Showing papers in "Biological Reviews in 1976"

Biometric analysis of geographic variation and racial affinities

Abstract: Summary 1. The study of geographic variation and the racial affinities between populations is of central importance to systematics and evolutionary theory. When using phenotypic variation to measure the similarity between the populations of a species one should analyse the variation in several characters simultaneously. This is a statistical procedure and is known as multivariate analysis. Multivariate analysis of phenotypic variation, unlike some other methods, has the advantage of not being dependent on living specimens. 2. To obtain an adequate sample at each locality, and an adequate distribution of localities within a given geographic area, can be a major problem. The pooling of data from adjacent localities is discussed. 3. There are several sources of phenotypic variation within a species, e.g. sexual and ontogenetic variation. Failure to eliminate the non-geographic sources of variation can confuse the assessment of the similarity between populations. 4. Correlation between characters can reflect similar genetic control and/or similar patterns of geographic variation, the biological interpretation being influenced by whether the data come from one locality or many. 5. The influences of environmental induction and genetic control cannot easily be separated. Also, some characters may not be entirely homologous throughout the range of the species. 6. Most studies rely on far too few characters of a too restricted type to give an ‘overall’ assessment of the phenotypic similarity. This is one of the most neglected aspects of the study of geographic variation. 7. The various forms of clinal and categorical variation, the precise nature and position of sharp transition (hybrid) zones, the relationship between non-adjacent as well as adjacent populations and the phenotypic divergence between island populations, etc., all come under the heading of geographic variation. The ideal technique should be able to elucidate all types of geographic variation but some techniques can only be used effectively with a few of them. Moreover, techniques may be limited in their application because they require the data to conform to certain models, e.g. normal distribution. 8. The degree of phenotypic similarity between populations can be measured by a wide range of similarity coefficients. Comparison between even a small series of populations produces a large set (or matrix) of similarity coefficients that is difficult to interpret. However, the relationships between populations can be summarized in several ways and these may be loosely grouped into four categories; (i) network diagrams, (ii) contours and isometric plots, (iii) hierarchical clusters, and (iv) ordination methods. These methods are explained and their advantages and limitations discussed. 9. The hierarchical (dendritic) model of cluster analysis is unsuitable for analysing all but a few types of geographic variation. 10. There are several types of ordination technique. They all aim to summarize the variation of many characters in a reduced number of axes. One can either emphasize the biological interpretation of each separate axis, or treat the analysis as a classifying technique and assess the grouping of the populations in the space defined by the axes. Considerable care is needed in interpreting the results of both of these approaches. If correctly applied, ordination techniques generally can be used to analyse all the forms of geographical variation and are therefore recommended. Contrary to current practice they can be used with a large number of characters. The advantages and limitations of the various ordination techniques are discussed. 11. Contours and their three-dimensional isometric plots can be used to portray geographic variations in the information obtained from a multivariate analysis. However, contours and isometric plots are limited in their applicability and the amount of information they can convey. 12. The sophistication of some multivariate methods should not be allowed to cloak the scientific inadequacies of a study. The use of more than one technique and variety in the choice of pertinent parameters may be of value in indicating the reliability of the results.

The effects of sodium chloride on higher plants

Abstract: Summary (1) This review concentrates on the effect of sodium chloride on the growth of higher plants, being primarily concerned with relatively high concentrations i.e. 50 mmol 1-1 and above, though something is also said about those instances when sodium acts as a micronutrient. Emphasis is placed on particular species or genera for which enough information is available to discuss possible mechanisms. (2) Trace amounts of sodium are required for the growth of plants using the C4 pathway of carbon fixation and may also be important in plants with Crassulacean acid metabolism. (3) The increased growth of Beta vulgaris brought about by sodium chloride can in part be explained by a sparing effect on potassium. However, growth is still increased when sufficient potassium is available. Complementary studies with rubidium indicate that the hormone balance in the plant may be changed. Sodium chloride also increases the level of sucrose in storage roots and allows beet plants to withstand water stress more readily, possibly by increased turgor pressure. (4) Sodium chloride increases production of dry matter in C4 species of Atriplex under conditions of low relative humidity because water loss is reduced and photo-synthesis hardly affected. (5) Succulence in many plants is stimulated by salinity. The essential basis of the phenomenon is an increased water potential gradient between the leaf and the external medium. In some instances, it is the accumulation of chloride which is important; in others it is the accumulation of cations, when potassium can be as effective as sodium. (6) Salinity reduces the final area achieved by growing leaves. Most of the studies have been made on Phaseolus vulgaris and an important early event is the reduction in the rate of expansion of the epidermal cells and this may be accompanied by a decrease in their number. Reduction of epidermal cell size is a result of water stress; sodium chloride may directly affect cell division, though water stress cannot be ruled out. Whether salinity brings about inhibition of cell division depends upon the calcium content of the medium – a high content is accompanied solely by a reduction in epidermal cell size. (7) Hormones, as yet unspecified, may play an important part in response of a growing leaf to salinity. However, there is no evidence that sodium chloride per se has an effect on hormone balance within the plant. So far, any measured changes in levels of specific hormones can be ascribed to the osmotic effects of the saline medium. (8) Two estimates by flux analysis of cytoplasmic concentration of sodium in plants growing in conditions of high salinity give a value of around 150 mmol 1-1. There is no similar information for chloride. Other techniques (histochemistry and X-ray micro-probe analysis) give questionable information. (9) There is now extensive information to show that enzymes of halophytes (other than ATPases) do not differ significantly from those of other higher plants with respect to their sensitivity in vitro to sodium chloride. There is a need for further work with respect to the activity of enzymes in the presence of those metabolites which have the highest cytoplasmic concentration. (10) Sodium-stimulated ATPases have been isolated from plant cells but their distribution amongst higher plants is restricted. (11) There are a number of reports of changed metabolism brought about by saline treatments but it is not clear how far the effects of sodium chloride and water stress are confounded. (12) Sodium appears to increase the sucrose levels in sugar beet by an inhibitory effect on product starch-granule-bound ADP-glucose starch synthase. (13) Reversal of a sodium pump located at the plasmalemma might have an effect on cell turgor. (14) Sodium (like other monovalent cations) causes loss of materials from plant cells, possibly through an effect on carrier proteins; calcium prevents this from happening. Calcium also allows plants to grow better in saline conditions by a depression of sodium uptake by and transport within the plant. The properties and composition of the membranes of mesophytes and halophytes need to be compared. (15) A saline medium exerts a major effect on plant growth through water stress to which a halophyte must adapt. As well as this, the cytoplasmic concentration of sodium chloride must be kept lower than the total cellular concentration of the salt. Unless this happens, it is likely that enzymic activity will be reduced due, in some instances, to an unspecific effect of a high concentration of monovalent cations and/or chloride and in other instances to competition between sodium and other cations, specifically potassium, for activation sites on enzymes, e.g. pyruvate kinase. (16) Further work is required to separate the osmotic effects from the specific effect of sodium chloride after it has entered the plant. As well as this, it has become clear that more information is needed about the mineral nutrition of halophytes.

The pathology of marine algae

Abstract: I11 . Non-infectious diseases . . . . . . . . . . . . . (I) Diseases caused by deleterious changes of the natural environment (2) Diseases caused by man’s activities: pollution damage . . . . . . . . . . (a) Oil and dispersants . . . . . . . . . . . . (b) Pesticides and related compounds (c) Heavy metals and radio-isotopes (d) Sewage and industrial effluents (e) Thermal pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Progress in the use of chromosomal translocations for the control of insect pests

Abstract: CONTENTS

A study of the mechanism of contraction in vertebrate smooth muscle.

Abstract: CONTENTS A . Ultrastructure I . Introduction . . . . . . . . . . . . . . I1 . The organelles and filamentous proteins of the intact muscle . . . . . I11 . Thin filaments . . . . . . . . . . . . . . IV . Thick filaments . . . . . . . . . . . . . . (a) Introduction . . . . . . . . . . . . . . (b) Early observations . . . . . . . . . . . . . (c) X-ray studies . . . . . . . . . . . . . (d) Ribbons . . . . . . . . . . . . . . . (e) Filaments . . . . . . . . . . . . . . (f) T h e effect of stretch and hypertonicity on filament shape (g) The effect of temperature on the appearance of thick filaments . . . . . . . . . (h) X-ray observations on muscles equilibrated at 37 \"C . . . . . . (i) The influence of change in ionic content on cell volume and the structure of thick filaments . . . . . . . . . . . . . . ( j ) Thick filaments in homogenates and extracts . . . . . . . . V . Intermediate filaments . . . . . . . . . . . . . VI . Dense patches and dense bodies . . . . . . . . . . . . . . . . . . . . . . VII . The mechanism of contraction

Salt tolerance in crustacea and the influence of temperature upon it

Abstract: I11 . The influence of temperature on salt tolerance . . . . I . Introduction . . . . . . . . . . 2 . Classification of beneficial effects of temperature on salt tolerance 3 . Discussion . . . . . . . . . . . (a) Classification . . . . . . . . . (I) General remarks . . . . . . . . (2) Evaluation of the temperature effects . . . . (b) Other ecological implications . . . . . . (I) Colonization of dilute media in the tropics . . . . (2) Migration . . . . . . . . . IV . General conclusion . . . . . . . . . .

The neuronal control of locomotion in the earthworm

Abstract: CONTENTS

Biosynthesis of small molecules in chloroplasts of higher plants

Abstract: Summary 1. Chloroplasts of higher plants contain enzymes which permit them to synthesize many kinds of small molecules in addition to carbohydrates. 2. Either aqueous or non-aqueous techniques may be used to isolate chloroplasts. Aqueous methods permit the isolation of chloroplasts showing high rates of photosynthesis; the organelles can be purified by means of density gradients. Non-aqueously isolated chloroplasts cannot photosynthesize, but show good retention of low-molecular-weight substances and soluble enzymes. 3. Whole cells photoassimilating 14CO2 show considerable formation of 14C-labelled amino acids and lipids, but isolated chloroplasts exhibit very poor synthesis of amino acids and lipids from 14CO2. 4. Chloroplasts play an important role in reducing nitrate to ammonia. There is controversy about the presence in chloroplasts of nitrate reductase and about the mechanism of the light-dependent reduction of nitrate to nitrite; however, it is generally agreed that non-cyclic electron transport directly supports reduction of nitrite to ammonia via a chloroplastic nitrite reductase. 5. Chloroplasts actively assimilate inorganic nitrogen into amino acids. The assimilation reaction is either the reductive amination of α-ketoglutarate to glutamate or the ATP-dependent conversion of glutamate to glutamine. The enzyme glutamate synthase has recently been found to be present in chloroplasts and may play an important function in nitrogen assimilation. 6. Numerous transaminases (aminotransferases) are present in chloroplasts. 7. The source of α-keto-acid precursors of chloroplastic amino acids is unknown. It remains to be established whether chloroplasts import the required keto acids or whether some of them might be generated via an incomplete tricarboxylic-acid cycle located in the chloroplast. 8. Chloroplasts contain characteristically high levels of mono and digalactosyl diglycerides, sulpholipid and phosphatidyl glycerol. They also have large amounts of polyunsaturated fatty acids. 9. Fatty acids are synthesized by the concerted action of fatty-acid synthetase, elongases and desaturases. Two pathways have been implicated for the formation of α-linolenic acid. 10. The galactosyldiglycerides are synthesized by successive galactosylation of diglyceride. The enzymes responsible are probably located in the chloroplastic envelope. 11. The other major chloroplastic acyl lipids (sulpholipid, phosphatidylglycerol and phosphatidylcholine) have not been, as yet, synthesized de novo by means of isolated chloroplast fractions. However, indirect evidence indicates that the first two are probably formed there. 12. Chlorophyllide synthesis involves the formation of δ-aminolaevulinic acid (δALA) followed by conversion of δALA to protoporphyrin IX, which is then transformed into protochlorophyll. 13. Recent evidence favours the view that δALA synthesis is not mediated by δALA synthetase but by another pathway in which δALA can be derived from α-ketoglutarate or glutamate. It has not been established whether this pathway is localized in plastids. 14. Conversion of δALA to protoporphyrin IX is mediated by soluble enzymes of the plastid stroma. Membrane-bound enzymes mediate the conversion of protoporphyrin to protochlorophyll. 15. Carotenoids are synthesized from acetyl CoA via geranylgeranyl-pyrophosphate and phytoene intermediates. Evidence has been obtained for both neurosporene and lycopene as precursors of the cyclic carotenoids. 16. The overall pathway of carotenoid formation is subject to photoregulation, particularly during the development of the chloroplast. 17. Carotenes are precursors of xanthophylls, the inserted oxygen being derived from molecular oxygen. 18. Chloroplasts may synthesize or interconvert gibberellin hormones.

The forms, uses and significance of genetic variation in endocrine systems.

Abstract: CONTENTS

Developmental biology and the study of malformations

Abstract: Experimental work on abnormal conditions of incubation in the chick has been undertaken to acquire a scientific approach to malformations. More precise experiments on causing abnormalities had a common origin with experimental embryology. Progress in experimental teratology during the last 50 years is reviewed in a commentary on the 4 principles formulated by Stockard in 1921. The results of cytogenetical studies in man and in other organisms have led to the tracing of some relationships between them. Present knowledge concerning malformations of the neural tube originating either experimentally spontaneously or phenotypically has been presented and the teratological implications of some recent theories on the expression of the genotype are discussed in particular reference to problems of hormones as teratogens the implication of carbohydrate metabolism and teratogenesis. It is speculated that teratogenesis is possibly related to cationic balance in early development and that 1 factor retarding progress in the understanding of malformations is the tendency toward the development of teratology in an adequately close relationship with other branches of cell biology.

The functional development of mammalian mitochondria.

Abstract: (ii) Resistance to chloramphenicol . . . . . 193 (iii) Pet 494-1 mutant ofyeast . . . . . . 194 (b) Cell hybridization . . . . . . . . 194 (2) Immunology andsurface probes . . . . . . 195 (3) Lipid substitution . . . . . . . . . 196 (4) Selective inhibitors . . . . . . . . . 196 (a) Theory and precautions . . . . . . . . 196 (a) DNA and RNA synthesis * 197 (c) Protein synthesis . . . . . . . . . 197 VII . Differentiation of adrenal cortical cells . . . . . . 198 . 198 (a) R6le of the mitochondrion . . . . . . . zoo (3) Mitochondrial turnover . . . . . . . . 201 VIII . Mitochondria and cancer . . . . . . . . . 201 (3) Protein synthesis . . . . . . . . . . 187 187 IV . Co-operation between mitochondrial and extra-mitochondria1 systems