegetation and soil data from six flatwoods remnants on the Illinoian till plain in Illinois were examined to identify what biotic and edaphic factors control vegetation structure, composition and diversity. Soils at all sites are characterized by the presence of argillic horizons (clay pans). Two soil ordination groups were clearly defined, each characterized by separate parent material origin. Canopy and subcanopy composition are dissimilar at sites with soils developed entirely in loess; more similar at sites on lacustrine soils (including aeolian sand). Quercus stellata was by far the most important tree species followed by Q. marilandica. Q. stellata importance was negatively correlated with Ca, particularly in the A-horizon. Q. marilandica shows a positive correlation to sand and magnesium, particularly in the B-horizon, and was most frequent and abundant on soils where Ca/Mg ratios are lower than 1.0 in the E and B-horizons. Total stem density of woody plants was positively related to total available soil moisture as influenced by depth to clay pan and soil textural differences. Ground-cover species richness and diversity were inversely related to tree and total woody stem density and tree diversity. One site with a recent history of annual prescribed fire had significantly greater ground-cover species diversity compared with the unburned sites. Evidence from stand structure and similarity between canopy and subcanopy strata, if reflecting stationary processes of canopy recruitment, suggest that disturbances historically maintained the importance of oak species in most flatwoods. Prescribed fire appears to maintain or enhance ground-cover species diversity at one flatwoods remnant, suggesting that fire could be used to maintain compositional stability at sites with greater available soil moisture.