Showing papers in "Biological Reviews in 1968"

The ecology of arctic and alpine plants

Abstract: Summary ‘How are plants adapted to the low temperatures and other stresses of arctic and alpine environments ?’ At present it is not possible to answer this question completely. Much work remains to be done, particularly on low-temperature metabolism, frost resistance, and the environmental cues and requirements for flowering, dormancy, regrowth, and germination. However, in brief, we can say that plants are adapted to these severe environments by employing combinations of the following general characteristics: 1. Life form: perennial herb, prostrate shrub, or lichen. Perennial herbs have greatest part of biomass underground. 2. Seed dormancy: generally controlled by environment; seeds can remain dormant for long periods of time at low temperatures since they require temperatures well above freezing for germination. 3. Seedling establishment: rare and very slow; it is often several years before a seedling is safely established. 4. Chlorophyll content: in both alpine and arctic ecosystems not greatly different on a land-area basis from that in temperate herbaceous communities. Within a single species there is more chlorophyll in leaves of arctic populations than in those of alpine populations. 5. Photosynthesis and respiration: (a) These are at high rates for only a few weeks when temperatures and light are favourable. (b) Optimum photosynthesis rates are at lower temperatures than for ordinary plants; rates are both genetically and environmentally controlled with phenotypic plasticity very marked. (c) Dark respiration is higher at all temperatures than for ordinary plants; rate is both genetically and environmentally controlled, with phenotypic plasticity very pronounced, i.e. low-temperature environment increases the rate at all temperatures. (d) Alpine plants have higher light-saturation values in photosynthesis than do arctic or lowland plants; light saturation closely tied to temperature. (e) There is some evidence that alpine plants can carry on photosynthesis at lower carbon dioxide concentrations than can other plants. (f) Annual productivity is low, but daily productivity during growing season can be as high as that of most temperate herbaceous vegetation. Productivity can be increased by temperature, nutrients, or water. 6. Drought resistance: most drought stress in winter in exposed sites is due to frozen soils and dry winds. It is met by decreased water potentials, higher concentrations of soluble carbohydrates, and closed stomates. Little drought resistance in snowbank plants. Alpine plants adapted to summer drought stress can carry on photosynthesis at low water potentials; alpine or arctic plants of moist sites cannot do this. 7. Breaking of dormancy: controlled by mean temperatures near or above 0° C., and in some cases by photoperiod also. 8. Growth: very rapid even at low positive temperatures. Respiration greatly exceeds photosynthesis in early re-growth of perennials. Internal photosynthesis may occur in hollow stems of larger plants during early growth. Nitrogen and phosphorus often limiting in cold soil. 9. Food storage: characteristic of all alpine and arctic plants except annuals. Carbohydrates mostly stored underground in herbaceous perennials. Lipids in old leaves and stems of prostrate evergreen shrubs. Depleted in early growth, and usually restored after flowering. 10. Winter survival: survival and frost resistance are excellent after hardening. Cold resistance closely tied to content of soluble carbohydrates, particularly raffinose. 11. Flowering: flower buds are pre-formed the year before. Complete development and anthesis dependent upon temperature of the flowering year and also, in some cases, upon photoperiod. 12. Pollination: mostly insect-pollinated in alpine regions and even in Arctic, but to a lesser extent. Wind-pollination increasingly more important with increasing latitude. Diptera more important than bees in the Arctic and in the highest mountains. 13. Seed production: opportunistic, and dependent upon temperature during flowering period and latter half of growing season. 14. Vegetative reproduction: by rhizomes, bulbils, or layering. More common and important in Arctic than in alpine areas. 15. Onset of dormancy: triggered by photoperiod, low temperatures, and drought. Dormant plant extremely resistant to low temperatures.

The fibroblast and wound repair

Abstract: Summary This review of connective tissue repair has attempted to place into historical perspective information obtained by newer approaches. The literature review is incomplete, as it was unfortunately necessary to leave many interesting studies out of the discussion. Emphasis has been placed upon what is known of the inflammatory response, the fine structure of the connective tissue cells in healing wounds and with correlated chemical findings in these tissues. An optimal inflammatory response appears to be an important, rapid, non-specific stimulus for fibroplasia. It is not clear how inflammation exerts this effect. The inflammatory cells and their enzymes markedly alter the extracellular matrix of injured tissue. The matrix of connective tissue may itself participate in the control of its own synthesis and degradation. It is possible that modification of this environment by injury and/or inflammation with ensuing matrix alteration may provide a stimulus for cell migration and protein synthesis. The converse may also be true, that is, a given level of matrix concentration may have an inhibitory effect upon the connective tissue cells. The inter-relationships between the connective tissue matrix and the cells, and the possibilities of feedback mechanisms playing a role in maintaining a balance between these two are important areas for future investigation. In this regard, additional questions may be asked concerning the role of the fibroblast in remodelling and degradation of connective tissue. It is not yet clear how important a balance between collagenolytic enzymes and the solubility states, or stability, of collagen are in each connective tissue. It will be interesting to determine which cells make collagenolytic and/or proteolytic enzymes upon appropriate stimulus. It is possible to distinguish between the fibroblast and the monocyte, or potential macrophage with the electron microscope. The rough endoplasmic reticulum with its large numbers of attached ribosomes is extensively developed in the fibroblast in contrast to the monocyte. The endoplasmic reticulum sequesters collagen precursors and other secretory proteins for transport either directly to the extracellular space, as appears to be the case for collagen, or to the Golgi complex as is the case for other exportable proteins. Collagen precursors are secreted into the environment and are not shed from within the cell surface. A number of cytoplasmic alterations have been described for fibroblasts and other cells during various pathological states. The significance of these alterations is not clear. It will be important to distinguish between specific and non-specific responses to injury, if these alterations are to aid us in understanding the various cellular responses. The source of the fibroblasts in granulation tissue appears to be mesenchymal cells from adjacent tissues rather than blood-borne precursors. Although contact inhibition can be demonstrated in vitro, it is not clear how important this phenomenon is in vivo, nor are the reasons for the ability of some tissues to heal by regeneration rather than by scar tissue formation understood. These and many other questions remain to be answered. The healing wound is multifaceted and presents the opportunity for systematic investigation into the problems of cell proliferation, cell and matrix interactions, and protein synthesis in vivo and it also can help to further our understanding of the ubiquitous fibroblast and its complex extracellular matrix.

The resistance of insect parasitoids to the defence reactions of their hosts

Abstract: Summary Some of the following propositions are to be read as suggestions or hypotheses, supported by circumstantial or direct evidence, but not yet rigorously demonstrated. An estimate of the significance to be attached to each should be gathered from the body of the paper rather than from the following brief statements. 1.The problem is posed: how do endophagous parasitoids counteract the haemocytic defence reactions of their usual hosts? 2.It has been demonstrated that the egg and young first-instar larva of Nemeritis canescens have a coating on their surface which enables them to escape the attention of the haemocytes of their usual host, and to develop without exciting a defence reaction. The coating is applied to the egg before it is laid, and to the cuticle of the larva before it hatches. A little evidence suggests that some other ichneumon wasps of the subfamily Ophioninae may use this mechanism of resistance. 3.Older first-instar larvae, and the second and later instars, of many parasitoids, both hymenopterous and dipterous, probably overcome the haemocytic reaction of their host by rapid feeding, which depletes its blood both of cells and of nutrients, and so drains its resources that haematopoiesis is prevented and encapsulation becomes impossible. 4.The common habit of parasitoids of lingering in the first instar, before ingesting much food, while the host goes on developing to another stage or undergoes diapause, may enable the larva to retain a protective coating that would have become ineffective if it had grown. When at length the larva does feed and grow, the preceding mechanism (3) comes into play. 5.The teratocytes and pseudogerms formed by many species in several families of Hymenoptera absorb nutrients on a large scale from the blood of the host. They act quickly, as soon as the larva hatches. I suggest that by their attrition of the host's reserves of food, and its consequent debility, they prevent an effective haemocytic reaction to the young parasitoid. 6.Some dipterous and hymenopterous parasitoids first inhabit the intestine of their host, and do not penetrate the body cavity until they are ready to overwhelm the defence reactions by rapid and gross feeding. 7.Parasitoids that live temporarily inside an organ of the host may there acquire a coating which protects them from reaction by the blood cells. 8.Species of parasitoids that occupy an organ of the host for a long period, and develop inside it, escape a defence reaction because they live within the connective tissue covering the organ, to which the blood cells do not react. 9.Eggs of hymenopterous parasitoids laid within the embryos of their hosts may be treated by the embryonic blood cells as a developing organ, and become covered with connective tissue as those organs are. Thereafter they would not be recognized as foreign bodies. 10.Parasitoid eggs laid in the eggs or the young larvae of their host may be coated with host substances, or covered by connective tissue (9), before the blood of the host be comes capable of vigorousdefence reactions. They would there after escape recognition as foreign bodies. This may be the advantage of the habit of the so-called egg-larval parasitoids. 11.Reasons have been given by Schneider (1950) for his belief that the serosa of the ichneumon wasp Diplazon fissorius secretes something that locally inhibits the defence reactions of its hosts. The trophamnion and pseudoserosa of some parasitoid eggs may have this function. 12. Some parasitoids, especially second- and third-instar larvae of Tachinidae, physically repulse the haemocytes of their host, moulding them into a capsule that serves the maggot as a respiratory sheath.

The cytoplasmic control of nuclear activity in animal development.

Abstract: Summary 1.This article reviews the occurrence, mechanism, and functional significance of the cytoplasmic regulation of nuclear activity during cell differentiation and especially during early animal development. 2.Nuclei from brain, and from other kinds of adult cell normally inactive in DNA synthesis, are rapidly induced to commence DNA synthesis by components or properties of intact egg cytoplasm. The components of egg cytoplasm which induce DNA synthesis are not species-specific and they are likely to include DNA polymerase. It is known that DNA polymerase exists in egg cytoplasm before it becomes associated with nuclei in which it is effective. The induction of DNA synthesis in brain nuclei by living egg cytoplasm is always preceded by a pronounced nuclear swelling, a dispersion of chromosomes or chromatin, and the entry of cytoplasmic protein into the nucleus. 3.RNA synthesis can be experimentally induced or repressed by living cytoplasm. The cytoplasm of unfertilized and fertilized eggs appears to contain components which can reversibly and independently repress the synthesis of ribosomal RNA, transfer RNA, and heterogeneous RNA. RNA synthesis can be induced by introducing nuclei inactive in this respect into the cytoplasm of cells very active in RNA synthesis. The induction and repression of RNA synthesis is preceded by a marked swelling of the nucleus and the dispersion of its chromosome material. 4.The cytoplasmic control of chromosome condensation before division has been demonstrated by introducing sperm or adult brain nuclei into the cytoplasm of oocytes undergoing meiotic maturation. 5.The evidence that regional differences in the composition of eggs and other cells are associated with changes in nuclear and gene activity is reviewed in Section 111. While it is certain that these regional differences are of great importance in cell differentiation, evidence that they have a direct effect on nuclear activity has been obtained in a few instances only. In some species it has been shown that the cytoplasmic components related to germ-cell differentiation include RNA and, frequently, granules. 6.It is concluded that whenever nuclei are introduced experimentally into the cytoplasm of another cell, they very quickly assume, in nearly every respect, the nuclear activity characteristic of the host cell. In many instances, altered function has been demonstrated in nuclei which subsequently support normal development. The induced nuclear changes are therefore regarded as normal and it is believed that they are achieved through the same mechanism as that by which the host cell nucleus originally came to function in its characteristic way. Examples are cited to show that changes in gene activity very frequently arise immediately after mitosis. The changes induced experimentally in transplanted nuclei resemble in very many respects those undergone by nuclei which are naturally reconstituted after mitosis, and it is argued that the two processes are functionally equivalent, It is suggested that during telophase of mitosis, chromosomes are reprogrammed in respect of potential gene activity by association with cytoplasmic proteins. Inter-phase nuclei seem not to show changes of gene activity except when they undergo a pronounced enlargement after entering a new cytoplasmic environment.

Molar occlusion in late triassic mammals

Abstract: Summary 1. A new genus and species of late Triassic mammal, Megazostrodon rudnerae, from Lesotho in southern Africa is described. The molars are similar to those of the British Eozostrodon parvus except that they are slightly larger and the upper molars have a large external cingulum supporting well-developed cusps. 2. Molar occlusion is discussed in two groups of late Triassic mammals: Eoxostrodon and the closely related Megazostrodon on one the hand and the unnamed primitive symmetrodonts on the other. It is shown that in Eoxostrodon the upper and lower molars did not have matching occlusal surfaces upon eruption but that wear produced matching occlusal surfaces. These surfaces are confined to the internal surface of the upper molars and the external surface of the lower molars and form a series of wide-angled triangles. The main cusp of an upper molar occluded between the main and posterior subsidiary cusp of the lower molar and the main cusp of the lower molar occluded between the main and anterior subsidiary cusp of the upper molar, 3. It is shown that the molars of Docodon and HaIdanodon were possibly derived from those of a primitive mammal such as Eozostrodon. The transition involved the development on the upper molars of an internal extension which, as it increased in size, established contact with the dorsal surfaces of two adjacent lower molars. The process involved is fundamentally different from that leading to tribosphenic molars. 4. In Megaxostrodon the main cusp of the upper molars occluded between the posterior and anterior subsidiary cusps of two adjacent lower molars, i.e. more posteriorly than in Eozostrodon. Primitive Rhaetic symmetrodonts were derived from mammals which had this type of occlusion and which were also closely related to Eoxostrodon and Megaxostrodon. The transition involved a rotation of the subsidiary cusps of the upper molars externally and those of the lower molars internally. This rotation increased the shearing surfaces between occluding upper and lower molars. Cusp rotation was carried further in the acute-angled symmetrodonts (Peralestes and Spalacotherium) and pantotheres. It appears that marked cusp rotation was coupled with the acquisition of transverse movements of the lower jaw during mastication. Transverse movement was apparently not possible in cynodonts, in Eoxostrodon (and related forms) and in Docodon. 5. The evolution of therian molars involves cusp rotation as originally proposed by the Cope—Osborn theory. Criticisms of the Cope—Osborn theory are re-evaluated in light of the new late Triassic material. 6. In Rhaetic symmetrodonts, molar wear produces matching occlusal facets, but the amount of attrition necessary to produce these facets was considerably less than in Eoxostrodon. In acute-angled symmetrodonts and in pantotheres, the molars erupt with more precise occlusal surfaces and attrition was not necessary to produce matching surfaces. 7. On the basis of the structure of the molar teeth it was concluded that Eozostrodon, Megazostrodon and Erythrotherium were closely related to the Rhaetic symmetrodonts. Slightly different occlusal relationships between upper and lower molars indicated that in these early mammals constant occlusal relations were being established. 8. Primitive cynodonts, such as Thrinaxodon, are characterized by alternate tooth replacement; there is a total lack of a constant occlusal relationship between upper and lower postcanine teeth. In Thrinaxodon individual postcanines were replaced several times. The crown structures of successive generations of postcanines were different so that a freshly erupted postcanine tooth had a crown structure quite distinct from the tooth which it replaced. It has been shown that the crown structure of one of the generations of postcanine teeth of Thrinaxodon is almost identical to that of Eozostrodon except that Thrinaxodon postcanines have a single root, On the basis of this similarity and the over-all structure of the primitive cynodont skull, it was concluded that Rhaetic mammals (excluding ictidosaurs and haramyids) could be derived from primitive cynodonts. 9. All the orders of Jurassic mammals (with the possible exception of multituber-culates) were probably derived from late Triassic mammals. The apparent close relationship of late Triassic mammals is evidence of a monophyletic origin of this class.

Calcium and plant growth

Abstract: Summary Calcium as a plant nutrient is characterized by its relatively high content in the plant coupled with a requirement not much higher than that of a micro nutrient element and an exceedingly uneven occurrence in soils. The difficulties in defining its actions are accentuated by a weak biochemical activity. In ecological conditions the secondary consequences of variations in calcium content may be more striking than the direct ones. Electron-microscopical studies have revealed that calcium is required for formation and maintenance of lamellary systems in cell organellae, a fact which might suffice to explain its indispensability for meristematic growth. Calcium is required for cell elongation in both shoots and roots; the common experience that it inhibits shoot elongation is certainly due to calcium additions far above actual requirement. It must be assumed for a rational interpretation of cell elongation that the fundamental mechanism is the same in shoots and roots. The one action which can be ascribed with certainty to calcium is a stabilizing of the cell wall with an increase in rigidity, an effect which, with over-optimal supply, may lead to growth inhibitions. The function is, however, necessary for the normal organization of cell walls. Calcium has, on the contrary, no significant effect on the synthesis of cell wall compounds but appears to act on their proper incorporation into the cell wall. The growth-active calcium may be bound not only to pectins but also to proteins and nucleoproteids in or in close contact with the cell wall. The supposition that calcium interacts directly with auxin in the cell wall has not been verified and does not seem very probable. There are reasons to believe that the points of action of calcium and auxin in the cell wall differ, auxin inducing growth by wall loosening and calcium establishing new wall parts. For submerged organs it may be necessary to consider an indirect effect of calcium on growth by its regulation of cytoplasmic permeability and thus affecting the exudation of growth-active compounds. The ecological problem is to characterize calcifuges (acid soil plants) from calcicoles (base soil or calcareous soil plants). Growth inhibitions on acid soils depend upon poisoning by A13+ and Mn2+. Opinions differ as to what extent this can be antagonized by calcium. Lime-induced chlorosis in calcifuges depends upon iron deficiency or iron inactivation in the plant. No acceptable explanation is given, but it might be related to an interaction of calcium carbonate, phosphorus, and iron. A hypothesis that it is linked to formation of organic acids is not tenable in the given form. Plants react to the calcium ions in the concentrations found in soils. Calcifuges have a low calcium-optimum for growth and show growth inhibition at high concentrations. Calcicoles have a high optimum for growth. Calcifuges are resistant to aluminium poisoning. Attempts made to explain the differences in calcium uptake and generally in salt uptake are tentative only, and relevant data are lacking.

Biochemical studies on the production of marine zooplankton.

Abstract: Summary 1 Phytoplanktonic algae vary in their value as food for zooplankton and no single algal food can meet the full nutritional needs of zooplanktonic animals Perhaps this is because optimal amounts of essential micronutrients are not all present in any one alga The dietary requirements of planktonic Crustacea, as far as they are known, bear some resemblance to those of vertebrates 2 The proteins of phytoplankton are similar in amino acid composition to those of zooplankton This circumstance should favour efficient synthesis of protein by the animal, for assuming amino acids are all released in the gut and absorbed at approximately the same rate, they will be presented to the tissues in roughly the right relative amounts for protein formation 3 Zooplankton are able to alter the characteristics of the fatty acids present in their diet by elongating the carbon chain length and by increasing the degree of unsaturation 4 Measurements of phosphorus and nitrogen excretion indicate that zooplankton are metabolically very active Some of the very high rates of phosphorus excretion are questioned and it is suggested that some portion of the phosphorus compounds liberated by zooplankton have passed straight through the gut without being assimilated It is unlikely that all forms of organically bound phosphorus are equally rapidly assimilated and turned over by zooplankton 5 Estimates of the rate of ammonia excretion by zooplankton differ markedly This may be a matter of size/surface area of the animals concerned—smaller animals excreting more rapidly than larger animals It has been claimed that a-amino nitrogen is released in considerable quantities by zooplankton but the evidence is not yet compelling 6 There is considerable disagreement on the efficiency of food assimilation and conversion by zooplankton One view is that irrespective of the quantity of phyto-plankton ingested assimilation is uniformly high The opposing view holds that when rapid ingestion of phytoplankton occurs the percentage assimilated falls More information on the feeding behaviour of zooplankton and on the physiology of their digestive processes is required before this controversy can be satisfactorily resolved

Glycoproteins in membranes.

Abstract: Summary 1. Although the classical models of biomembranes have emphasized the lipid and protein nature of these structures, a small quantity of carbohydrate is present as glycoprotein and glycolipid in animal cell membranes. In this article an attempt has been made to indicate that such carbohydrate materials should be considered in any complete model of animal cell membranes. 2. Various techniques that have demonstrated the presence of glycoproteins in animal cell membranes have been discussed here. In particular, cell electrophoresis, especially when the measurements are combined with specific enzyme treatments of the cells, has indicated that the net negative surface charge on intact viable cells is mainly due to sialic acid-containing glycoproteins and not as previously thought to ionizable phosphate groups associated with a complex phospholipid system. 3. Membrane-bound antigens are complex carbohydrate-containing macro-molecules whose antigenic activity is principally associated with the configuration at terminal positions on the carbohydrate chains. Enzymic degradation of the glycoproteins of the intact plasma membrane of cells, especially erythrocytes, causes profound changes in the immunological behaviour of the cell. 4. Histology and electron microscopy have indicated the presence of carbohydrate at the periphery of many mammalian cells. Some workers consider that such materials are present in a ‘cell coat’ covering the plasma membrane, rather than in the membrane itself. 5. Studies on the isolation and characterization of membrane glycoproteins have indicated that small oligosaccharide units linked to O-seryl or glutamyl residues in proteins are important structural units in plasma membranes. 6. The glycosylation of proteins takes place within the membranes of the endo-plasmic reticulum. Studies of glycoprotein biosynthesis suggest that the cell is able to synthesize a diversity of cell-surface structures from a relatively small number of monosaccharides. 7. Modification of the sialic acid-containing glycoproteins of the plasma membrane affects the transport of materials in and out of cells. Glycoproteins are present in membranes other than the plasma membrane, and are therefore considered to be integral components of membranes; hence the designation ‘cell coat’, whilst useful as a descriptive term, should not be taken to indicate that glycoproteins are constituents of a functional entity separate from that of the membrane. 8. Evidence that membrane glycoproteins may act as sites of interaction between cells is discussed. The involvement of glycoproteins in such a role would explain why the cell had developed a biosynthetic process capable of producing varied surface oligosaccharide structures from monosaccharides.

The mode of action of vitamin D.

Abstract: Summary 1. The purpose of this review article is to re-evaluate and integrate many of the observations related to the physiological effects of vitamin D, using as a working hypothesis the concept that the vitamin may be acting analogously to a steroid hormone in terms of its ability to interact with genetic information and ultimately elicit a physiological response. Prior to this time the problem of the mechanism of action of vitamin D has primarily been approached from the point of view that the vitamin was acting as a cofactor for some specific enzymic reaction. 2. The physiological activities of vitamin D are integrated with those of parathyroid hormone to provide a homeostatic control for the regulation of primarily calcium and secondarily phosphate metabolism. It is proposed that the role of vitamin D in this homeostatic control mechanism is older and more fundamental than that of parathyroid hormone. The interaction of vitamin D on skeletal calcium metabolism may have evolved before the effects of the vitamin on intestinal calcium absorption. 3. There are several physiological defects of calcium metabolism—rickets, osteo-malacia, vitamin D-resistant rickets and idiopathic hypercalcaemia—all of which may be a consequence of an aberration in one or another of the interlocking steps of the vitamin D-dependent and calcium-dependent homeostatic control mechanism. 4. The most thoroughly established action of vitamin D in vivo is to promote or facilitate the intestinal absorption of calcium. Although the exact biochemical details of this process are not available, this may involve vitamin D-mediated synthesis of the appropriate enzyme systems or the alteration of membrane structure necessary for calcium absorption. It is not yet unequivocally established whether calcium absorption is an energy-dependent active transport process or is a passive carrier-mediated or simple diffusion process. 5. The exact action of vitamin D on bone metabolism is not as well established, but the primary effect of the vitamin is likely to mediate bone resorption. The vitamin D-dependent activities of the cell in both the intestine and bone are to absorb calcium and transfer it to the blood. 6. No direct effects of vitamin D on intestinal absorption of phosphate have been found. Furthermore the validity of a vitamin D-mediated renal reabsorption of phosphate is questioned, for the major effects of vitamin D are cation oriented. If the renal effects of vitamin D are true, it is postulated that the mechanism of action of the vitamin here on the anion, phosphate, is fundamentally different from its cation oriented mechanism. 7. There is a lag in the action of vitamin D on the vitamin mediated: (a) transport of calcium both in vivo in rats and chicks, and in vitro with everted intestinal slices; (b) the apparent increased permeability of intestinal mucosa; (c) increased levels of citric acid in serum or bone; (d) the increased incorporation of radioactive inorganic phosphorus into intestinal mucosa phospholipids. As shown by the use of radioactive vitamin D, this lag is not due to a lack of the vitamin in the target organs. 8. Whereas large, unphysiological doses of radioactive vitamin D localize in all tissues and all subcellular fractions, small physiological doses of radioactive vitamin D localize predominantly in the nucleus of the intestinal mucosa. The amount of vitamin D localized in the nucleus would appear to be too low for the vitamin to function as a cofactor, and is more indicative of an interaction on or with deoxy-ribonucleic acid. 9. Actinomycin D, an inhibitor of DNA-directed RNA synthesis, inhibits the action of vitamin D in mediating intestinal calcium absorption and bone resorption. Vitamin D also stimulates messenger-RNA synthesis in intestinal mucosa within 1/2 hr. of vitamin treatment. Vitamin D may play a crucial role, along with parathyroid hormone and calcium, in a DNA, gene-dependent, homeostatic control mechanism for cal, ium metabolism. In this system the vitamin D molecule has certain very specific structural requirements which are probably a reflection of the specificity of its receptor molecule, rather than structural requirements for a cofactor-enzyme relationship.

Motility in procaryotic organisms: problems, points of view, and perspectives.

Abstract: Summary 1. Procaryotic motility mechanisms are more difficult to investigate than those of the generally larger, hence more easily observable, eucaryotic forms. Furthermore, although the function—namely translational locomotion—is the same, the biomechanisms by which this end is accomplished may be, in fact, quite distinct in the two forms. 2. Observational techniques for studying procaryotic motility are relatively crude and qualitative. Progress toward a greater understanding of motility phenomena will be made correlative with advances derived from devising specific techniques involving approaches adapted from electrical engineering, biophysics and cybernetics. 3. There is a great amount of information at hand concerning the qualitative and quantitative chemical composition of procaryotic flagella, but there is no assurance that preparatory techniques include either the entire organelle on the one hand, or do not introduce subtle errors on the other hand. Similarly, the structural features of flagella as derived from electron-microscope studies of fixed preparations may be themselves influenced by the techniques employed to reveal them. 4. Chemotactic responses of bacteria have been noted almost since the beginning of bacteriology as a formal scientific endeavour, yet the study of transduction of environmental stimuli, using motile bacteria as experimental subjects, is a relatively recent development. We have proposed that the cytoplasmic membrane may act as a non-specific receptor-transmitter of such signals in motile bacteria. If this is found to be the case, perhaps a sensory code may be more amenable to discovery here than with more complex forms of organisms. 5. Knowledge of the physical aspects in procaryotic flagellar movement is extremely fragmentary. There is some information on the movements of living functional flagellar fascicles, but this form of movement of an individual flagellum is purely speculative. We have proposed that the procaryotic flagellum is a rigid or semi-rigid helix, which does not transmit helical waves of contraction, and that its movements are governed by a specialized area of the cytoplasmic membrane. The flagellum may rotate or wobble within the flagellar basal bulb to produce the motion necessary for propulsion. This view ‘explains’ many of the known properties of procaryotic flagella. 6. The basis of gliding motility remains unknown even after a great deal of experimental work. In our view, the secretion of slime is necessary for adhesion to a solid surface, and movement is believed to be mediated by a mechanism involving contractile waves. 7. Studies on procaryotic motility may yield valuable information on certain areas of general biological interest. Among these are: (a) the transduction of environmental stimuli and the sensory code; (b) the development of reproducible observational techniques for quantitative data on the hydrodynamic and biophysical parameters of cell motion in procaryotic forms; (c) the phenomenon of unicellular ‘behaviour’ and the survival value and evolutionary significance of motility; and (d) the elucidation of the mechanism of gliding with perhaps an assessment of its utility in a wide variety of micro-organisms. All of these areas are ripe for imaginative and innovative experimentation; let us hope it will be forthcoming!

The urophysis and the caudal neurosecretory system of fishes

Abstract: Summary 1 The caudal neurosecretory system is defined in teleosts as a complex of secretory neurones (Dahlgren cells) in the caudal spinal cord leading by a tract to neurohaemal tissue organized as a typical neurosecretory storage-release organ: the urophysis 2 The teleost urophysis is generally a distinct, easily recognizable, lobate structure of variable external form Significant morphological variations lie in the organization of the neurosecretory fibres in relation to the vascular bed and in the degree of penetration of the meninx by the neurosecretory fibres to form an organ external to the spinal cord proper 3 The elasmobranch caudal system is composed of large cells with short axons projecting to a diffuse vascular bed; there is no organized urophysis 4 The caudal neurosecretory system and its urophysis appear late in post larval development by comparison with the hypothalamic neurosecretory system The Dahlgren cells originate from the ependyma in development and also during regeneration of the caudal system in adult life 5The elasmobranch system may represent the more primitive condition, and stages in the evolution of the advanced urophysial types can be visualized The particular histology shown by the caudal system appears to have taxonomic significance 6The cytology of the Dahlgren cell and its neurosecretory material is described The proteinaceous neurosecretory material has an affinity for acid stains but not for the Gomori stains or reagents demonstrating SH/SS groups The inclusions visible at the light-microscope level are aggregates of elementary neurosecretory granules, 800–2500A diameter, which originate from Golgi centres The possible participation of preterminal axonal regions–and tubular systems evident therein—in the formation of neurosecretory material is considered 7The structure of the axon terminals raises questions about the way in which neurohormone may be released into the blood Small vesicles have been variously interpreted as cholinergic synaptic vesicles and as products of the fragmentation of membranes of elementary neurosecretory granules Evidence for the release of ‘neuro-secretion centripetally’ into the cerebrospinal fluid also exists 8Functional analysis of the caudal neurosecretory system has proven most difficult, The bulk of earlier data and more recent information indicate a role in ionic regulation Increased sodium uptake by the gills of goldfish has been reported, as a result of administration of urophysial extract, and electrophysiological studies indicate a responsiveness of the system to variations in blood sodium ion concentration The urophysis also has a definite pressor effect in eels and will stimulate water retention in anurans The early claim of Enami that the system was involved in buoyancy regulation has never been substantiated It must be admitted that the function of this system, virtually ubiquitous in teleost and elasmobranch fishes at least, has been anything but established and still represents a major challenge to comparative physiologists

The Blochmann bodies: hereditary intracellular symbionts of insects.

Abstract: Summary The term ‘Blochmann body’ was originated by Wheeler in 1889 for bacteria-like particles in the cytoplasm of cockroach eggs. These particles can be traced during embryonic development to definitive somatic cells, the mycetocytes. These and similar particles of other insects have so far not been cultivated in bacteriological media nor injected into host animals to produce either pathological or benign infections. Their structure and composition indicate them to be evolutionary descendants of free-living micro-organisms; operationally, they appear to belong to the class of cell particles designated by Lederberg (1952) as plasmids. Genetic studies have shown the Blochmann bodies to be transmitted through the maternal line. Their presence in the egg cytoplasm at some stage in oocyte development is easily demonstrated, but studies by a number of workers have so far yielded a variety of conflicting claims or suggestions as to how the particles get into the germ cells or oocytes. The Blochmann particles of cockroaches, besides existing in the mycetocytes and eggs, occur embedded in a dense tangle of microvilli which are extensions of the plasma membrane of young oocytes. Essentially particle-free strains of cockroaches can be produced by feeding aureo-mycin or high levels of urea, or by withholding manganese. The effect is produced only by treating females, and is delayed one generation. In generations following the first (symbiont-free) generation, the Blochmann symbionts gradually reappear, suggesting that elimination was not absolute. Blochmann bodies in both the mycetocytes and the ovarioles of the cockroach carry out oxidative metabolism, as indicated by their ability to reduce tetrazolium. Glycolysis has not been demonstrated. The generalization that symbionts of the Blochmann type represent an adaptation to compensate for dietary deficiencies is inapplicable, since deficiencies have not been demonstrated for the diets of cockroaches, weevils, or homopterans—the major insect groups in which the symbionts occur. The symbionts of cockroaches and homopterans appear to be involved in the utilization of nitrogenous waste products in synthetic metabolism. In most instances the Blochmann bodies lack the central nucleoid body characteristic of the growing phase of free-living bacteria, thus resembling the Kappa and Mu particles of Paramecium and the endosymbiont of the protozoan Crithidia oncopelti. Both histochemical tests and electron-microscope studies indicate a DNA component that is widely dispersed within the particle. Blochmann bodies are without internal cristae. The cockroach symbionts contain muramic acid, a diagnostic feature of the cell wall of bacteria. Response to various nuclear and cytoplasmic reagents is intermediate between those of typical mitochondria and free-living bacteria. The envelopes of the Blochmann particles are generally thinner than those of free-living bacteria. The function of plasmids of the Blochmann type may be that they, like the bacteroids of the Rhixobium-legume symbiosis, extend the range of metabolic potential of the host cell by a process of mutual host-symbiont adjustment. Possible roles could be subsumed under the headings of bacteria-like, mitochondria-like, or nucleus-like functions.

Selection, prediction and response.

Abstract: Summary 1. The biometric approach to selection experiments has been outlined, and found to be rather deficient because it is based on excessively restrictive models which cannot take into account the complex architectures of quantitative traits as are being revealed today. 2. The nature of polygenes is discussed in detail from the theoretical point of view. In out breeding species, some form of the balanced polygenic complex is likely, showing polymorphism for the constituent genes. Although polymorphism is implicit in the argument, definitive evidence for poiymorphisms has only just appeared. 3. There is no evidence that polygenes differ from any other gene. 4. Several artificial selection experiments are described, in particular in Drosophila. By means of appropriate breeding techniques after obtaining responses to selection, genetic activity controlling quantitative traits can be located to chromosomes, and even specific loci found. Such few studies as have been carried out reveal, in general, the types of genetic architecture predicted on theoretical grounds. 5. Selection for behavioural traits is considered briefly and it appears that no new principles are needed, except that careful environmental control and objective measurement present problems. 6. The results of selection for quantitative traits in micro-organisms reveal similarities to results in higher organisms in the few cases where definitive work has been carried out. 7. Work on the simulation of models by computers has not greatly advanced selection experiment theory, mainly because, with few exceptions, linkage has been ignored. 8. The existing theory on which selection experiments are based is inadequate for several reasons. It cannot predict the rate of response to selection nor the ultimate limits to selection, the nature of correlated responses to selection, nor the nature of gene segregation underlying the observed variability. 9. Strains set up from single inseminated founder females from the same population of Drosophila have been shown to differ genetically for several quantitative traits. Therefore the base population is polymorphic for genes controlling these traits. This was exploited by carrying out directional selection on lines derived from those strains showing a high incidence of scutellar chaetae. This led to far more rapid responses to selection than lines derived from strains where the incidence of scutellar chaetae was lower. 10. Ultimately, one can envisage the selection experiment as it is known today being partly replaced by the manipulation of located genes controlling quantitative traits into certain combinations.

Membranous structures in yeasts.

Abstract: Summary 1. Most yeast cells carrying out active respiration have spherical or ellipsoidal mitochondria, with plate-like cristae. 2. Cytoplasmic petite strains of Saccharomyces cerevisiae have aberrant mitochondria, often containing whorled membranes. Mutants with deficiencies in the tricarboxylic acid cycle have mitochondria which appear normal when the cells are grown in low levels of glucose. 3. Cells of normal and petite S. cerevisiae grown strictly anaerobically show no recognizable mitochondrial profiles. 4. Carbon substrates which can only be respired promote the development of well-defined mitochondria. In certain facultatively anaerobic yeasts respiration is suppressed by glucose and the mitochondria under these conditions are large, pleomorphic and few in number. Other fermentable carbohydrates do not give this repression. 5. A number of antibacterial antibiotics, which inhibit mitochondrial protein synthesis, cause a disorganization of the mitochondrial cristae. 6. In yeast cells adapting from anaerobic to aerobic conditions mitochondria appear to develop from proliferations of the endoplasmic reticulum, which become progressively more organized. 7. Vacuoles often contain granular material, but in S. cerevisiae the vacuole, which has been described as a lysosome, frequently contains myelin-like lipid inclusions. The material in these inclusions is apparently derived from spherosomes. 8. Endoplasmic reticulum, orientated parallel to the plasmalemma, may be associated with fermentative ability in certain facultatively anaerobic yeasts. Endoplasmic reticulum is also actively involved in the budding process. 9. Normally the yeast-cell plasmalemma shows only minor convolutions, but in chloramphenicol-grown Rhodotorula glutinis the plasmalemma produces vesicular structures termed ‘paramural bodies’. 10. The yeast nuclear membrane has about 200 pores occupying 6–8 % of the total surface area. The nuclear membrane remains intact during mitotic division in yeasts until the daughter nuclei separate.

Quelques aspects moléculaires de la cytologie et de l'embryologie*

Abstract: Summary Our present knowledge of the cell structure, which is largely based on electron microscopy, is compared with what was known a few decades ago, when only light microscopy was available to the cytologist The importance of cytochemical methods for the detection and localization of macromolecules (nucleic acids, proteins) is stressed But it is pointed out that further analysis, with biochemical techniques, was required in order to understand the actual mechanisms of macromolecule synthesis in the cell (in particular, the relationships existing between nucleic acids and protein synthesis) The importance of genetical analysis in simple systems such as viruses and bacteria for the development of ‘molecular’ biology is then emphasized: in particular, the work of Avery identifying the ‘transforming principle’ with DNA, of Beadle leading to the ‘one gene, one enzyme’ theory, of the virologists who demonstrated that it is the nucleic acid component of viruses which carries the genetical information, have been of fundamental importance for the development of modern biology No less important has been the work of the X-ray crystallographers (Crick and Watson, Perutz, Kendrew, etc) who established the fine structure of nucleic acids and of proteins A brief review and a schematic representation of present ideas regarding the control exerted by DNA on the synthesis of specific proteins are then given: the main characteristics of the different kinds of RNA's, their interactions for the formation of polysomes, the role of the latter in protein synthesis, the main principles of the genetic codes, are briefly summarized But cells are, in many respects, more complicated than bacteria The concepts of molecular biology cannot be applied to cell differentiation without a recognition of the greater complexity of animal and plant cells They represent, however, a most useful and powerful guide for research in that area: for instance, many aspects of morphogenesis in the unicellular alga Acetabularia and in amphibian eggs can be explained on the assumption that messenger RNA's are produced by the nucleus and stored, in a stabilized form, in the cytoplasm during days or even weeks This stability of messenger RNA's in eggs and algae is at variance with their short life in bacteria The behaviour of non-nucleate fragments of Acetabularia is surprising in many respects: they are the site, not only of the synthesis of specific proteins, but even of RNA and DNA net synthesis Such a synthesis of macromolecules, in the absence of the nucleus is probably linked to the presence of the chloroplasts in this alga: they contain DNA, can synthesize RNA and proteins, and can even increase in number in the absence of the nucleus The presence of large amounts of DNA in the cytoplasm of many animal eggs raises a number of questions and might account for the extremely important role of the cytoplasm in the very early stages of embryonic development It is concluded that none of the great problems of cell biology will be solved without the help of the techniques and the theoretical ideas which have been so fruitful for the simpler systems used by the molecular biologists