Showing papers in "Biological Reviews in 1995"

Sexual selection, honest advertisement and the handicap principle: reviewing the evidence

Abstract: SUMMARY (i) To find out whether a mating preference could have initially evolved for adaptive reasons, one must determine whether the preferred trait could have provided useful information about mate quality at the time when the preference first arose. (ii) One way to do so is to determine whether the preference evolved before or after the preferred trait. If the preference evolved first, then it cannot initially have served an adaptive function in mate choice, rather it must have arisen by random drift, or as a pleiotropic consequence of selection acting on other aspects of individual perceptual abilities. (iii) A number of studies have shown that females exhibit a mating preference (e.g. for movement) in non-sexual contexts also, which suggests that it may have evolved for reasons unconnected to mate choice. In addition, phylogenetic analyses have revealed that in several cases, females of a certain taxon exhibit a preference for a male trait that is absent in a sister taxon and in outgroup taxa, and that this preference is shared by females of the sister taxon tacking the male trait. The principle of parsimony suggests that such a preference has been inherited from a common ancestor, while the preferred trait arose only once in the lineage exhibiting the trait, i.e. that the preference predates the attractive trait. (iii) While the above evidence indicates that females may possess ‘hidden’ preferences for male traits that are not exhibited by members of their own species, and that in at least some cases males have later evolved display traits that exploit preexisting preferences of this kind, there have been too few historical studies of preference evolution to allow one to assess the frequency of such exploitation. Moreover historical studies cannot provide strong support for the adaptive origin hypothesis, because coevolution of trait and preference (as opposed to exploitation of a pre-existing bias) is compatible with Fisherian models of preference evolution as well as with honest advertisement and the handicap principle. One can conclude only that while some mating preferences did not originally evolve for adaptive reasons, others may or may not have done so. (iv) To find out whether a mating preference is currently maintained by natural selection because the preferred trait provides useful information about mate quality, one must investigate the phenotypic and genotypic correlates of display, and the fitness consequences of mate choice. (v) A review of the published data reveals some support for the ideas of adaptive choice and honest advertisement. In a number of species, preferred display traits are correlated with putative measures of quality, and in a small proportion of these, there is evidence that reproductive success and/or offspring performance are higher for individuals mated to attractive partners. Very few studies report a failure to find any such correlates of display or any such benefits. (vi) While the above result suggests that honest advertisement does sometimes occur in extant populations (which does not necessarily imply that preferred traits originally evolved as reliable indicators of mate quality), the possibility of publication bias means that one cannot assess how widespread it is. More data is needed to remedy this problem, particularly regarding the fitness consequences of mate choice for females. Experimental rather than observational methods are the best means to gather such data. Studies that look for correlates of display, for instance, should rely on experimentally induced rather than natural variation in ‘quality’. (vii) The most common correlates of male display are age and dominance. The latter observation suggests that there may often be interactions between the processes of intersexual and intrasexual selection. (viii) There is considerably more evidence to support the idea of female choice for direct than for indirect benefits. At the same time, however, it is apparent that mating decisions are commonly influenced by more than one measure of quality, so that these two kinds of choice need not be independent. To assess this possibility will require more studies of the relationship between male attractiveness and offspring performance. (ix) Mate choice is frequently based on more than one display trait, and each trait is frequently influenced by more than one aspect of quality. ‘One quality, one trait’ views of honest advertisement are simplistic, and must be abandoned. (x) Honesty in sexual displays is sometimes maintained by cost (as in strategic handicap models) and sometimes, with approximately equal frequency, by physical necessity (as in revealing handicap models). In some cases, both mechanisms are involved in a single signalling system. To further distinguish between these possibilities will require experimental investigation of display cost, based on manipulation of display traits.

Potential mechanisms for sex ratio adjustment in mammals and birds.

Abstract: Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.

Three‐way interactions between plant pathogenic fungi, herbivorous insects and their host plants

Abstract: 652 ( I ) Direct effects of non-vector herbivores 653 (2) Indirect effects, mediated through the host 654 IV. Root-shoot interactions 657 ( I ) Above-ground insects and below-ground fungi 657 (2) Above-ground fungi and below-ground insects 658 . . . . . . 11. The effect of plant pathogenic fungi on herbivorous insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) Effects on insects . . . . . . . . . . . . 648 . . . . . . 111. The effect of herbivorous insects on plant pathogenic fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Induced resistance a common defence against pest and pathogen ? . . . . 660 ( I ) The hypersensitive response and lignification . . . . . . . . 661 ( 2 ) Phytoalexins . . . . . . . . . . . . . . 662 (3) Proteinase inhibitors 663 (4) Lipoxygenase 664

The Benefits of Mutualism: A Conceptual Framework

Abstract: Summary There are three general mechanisms by which phenotypic benefits are transferred between unrelated organisms. First, one organism may purloin benefits from another by preying on or parasitizing the other organism. Second, one organism may enjoy benefits that are incidental to or a by-product of the self-serving traits of another organism. Third, an organism may invest in another organism if that investment produces return benefits which outweigh the cost of the investment. Interactions in which both parties gain a net benefit are mutualistic. The three mechanisms by which benefits are transferred between organisms can be combined in pairs to produce six possible kinds of original or ‘basal’ mutualisms that can arise from an amutualistic state. A review of the literature suggests that most or all interspecific mutualism have origins in three of the six possible kinds of basal mutualism. Each of these three basal mutualisms have byproduct benefits flowing in at least one direction. The transfer of by-product benefits and investment are common to both intra- and interspecific mutualisms, so that some interspecific mutualisms have intraspecific analogs. A basal mutualism may evolve to the point where each party invests in the other, sometimes obscuring the nature of the original interaction along the way. Two prominent models for the evolution of mutualism do not include by-product benefits: Roughgarden's model for the evolution of the damsel-fish anemone mutualism and the ‘Tit-for-Tat’ model of reciprocity. Using the conceptual framework presented here, including in particular by-product benefits, I have shown how it is possible to construct more parsimonious alternatives to both models.

Plant Storage Proteins

Abstract: (e ) Structural and evolutionary relationships of prolamins (f) What are prolamins? . . . . . . . ( 2 ) The zS albumins . . . . . . . . (a) Distribution and properties . . . . . (b) Evolutionary relationships . (c) Biological activity . . . . . . . ( d ) Use in genetic engineering (3) Globulins . . . . . . . . . (a) I IS globulins . . . . . . . . (b) 7 s globulins . . . . . . . . (c) Are 7 s and I IS globulins related? . . . . (d) Cereal globulins . . . . . . . (4) Synthesis and deposition . . . . . . . (a ) Protein folding and assembly in the ER . . . (b) Protein targeting and protein body formation . . ( 5 ) Comparative properties of seed storage proteins . . (a) Solubility . . . . . . . . . (b) Amino acid composition and protein structure . . (c) Subunit structures . . . . . . . (d) Polymorphism and post-translational processing . I11 . Seed storage proteins of Gymnosperms . . . . . IV . Tuber storage proteins . . . . . . . . ( I ) Potato . . . . . . . . . . ( 7 ) Sweet potato . . . . . . . . . (3) Taro . . . . . . . . . . (4) Yams . . . . . . . . . . ( 5 ) General features of tuber storage proteins . . . . . . . . ( I ) Soybean VSPs . . . . . . . . . ( 2 ) VSPs in deciduous trees . . . . . . . . . . . . . . . ( I ) Bryophytes . . . . . . . . . ( 2 ) Pteridophytes . . . . . . . . . (b) The prolamins of oats . . . . . .

Historical Burden In Systematics And The Interrelationships Of ‘Parareptiles’

Abstract: Summary The interrelationships within the clade comprised of turtles, pareiasaurs, and procolophonid-like taxa are investigated via a cladistic analysis incorporating 56 characters. A single most parsimonious tree was found (80 steps, c. i. = 0·8) in which the successive outgroups to turtles are: pareiasaurs, Sclerosaurus, lanthanosuchids, procolophonoids (=Owenetta, Barasaurus and procolophonids), and nyctiphruretians (= nycteroleterids). Thus, as suggested recently by other workers (Reisz, in Fischman, 1993) turtles are the highly modified survivors of a radiation of poorly-known reptiles commonly called ‘parareptiles’. Pareiasaurs are united with turtles on the basis of twenty unambiguous derived features which are absent in other basal amniotes (=‘primitive reptiles’) and reptiliomorph amphibians: for example, the medially located choana, enlarged foramina palatinum posterius, blunt cultriform process, fully ossified medial wall of the prootic, opisthotic-squamosal suture, lateral flange of exoccipital, loss of ventral cranial fissure, thickened braincase floor, ‘pleurosphenoid’ ossification, reduced presacral count, acromion process, trochanter major, reduced fifth pedal digit, and presence of transverse processes on most caudals. Recent phylogenetic proposals linking turtles with captorhinids, with dicynodonts, and with procolophonoids are evaluated. None of the proposed traits supporting the first two hypotheses is compelling. The procolophonoid hypotheses is supported by only one synapomorphy (the slender stapes). All other synapomorphies proposed in favour of the above groupings either occur in many other primitive amniotes, or are not primitive for turtles, or are not primitive for the proposed chelonian sister-group. Nyctiphruretus and Lanthanosuchids and nycteroleterids, often considered to be seymouriamorph amphibians, are demonstrated unequivocally to be amniotes. The ‘rhipaeosaurs’, currently considered to be pareiasaur relatives, are shown to be a heterogenous assemblage of seymouriamorphs, therapsids and nycteroleterids. The phylogeny proposed here indicates that many of the traits of the earliest known turtle, Proganochelys, previously interpreted as unique specialisations, also occur in pareiasaurs and other near outgroups of turtles, and must instead represent the primitive chelonian condition: for example, the wide parietals and the short quadrate flange of the pterygoid. The sequence of acquisition of chelonian traits is discussed: many features once thought to be diagnostic of turtles actually characterize larger groupings of procolophonomorphs, and must have evolved long before the chelonian shell appeared. These traits include most of the chelonian-pareiasaur synapomorphies listed above, and many others which characterize more inclusive groupings found in this analysis. In putting Proganochelys much closer to the main line of chelonian evolution, in elucidating the sequence of acquisition of chelonian traits, and in reducing greatly the number of differences between turtles and their nearest relatives, this study helps bridge one of the major gaps in the fossil record. The failure of previous cladistic analyses to identify correctly the nearest relatives of turtles is attributed to biased character selection, caused by an over-reliance on cranial characters deemed ‘important’ by earlier workers, and by a tendency to shoehorn ‘parareptile’ taxa into phylogenies derived from analyses restricted to ‘mainstream’ groups such as synapsids, diapsids, turtles, and ‘captorhinomorphs’. Many of the synapomorphies that resolve turtle origins are postcranial, and the three nearest outgroups to turtles are all highly bizarre groups which were dismissed as ‘too specialized’ by early workers and continued to be inadequately assessed even by workers using a cladistic framework.

Population cycles in northern small mammals.

Abstract: I. The regular multiannual oscillations of small mammals at northern latitudes have been a subject of intensive study from the beginning of this century. The existence of a subjective bias in the research due to different schools of study together with a long series of failures and seemingly contradictory results in experiments testing a multitude of hypotheses have brought confusion to the field of study. Much of this confusion has resulted from a failure to recognize sharply the problem studied, which in turn has masked the progress made during the years. Northern mammal cycles are not a single problem but a composition of many related problems. Every problem may have a single-factor explanation, but even with a single-factor explanation, one factor is not necessarily an answer to all of the related problems. 2. At present, we can state that the cyclicity is caused by a predator-prey interaction. Both the 8-11-year and the 3-5-year cycles may be special cases of a more general cycle, most likely caused by a herbivore-resident specialist predator interaction, where the period of the cycles is determined by size-related constraints affecting the increase rate of the populations. The factors determining the amplitude of the cycles probably vary regionally and/or temporally. The operation of generalist and nomadic predators is largely responsible for the regional and geographic synchrony in cycles, although climatic factors may also contribute to the geographic synchrony. The northern distribution of animal communities; both these factors affect the density of generalist predators, which act as a stabilizing factor in the system. The age-related survival pattern seems to be mainly caused by predation, and the cyclically fluctuating reproductive output and mean body mass may be caused by changes in prey behaviour in response to fluctuating predation risk. Thus, we can already give a plausible explanation for most problems related to northern mammal cycles. 3. In all problems discussed, predation seems to be involved, and in most problems, it seems to be the factor which explains the observed patterns. Thus, as a generalization, it can be said that predation seems to be the key factor in the explanation of the northern multiannual cycles of small mammals. 4. There seems to be a linkage between diversity and cyclicity, probably because the diversity of the community (the number of prey species available) may determine the diet choice of a predator, which in turn determines whether the predators have a stabilizing or a destabilizing impact on prey populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Filter‐feeding in marine macro‐invertebrates: pump characteristics, modelling and energy cost

Abstract: 11. Sponges . . . . . . . . . . ( I ) Functional morphology: filter mechanism ahd pump design (2) Energy cost and pump model 111. Polychaetes . . . . . . . . . ( I ) Sabella penicillus . . . . . . . (2) Chaetopterus variopedatus . . . . . . ( 3 ) Nereas diversicolor . . . . . . . IV. Bivalves . . . . . . . . . . ( I ) Water pumping and particle retention (2) Pump characteristics, energetics and modelling (3) Interpretation problems . . . . . . V. Ascidians . . . . . . . . . . . . . .

H. a. gleason's ‘individualistic concept’ and theory of animal communities: a continuing controversy

Abstract: Summary A tradition of natural history and of the lore of early twentieth-century ecology was that organisms lived together and interacted to form natural entities or communities. Before there was a recognizable science of ecology, Mobius (1877) had provided a name ‘biocoenosis’ for such entities. This concept persisted in the early decades of ecological science; at an extreme it was maintained that the community had integrating capabilities and organization like those of an individual organism, hence the term organismic community. In the 1950s- 1970s an alternative individualist concept, derived from the ideas of H. A. Gleason (1939), gained credence which held that communities were largely a coincidence of individualistic species characteristics, continuously varying environments and different probabilities of a species arriving on a given site. During the same period, however, a body of population based theory of animal communities became dominant which perpetuated the idea of patterns in nature based on biotic interactions among species resulting in integrated communities. This theory introduced an extended terminology and mathematical models to explain the organization of species into groups of compatible species governed by rules. In the late 1970s the premises and methods of the theory came under attack and a vigorous debate ensued. The alternatives proposed were, at an extreme, null models of random aggregations of species or stochastic, individualistic aggregations of species, sensu Gleason. Extended research and debate ensued during the 1980s resulting in an explosion of studies of animal communities and a plethora of symposia and volumes of collected works concerning the nature of animal communities. The inherent complexity of communities and the traditional differences among animal ecologists about how they should be defined and delimited, at what scale of taxa, space and time to study them, and appropriate methods of study and analysis have resulted in extended and as yet inconclusive discussions. Recent differences and discussions are considered under five general categories, evolution and community theory, individualistic concept, community definition, questions from community ecology and empirical studies. Communities are seen by some ecologists as entities of coevolving species and, in any case, it is necessary to integrate evolutionary ideas with the varied concepts of community. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities. Among the major difficulties of addressing the problems of community are problems of definition and terminology. One commentator noted that community ecology may be unique in the sciences because there is no consensus definition of community. One consequence of the lack of consensus definition is evident in the varied and diffuse questions posed in studies of community. Some critics of community ecology fault it for posing unanswerable questions. Recent empirical studies include various assessments about community ranging from deterministic, integrated and organismic to individualistic with various suggestions for compromise. The early emphasis on birds in studies of animal communities has expanded to obviate the argument that any position is constrained by the taxon studied. Insects, in general, are more prone to give rise to interpretation of a nonintegrated community. Parasite community studies have given rise to some distinctive categories and terminology. However, consensus is not achieved either within or among taxonomic groups or habitat groups. The extreme heterogeneity and complexity of communities (and of ecologists) has produced extended discussions of how to approach such multidimensional complexity. These discussions often turn on polarized positions of reductionism and experiment versus holism. Proponents of reductionism asserted that natural communities cannot be understood or their structure and organization predicted until experimental communities, or models thereof, are understood. Holists insisted that the inherent complexity and variability of communities cannot be elucidated in simplified experimental communities or in models. A more recent trend has urged pluralism, or, at least, mutual respect and dialogue, which are sometimes lacking, between proponents of these divergent approaches to communities. Recent work perpetuates the original dichotomy between integrated organismic community concept and individualistic non-integrated concept. The hope for a rule-governed community has extended to metarules and a new theory of community as divided into core species and satellite species is called into question. The problems of distinguishing between determinism and chance effects in community organization continue and the lost or fading hope of a general theory of community is revived in a search for rules that govern their assembly.

A PIVOTAL ROLE FOR TGF-β* IN ATHEROGENESIS?

Abstract: . . . . . . . . . . . . . . . . . . . ( I ) Introduction . . . . . . . . . . ( 2 ) Factors affecting the presence of latent TGF-fi . . . (3) Activation of TGF-P . . . . . . . . (4) Bioavailability of TGF-/I. . . . . . . . (5 ) The cellular response to TGF-,8 IV. Evidence for a role of TGF-P in atherogenesis ( I ) Role of TGF-P in the normal vessel wall (2) Potential defects in the biology of TGF-/I V. The hypothesis: TGF-P as a brake on atherogenesis VI. Potential therapies based on modulation of T G F P . . . . . . . . . . . . . . . . . . . . . . . . ( I ) Currently available modulators of TGF-/I activity . . . (2) Potential side effects of therapies based on TGF-P modulation V I I . Conclusion . . . . . . . . . . . VIII. Acknowledgements . . . . . . . . . IX. References . . . . . . . . . . . . . . . 571 575 575

Regeneration in the vertebrate central nervous system: phylogeny, ontogeny, and mechanisms.

Abstract: (i) Cyclostomes . . . . . . (ii) Teleost fish . . . . . . (iii) Urodele amphibia . . . . . (iv) Anuran amphibia , . , (v) Reptiles . . . . . . . (vi) Birds . . . . . . . ( b ) Mammals . . . . . . . (i) Spinal cord . . . . . . (ii) Olfactory neurons . . . . . (iii) Retina . . . . . . . (iv) Hypothalamic-hypophysial system . . (v) Hippocampus . . . . . . (vi) Unmyelinated monoaminergic axons . 111. Ontogeny . . . . . . . . IV. Mechanisms . . . . . . . . (u) Intrinsic neuronal factors . . . . (b ) Environmental factors . . . . . ( i ) Oligodendrocytes and myelin . . . (ii) Astrocytes and glial scarring . . . (iii) Macrophages/microglia . . . . (iv) Axon growth-promoting molecules . . (v) Axon growth-inhibitory molecules . . (c) Axon response. . . . . . . V. Summary . , , . . , . , VI. Conclusions . . . . . . . . VII. Acknowledgements . . . . . . VIII. References . . . . . . . . ( a ) Non-mammalian vertebrates . , . . . . . . . 5 97

The evolution of suspension feeding in gastropods

Abstract: CONTENTS I . Introduction . . . . . . . . 11. Modes of suspension feeding in gastropods . . 111. Morphology and description of ctenidial filter feeding IV. Ontogeny of suspension feeding gastropods . . . VI. Discussion and Conclusions . . . . . VII. Summary . . . . . . . . VIII. Acknowledgements . . . . . . IX. References . . . . . . . . V. Phylogeny of suspension feeding gastropods . . . . . . . . 549 5 52 5 5 3 557

The revival of interest in mechanisms of bacterial pathogenicity.

Abstract: 1. After a long barren period, the study of bacterial pathogenicity is now one of the most popular subjects in microbiology. This is because bacterial diseases remain a major problem in public health despite the advent of antibiotics, and the subject is a fertile field for the application of genetics and molecular biology. 2. Pathogenicity is a multifactorial property. The biological requirements are abilities to: infect mucous surfaces; enter the host through those surfaces; multiply in the environment of the host; interfere with host defences; and damage the host. Each requirement has many facets all of which can be accomplished by a variety of processes. 3. The molecular determinants of the five requirements for pathogenicity can be identified and the relation between their structure and function obtained by a seven step procedure. Genetic manipulation and observations on organisms grown in vivo play major roles in this procedure. Other vital aspects are the availability of good animal models and the design of biological tests for virulence determinants in vitro that are pertinent to the situation in vivo. 4. A survey of the state of studies on bacterial pathogenicity has highlighted some areas of immense erudition and exposed others that need more attention in the future. Research is often at the highest level of molecular biology for: adherence to and entry of epithelial cells; interference with humoral and phagocytic defences; toxins; and direct induction of cytokines and inflammation. The major gaps are: the determinants of competition with commensals on mucous surfaces; spread into deeper tissues; the host supplied nutrients and metabolism underlying growth rate in vivo; the determinants of interference with the immune response in important chronic diseases and carrier states; the determinants of immunopathological reactions that cause damage in chronic disease; and the determinants of change from carrier to invasive state. Areas that are receiving some attention but are worthy of more are: moving through mucus to gain access to mucous surfaces; opportunistic infections; the determinants of mixed infections; and the determinants of host and tissue susceptibility to infection. 5. Current interest in the regulation of production of virulence determinants and the influence on it of environmental factors has raised speculation on the role these factors play in vivo. However, it has not yet provided much information on the host factors specifically involved in particular bacterial infections. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities.(ABSTRACT TRUNCATED AT 400 WORDS)

Variability of neoplastic parameters in colon tumours, and its significance in diagnostic practice.

Abstract: SUMMARY We have reviewed the value of individual variability in the reaction of tissues to treatment with carcinogens, and the manifestation of this variability in different morphological (histological, morphometric, and ultrastructural), histochemical and immunohistochemical parameters generated in tumorous tissues. Moreover, we also reviewed data in the literature on individual variability in the manifestation of some biochemical and immunochemical markers which are accumulated in the serum of tumour-bearing patients and which are characteristic for the different phases of tumourigenesis. The high variability of different tumorous parameters suggests that none can be utilized alone as a conclusive marker of neoplasia and that only their combined use can give objective information. We also reviewed the impact of this variability in the evaluation of various pathological methods to detect different stages of neoplastic transformation in the colon. It has been concluded that none of the present approaches can be absolutely conclusive and without false results, and that objective information regarding early cancerous changes may be obtained only by use of combined analyses utilizing several laboratory methods. The diagnostic procedures should be based on the complex utilization of all appropriate methods using the quantitative interpretation of the obtained data. Multivariate analysis of many parameters should be very effective for the prediction of therapeutic results.

Treatability, toxicity and biodegradability test methods

Abstract: Summary 1. This review confirms that treatability and biodegradability test methods have been cited extensively in the literature. It is clear that the method selected depends on the specific objectives of the test, i.e. the determination of whether a substance is toxic, biodegradable or treatable. Factors that have to be considered when selecting the test methods are the cost of performing the test, the time and resources involved, and the accuracy required. It often appears that more extensive simulation studies are required after initial screening tests have been performed. 2. Many of the enzyme and bacterial growth tests which have been developed for monitoring or screening of toxicants and their persistence in water and wastewaters have been reviewed. Most of these tests are rapid, inexpensive, and reproducible. Most of the biochemical and microcalorimetric approaches, although promising, are still in their infancy as regards toxicity testing. Therefore, biological testing still appears to be most suitable for routine assessment. 3. Micro-organisms are particularly suitable for use in toxicity testing of chemicals as they are inexpensive to culture, have rapid growth rates, and usually provide reproducible results (Vaishnav & Korthals, 1990). Many bioassays have been developed to evaluate the toxicity and treatability of municipal and industrial effluents. Numerous single species tests have been recommended by several authors (Dutka et al., 1983; Beaubien et al. 1986). Such approaches are mainly based on the belief that, by selecting the most sensitive species and by using appropriate factors to allow for variability not included in the test, the highest levels of biological organization will be adequately protected. Single species tests are now quite well established, and when properly used, are easy to analyse and quantify. However, it has been pointed out (Levin, 1984) that the results obtained from single species tests cannot easily be applied to natural field conditions because the test organisms are extensively laboratory acclimated; also the test conditions provide for optimized growth and survival, a situation unlikely to be found in the field. Moreover, a fundamental problem with this approach is that it assumes that the ecosystem is a collection of single species exposed to toxicants under constant conditions (Cairns, 1982). 4. Multi-species toxicity tests, that is the use of mixed cultures or communities of micro-organisms for a testing protocol, are found to be generally much less sensitive than single species tests (Dutka & Kwan, 1984). It has been claimed that the use of multi-species tests give more realistic results than single species tests, and that single species adaptation and replacement is a common phenomenon in natural systems (Cairns, 1982). 5. Chemical persistence is the other major area of testing involving micro-organisms. Many biodegradation procedures have been developed (UK Standing Committee of Analysts, 1981), but surprisingly little interlaboratory validation of test methods has occurred. Those studies which have attempted validation of test methods have generally found good agreement using either non-biodegradable or readily degradable chemicals. Poor agreement has been reported when testing partially degradable or problematic chemicals, especially when non-acclimated inocula were used (Gerike & Fischer, 1979). Most biodegradation test methods have been criticized as the nonspecific analytical techniques employed require the use of relatively high substrate concentrations (Gledhill, 1987). Chemical degradation may be proportional to concentration, may not occur below certain threshold concentrations, may proceed via alternate metabolic pathways, or may occur more rapidly via alternate metabolic pathways. Studies by Alexander (1985) indicate the persistence of chemicals at realistic environmental concentrations may be different from that at higher concentrations. More realistic test systems which simulate actual ecosystems are needed for studies of microbial effects and persistence. Test systems should incorporate realistic chemical concentrations, natural microbial communities, and as much of the physical structure of the environment as practical. They should also be adapted to routine laboratory use. Most biodegradation studies look at the disappearance of parent material or the formation of metabolic products such as CO2. Because of the relative high variation and lack of reproducibility of results of die-away tests, and as CO2 may be a poor method of measuring toxicity because of the slow conversion of organic carbon to CO, both of these approaches have serious limitations when examination of typical environmental chemicals is desired. They also become too laborious, expensive or impractical to be useful in routine test procedures. 6. Respiration is a universally applicable parameter for assessing toxicity to aerobic bacteria. Inhibitory effects on respiration are rapid and can be measured with simple, inexpensive equipment. This method offers some advantages over others in that it requires little attention and also simulates quite closely the conditions found at a wastewater treatment plant. Among the disadvantages are the high concentration of test compound required. In the presence of a readily biodegradable substrate, bacteria respire rapidly, but when the substrate is exhausted the respiration rate falls rapidly to an endogenous level in which the cells are using stored substrate and other expendable cell constituents. If substrate is reintroduced after a long period of endogenous respiration, a lag period may be observed before active growth resumes. It has been found that bacteria are less susceptible to inhibitors when respiring in the endogenous phase, and the effects on growth and cell division cannot be ascertained on cells in this state. Thus in order to maximize sensitivity, the bacteria should be provided with a readily oxidizable substrate so that they are exogenously respiring, growing, and dividing when exposed to the potential inhibitor. Generally, a mixture of substrates similar to those on which the culture would grow naturally is suitable, for example synthetic sewage is commonly used as a growth medium for bacteria from sewage treatment plants. 7. Activated sludge is variable in populations of bacteria and varying results are often reported for inhibition of respiration studies. Within-plant variations can occur from day to day as a result of shifts in the bacterial population, probably caused by changes in the strength of components in the influent. Between plant variations can be greater. Sludge from different sources, and/or grown under different conditions, may also vary in response to inhibitors, because of varying degrees of reaction of some inhibitors with non-living sludge components (Kilroy & Gray, 1992b). Therefore, in practice, it has been usual to consider EC50 values from the inhibition of respiration of activated sludge tests in terms of order of magnitude. 8. Normal short-term tests, including inhibition of respiration of activated sludge, are deliberately carried out using unacclimated micro-organisms. This design is aimed to be as conservative as possible for the purpose of predicting effects in the environment, but may be unnecessarily stringent for the purposes of investigating the treatability of a chemical by activated sludge as no account is taken of the possible reduction in toxicity by acclimation of the micro-organisms to the chemical when it is continuously discharged to the wastewater treatment plant. Unlike chemical determinations where all the reagents can be clearly specified, the microbial inoculum or reagent used in biodegradability testing can rarely be fully characterized and is usually only vaguely specified (e.g. as activated sludge, sewage effluent). The BOD method is not labour intensive, but it does require a 5-day test period. While it does not allow a quick assessment of toxicity, it spans a number of generations of bacteria and is therefore more likely to identify effects on growth and cell division than short-term tests lasting for a few hours. If respiration is slow, the test can be extended (to 20 d) to allow for acclimation of the bacteria to the toxic chemical. Recovery of the respiration in the BOD test has been observed in the presence of some initially inhibitory chemicals by many workers (Busch, 1982; Trudgill et al., 1971). 9. Although useful, short-term tests are limiting as experimental conditions differ considerably from those in full-scale plants. The tests are batch processes in which the progress of a biochemical reaction is investigated over a relatively short time period, from 30 min in respiration inhibition studies to 5 d in BOD toxicity testing. The mixing characteristics therefore exhibit completely plug-flow dispersion. In contrast, the dispersion in full-scale continuous-flow plants approaches either completely mixed or dispersed plug-flow character. Substances described as non-toxic and degradable in short-term tests can exhibit other deleterious effects on full-scale operation. In short-term tests the biodegradation of a substance does not require that micro-organisms should be in any particular physical state, whereas the efficient operation of full-scale activated sludge plants requires that the flocculation of biomass occurs in order that separation of solids from the final effluent is achieved. Some substances can cause deflocculation without inhibiting respiratory activity. Other substances are responsible for the proliferation of filamentous micro-organisms resulting in sludge bulking. For these reasons simulation tests should be carried out in order to determine the treatability of chemical substances in activated sludge plants.

The Taxonomy of European Vascular Plants: A Review of The Past Half‐Century and the Influence of the Flora Europaea Project

Abstract: Summary Using the newly-completed second edition of the first volume of Flora Europaea as a starting-point, the author, a member (until 1993) of the Flora Europaea Editorial Committee, reviews the aims and scope of the whole project, and considers what influence it has had on European vascular plant taxonomy in the post-war period. The prospects of completing the second edition are discussed, and finally some comment is made on the present difficulties for taxonomists