Showing papers in "Biological Reviews in 1983"

The transport and function of silicon in plants.

Abstract: Summary A number of lines of evidence (Mr, number of -OH groups, measured fluxes at inner mitochondrial membranes) suggest the intrinsic PSi(OH)4 of about 10-10 m s-1 in the plant cell plasmalemma. While relatively low, such a PSi(OH)4 could maintain the intracellular concentration of Si(OH)4 equal to that in the medium for a phytoplankton cell of 5 μm radius growing with a generation time of 24 h. Such passive entry could not account for SiO, precipitation such as is required for scale (Chrysophyceae) or wall (Bacillariophyceae) production in terms of either the generation of a super-saturated solution or the quantity of SiO2 required; active transport occurs at the plasmalemma (and possibly at an internal membrane) of such cells. The energy required for silicification, even in a diatom with an Si/C ratio of 0.25, is only some 2% of the total energy (as NADPH and ATP) needed for growth; the energy cost of leakage of Si(OH)4 due to the intrinsic permeability of lipid bilayers to Si(OH)4 is never more than 10% of the cost of silicification. In vascular land plants the entry of Si from the soil into the xylem can involve a flux ratio (mol Si/m3 water) that is less than (e.g. Leguminoseae) equal to (e.g. many Gramineae) or greater than (e.g. Oryza, Equisetum) the concentration (mol m-3) in the bathing solution. Even the low influx of the Leguminoseae cannot be accounted for by the ‘lipid solution’ value of PS(OH)4, but requires entry coupled (phenomenologically) to water influx with a reflexion coefficient of about 0.9. The situation in most Gramineae is described by such a coupling with a reflexion coefficient near O, while the accumulation of Si (relative to water) in Oryza and Equisetum involves an apparent reflexion coefficient which is negative, i.e. an active transport system stoichiometrically related to water flux. Even in Leguminoseae with a transpiration-stream concentration of Si(OH)4 of only 20 mmol m-3 (cf. the soil solution at 200 mmol m-3), the fact that only I % of the water in the xylem is retained in the plant means that Si(OH)4 at transpirational termini approaches saturation; super-saturation, and precipitation of SiO, occurs in Gramineae and Equisetum. SiO2 precipitation occurs mainly near transpirational termini but can also occur in the xylem vessels and endodermis of roots, for example. Si(OH)2 mobility in the phloem seems to be very restricted. The energy costs of SiO2 relative to organic compounds as structural and defensive materials are in the ratio of 1:10-1:20 (on the basis of weight of material). The relative rarity of SiO2 as a structural material is discussed in the context of the evolution of Si(OH)4-transport mechanisms.

Larval ecology of marine benthic invertebrates: paleobiological implications

Abstract: Summary 1. Modes of larval development play important roles in the ecology, biogeography, and evolution of marine benthic organisms. Studies of the larval ecology of fossil organisms can contribute greatly to our understanding of such roles by allowing us to race effects on evolutionary time scales. 2. Modes of development can be inferred for well preserved molluscan fossils because the size of the initial larval shell (Protoconch I in gastropods, Prodissoconch I in bivalves) reflects egg size. Other morphological criteria are also available, and a comparative approach based on related taxa with known development may be the most reliable method. By combining larval and adult traits, it is possible to recognize modes of larval development in at least some fossil bryozoans, brachiopods, and echinoderms as well. (a) Planktotrophic larvae arise from small eggs, are released in enormous numbers with little parental investment per offspring, and suffer tremendous mortality during and shortly after a planktic existence. These larvae feed on the plankton during development, and are commonly capable of a prolonged free-swimming existence, and thus wide dispersal. (b) Nonplanktotrophic larvae (which include both planktic lecithotrophic forms and ‘direct developers’) generally arise from large eggs, with relatively few young produced per parent. Relative to planktotrophic larvae, nonplanktotrophic larvae generally receive greater parental investment per larva, and larval mortality is generally lower. These larvae rely on yolk for nutrition during development, and planktic durations are generally much briefer than for species with planktotrophic larvae, so that dispersal capability is considerably less. Energetic investment per egg is generally higher than in planktotrophs, but as there are lower fecundities as well it is difficult to generalize about the total energetic cost of one mode of reproduction against the other. 3. Owing to the high dispersal capability of planktotrophic larvae, it has been suggested that species with such larvae will be geographically widespread, geologically long-ranging, and exhibit low speciation and extinction rates. Species with nonplanktotrophic larvae will tend to be geographically more restricted, geologically short-ranging, and exhibit high speciation and extinction rates (again, as a consequence of their characteristically low larval dispersal capabilities). 4. Recognition of differential dispersal capabilities can play a role in paleobiogeo-graphic analyses. Concurrent study of the distribution of groups with contrasting modes of development will permit testing of hypotheses concerning timing, magnitudes and frequencies of migration and vicariance events. 5. Larval types are not randomly distributed in the oceans, but relationships with other aspects of the organisms' biology and habitats are very complex. Mode of development varies with: (a) Ecology. A simple r–––K model of adaptive strategies is clearly insufficient to explain the observed relationships: while many ‘equilibrium’ species have nonplanktotrophic larvae, and organisms living in less prdictable environments often have planktotrophic larvae, some of the most opportunistic marine species have nonplanktotrophic larvae. Nonetheless, planktotrophic development seems most suited for exploitation of patchy but widespread habitats. (b) Latitude. At shelf depths, planktotrophy is predominant in the tropics, and decreases sharply at high latitudes. (c) Depth. Incidence of planktotrophy decreases with depth across the continental shelf, at least in some taxa. Beyond the shelf, many deep-sea organisms are nonplanktotrophic (e.g. most bivalves, peracarid crustaceans), but planktotrophic development appears to be present in other groups (prosobranch gastropods, ophiuroids, and bivalves inhabiting transient habitats such as sunken wood and hydrothermal vents). These trends in developmental types will be accompanied by trends in evolutionary rates and patterns as outlined above. The study of larval ecology by paleobiologists will yield insights into the processes that gave rise to ancient evolutionary and biogeographic patterns, and will permit the development and testing of hypotheses on the origins of the patterns observed in modern seas.

On the definitions and functions of dominance and territoriality

Abstract: Summary 1. Dominance/subordinance is a relationship between two individuals in which one defers to the other in contest situations. Each such relationship represents an adaptive compromise for each individual in which the benefits and costs of giving in or not giving in are compared. Familiar associates in groups or neighbours on nearby territories may develop relatively stable dominant-subordinate relationships based on individual recognition. Although the aggressive aspects of dominance are usually emphasized, the less conspicuous actions of the subordinate individual are actually more important in maintaining a stable relationship. 2. In evolutionary terms, dominance essentially equals priority of access to resources in short supply. Usually the subordinate, who would probably lose in combat anyway, is better off to bide its time until better able to compete at another time or another place. Both individuals save time, energy, and the risk of injury by recognizing and abiding by an established dominant-subordinate relationship. 3. Dominance can be either absolute or predictably reversible in different locations or at different times. Of the various forms of dominance behaviour, rank hierarchies and territoriality represent the two extremes of absolute and relative dominance, respectively. A dominance hierarchy is the sum total of the adaptive compromises made between individuals in an aggregation or organized group. Many animals seem to be capable of both absolute and relative dominance, and within species-specific limits the balance may shift toward one or the other. High density, or a decrease in available resources, favours a shift from relative to absolute dominance. Some species may exhibit both simultaneously. Social mammals may have intra-group hierarchies and reciprocal territoriality between groups, while the males of lek species may exhibit ‘polarized territoriality’ by defending small individual territories, with the most dominant males holding the central territories where most of the mating takes place. 4. Territoriality is a form of space-related dominance. Most biologists agree that its most important function is to provide the territory holder with an assured supply of critical resources. Territoriality is selected for only when the individual's genetic fitness is increased because its increased access to resources outweighs the time, energy, and injury costs of territorial behaviour. 5. Territoriality was first defined narrowly as an area from which conspecifics are excluded by overt defence or advertisement. The definition has been variously expanded to include all more or less exclusive areas without regard to possible defence, and finally to include all areas in which the owner is dominant. I define territory as a fixed portion of an individual's or group's range in which it has priority of access to one or more critical resources over others who have priority elsewhere or at another time. This priority of access must be achieved through social interaction. 6. My definition excludes dominance over individual space and moving resources, and includes areas of exclusive use maintained by mutual avoidance. It differs from most other definitions in its explicit recognition of time as a territorial parameter and its rejection of exclusivity and overt defence as necessary components of territorial behaviour. There is an indivisible continuum of degrees of trespass onto territories, and functionally it is priority of access to resources that is important rather than exclusive occupancy. 7. There is a similarly indivisible continuum in the intensity of behaviour needed to achieve priority of access to resources. Deciding whether or not an exclusive area is defended leads to the pointless exercise of trying to decide which cues indicating the owner's presence are conspicuous enough to merit being called defence. Concentrating on overt defence emphasizes the aggressive aspects of territorial behaviour rather than the equally or more important submissive aspects such as passive avoidance.

Mating system and sexcul dimorphism in large terrestrial, mammalian herbivores

Abstract: Summary Sexual dimorphism in mammals is not entirely satisfactorily explained by the models that are advanced to account for it among birds. This may be because species-specific styles of being dimorphic, and of attaining mature dimorphic state, are not clearly recognized. Mature dimorphism is a syndrome involving body size, appearance and weaponry; each facet and the whole syndrome may have functions in both fighting and signalling. The mature dimorphic stage has to be reached by growth and change from juvenile and sub-adult states. The occurrence of the separate facets of the dimorphic syndrome are reviewed in species of Bovidae, Cervidae and Macropodidae, large, diverse families of eutherian (the first two) and metatherian mammals, which have broadly similar ecological adaptations. In each family the smallest species tend to be homomorphic, with small, inconspicuous weapons. Greatest dimorphism in size is found in medium-sized bovids and cervids, and the larger macropodids (in which no species exceeds 100 kg male weight); the range of species showing greatest dimorphism in size also shows the most exaggerated weapons. Mature dimorphism is reached by different patterns of growth, which may be determinate and similar in the sexes (leading to homomorphism), determinate but differing between the sexes, or indeterminate and differing, both of which lead to heteromorphism. The syndromes of dimorphism and patterns of growth are associated and a classification of styles of dimorphism is presented. The adaptiveness of the styles is suggested in terms of what is known of the socio-ecology, in particular the male reproductive strategies, of the species. The various styles of heteromorphy appear to be associated with males' way of achieving polygyny: such as by non-resource-based territoriality, by dominance-determined access to oestrous females, or by wandering and formation of a consortship with pro-oestrous females. The relevance of the species' ecology of use of resources to these styles of dimorphism and mate-acquisition is briefly discussed.

The natural control of populations of butterflies and moths

Abstract: Summary 1. Life-table data for 14 species of Lepidoptera are analysed by the k-factor technique of Varley & Gradwell (1960). Two factors are shown to be of particular importance in determining fluctuations in abundance from one generation to the next. These key factors are predators and the failure of females to lay their full complement of eggs. 2. Data from 24 studies are reviewed to identify any density-dependent factors that would be capable of regulating the populations about an equilibrium density. In eight studies no density-dependent relationships could be identified, and in a further 13 the only density dependence demonstrated was due to intraspecific competition for resources. It is argued that competition is incapable of regulating populations at low density. In the other three studies, natural enemies are thought to be acting in a density-dependent manner, but their ability to regulate the populations is questioned. 3. The frequency of over-population and of extinction is reviewed and both appear to occur frequently in Lepidoptera. This, coupled with the failure of most studies to demonstrate the existence of density-dependent processes capable of regulating populations, leads the author to reject the model of regulation about an equilibrium density in favour of a model of population limitation by a ceiling set by resources. 4. Fluctuations in resource availability may be important in determining variations in the abundance of many Lepidoptera, but at present too few ecologists have quantified the carrying capacity of habitats occupied by the species they study to generalize about this.

Protein synthesis and degradation during aging and senescence

Abstract: Summary 1. The published results on protein synthesis during aging are contradictory. Possible sources of error and variability include: an insufficient number of different animal ages used; use of whole organs that are cytologically highly heterogeneous; different animal strains; neglecting to measure the specific activity of the precursor pool for protein synthesis; and inadequate methodology for measurement of in vivo rates of protein synthesis. 2. In general, protein synthesis rates in mammals have been reported to decline 4–70% with age. In insects and other organisms, greater losses (60–90%) have been observed. 3. Limited evidence indicates that in some systems a decline in the rate of protein synthesis may be due to alterations (as yet of unknown nature) in the initiation components of the protein synthetic apparatus. Futhermore, some studies suggest that in some organisms aging affects the expression of specific parts of the genome. 4. The significance of results on protein metabolism obtained from some studies with nematodes is at present unknown, owing to problems associated with age-synchronization methods. Also, the in vitro fibroblast system for the study of human cellular aging has not been met with universal acceptance; it is generally believed that this system has not yet been established as a valid analogy to mammalian aging in vivo. 5. Failure to detect defective enzymes in many old organisms indicates at least that not all proteins are altered during aging. The complete thermal stability of purified enzymes from old organisms suggests that the observed thermolability of the same enzymes in crude cell extracts is not an intrinsic property of those enzymes. Post-translational modifications (partial denaturation) may constitute the primary mechanism for the production of altered cell polypeptides during aging. 6. The available evidence does not support the concept of an age-dependent decline in translational accuracy. The future purification to absolute homogeneity of an altered enzyme and its ‘young’ (unaltered) counterpart, and their sequencing, should resolve the question of translational errors. 7. Some degree of age-related ribosome loss appears to occur in fixed postmitotic cells. In general, the published polyribosomal profiles may represent artefacts due to insufficiently suppressed ribonuclease activity during extraction. 8. The published studies on protein degradation during aging are also contradictory. Some investigators have neglected the possibility of reutilization of labelled amino acid. It is possible that some of the observed age-related alterations in protein degradation rates are due to altered endocrine status of the animals used, rather than to defects in the protein degradative pathways. The studies utilizing cell culture systems are also contradictory, probably due to different experimental designs. 9. Limited evidence suggests that protein degradation may slow down with age in mammals and nematodes. An inefficient protein degradation system in old organisms could provide an explanation for the accumulation of altered macromolecules in some organisms. Virtually nothing is known about regulatory mechanisms of protein degradation during senescence. 10. There is a need to examine which proteins are synthesized and degraded at selectively different rates as a function of age and what their physiological role is. This approach would be more informative than the study of total protein turnover with age. 11. The physiological significance, and the causes of the observed declines in protein synthesis and degradation rates during aging and senescence, remain to be established.

Adhesion in byssally attached bivalves.

Abstract: Summary The byssus is a structure produced by marine bivalve molluscs to adhere, usually permanently, to substrata under water. As the adhesion of synthetic polymers to surfaces is predictably compromised by the presence of water, particularly bulk water, it is of particular interest to discover the mechanism of byssal adhesion. In most species, the byssus consists of at least four essential components: acid mucopolysaccharides, adhesive protein, fibrous proteins, and an oxidative enzyme, polyphenoloxidase. The function of the mucopolysaccharide component is still uncertain, but it can conceivably be used by the animal as a temporary adhesive, a surface modifying agent, and/or a stabilizing filler for the permanent adhesive. The adhesive protein known as the polyphenolic protein in Mytilus is but a thin plaque applied to the substrate surface by the foot of the animal. The molecular and physical properties of this adhesive protein conform remarkably well to what one expects of an ideal synthetic polymer, i.e. high molecular weight, abundance of large and polar side chains, near-zero surface contact angle, and total water-insolubility after setting. The fibrous proteins constitute the major portion of the thread or ribbon-like material connecting the animal to the adhesive plaque on the substrate surface. These proteins are packed in ordered crystalline arrays, e.g. β-pleated sheet and collagen helix (in mytilids) as is to be expected from structural tensile elements of Nature. The enzyme polyphenoloxidase is presumed to induce intermolecular cross-linking of proteins in the fibrous and adhesive portions of the byssus. In Mytilus the natural substrates of the enzymc may be the dopa-containing polyphenolic protein and accessory gland protein.

Functional aspects of drosophila courtship

Abstract: Summary 1. The elements that make up the courtship behaviour of males and of females are briefly described. It is pointed out that some of the terms used, such as female ‘repelling’ behaviour, are misleading as they do not reflect the known functions of the behaviours. 2. Evidence has been presented for a number of distinct pheromones with different functions during courtship. These claims are critically examined as the evidence is incomplete and at times conflicting. It seems unlikely that any pheromones other than those acting over a very short distance are involved in courtship. There is sound evidence for an aphrodisiac pheromone produced by all females which stimulates male courtship. A pheromone, which may be the same one, is produced by males less than 12 h old, which also stimulates male courtship. No function is ascribed to this pheromone. Fertilized females either produce less aphrodisiac pheromone or they may, in addition, produce one that inhibits male courtship. Mature males may also produce an inhibitory pheromone. Females produce a contact pheromone which is species-specific and involved in sexual isolation. It is not at present clear whether this is different from the aphrodisiac pheromone. 3. There is considerable variability in the importance of vision in courtship. Many species will mate satisfactorily in the dark, suggesting that visual stimuli are not critical. Most species use vision to orient towards one another and for males to track and follow females. Even in light-independent species such as D. melanogaster, specific visual signals may be used in courtship although they are not obligatory. Thus the red eye of the male is a sexual signal for females. Conversely, some light-dependent species do not appear to make use of visual signals as a major factor in courtship. Some, however, do perform behaviours that are clearly visual and which may act to emphasize markings on wings, head or body. 4. The majority of Drosophila species perform courtship songs by vibrating one or both wings. The songs produced by males sexually stimulate the females. They are species specific and there is considerable indirect and some direct evidence that the songs are involved in sexual isolation. Males of many species produce two different songs during courtship and it is probable that one is concerned mainly with sexual stimulation and the other with species recognition. Females of certain species of Drosophila and Zaprionus also sing during courtship and these songs may aid species recognition by males. In addition males and unreceptive females perform ‘aggressive’ songs. 5. Almost all studies of Drosophila courtship have been made in very confined conditions in the laboratory. Interpretation of some of the results obtained in this way may require modification in the light of ecological research and observation of courtships under more natural conditions.

The Norfolk Broadland : Experiments in the restoration of a complex wetland

Abstract: Summary 1. The Norfolk Broadland comprises wide river valleys, floored with deep deposits of peat and clay. Over forty mediaeval peat pits (the Broads) became flooded after the fourteenth century and were mostly connected with the rivers by navigation channels. Between about 1400 A.D. and 1800 A.D. the valleys supported a diverse wetland ecosystem, partly maintained by deliberate cropping of wetland plants. Some of the wetland was gravity-drained, but extensive aquatic habitats held diverse fens and submerged vegetation dominated by short-growing aquatic plants in very clear water. 2. The Enclosure Acts passed around 1800 encouraged more intensive agriculture in the catchments and this, coupled with pumped drainage, particularly of the wetlands in the lower valleys, caused them to sink and so they were embanked against the rivers. The drained land, intersected by channels (dykes), was grazed. 3. In the late nineteenth century, migration of human populations to the towns led to pressures for improved sewage disposal; consequently, increasing amounts of raw sewage, and later, treated effluent, both major sources of phosphorus, were discharged to the rivers and Broads, which were thus progressively eutrophicated. Agricultural changes led to an increased nitrogen supply. A tourist boating industry also began then. 4. Increasing eutrophication caused replacement of low-growing submerged aquatic plants together with ranker species, and then, particularly after the Second World War, submerged plants were lost from much of the waterway and replaced by phytoplankton-dominated communities. The mechanism by which this change took place concerns growth of epiphytic and filamentous algal communities and probably interactions with phytoplankton-grazing animals associated with the plants. The loss of submerged plants is linked with decreased invertebrate diversity, and changes in fish and water bird populations. Eutrophication is also associated with fish-kills caused by a toxic flagellate, Prymnesium parvum, oubreaks of avian botulism and decreased fen diversity through flooding with enriched water. Increased boat activity has led to severe bank erosion. Erosion is exacerbated by loss of bank protection, because of loss of submerged plants and of fringing reedswamp through damage by coypu, an exotic rodent. 5. Lack of labour-intensive management of the undrained fens has led to a decrease in their diversity, through natural succession to alder swamp. 6. Restoration of an aquatic-plant-dominated waterway is desirable and has been attempted through various means of reducing phosphorus input. Complete isolation of Broads or dykes has proved generally useful, but complications with release of phosphate from sediments, though such release eventually declines, are delaying success of an attempt to reduce phosphorus levels by precipitation of phosphorus from sewage effluent. Such a technique is the only one widely practicable in Broadland. 7. Simple nutrient reduction may not always be sufficient to cause a return from phytoplankton-dominated, turbid water to clear water with aquatic plants. It may be necessary to reinstate a higher intensity of zooplankton grazing than at present possible because of predation of grazers by fish in a habitat lacking refuges for the zooplankton from such predation. 8. The drained grazing marshes contain, in their dykes, a rich aquatic plant and invertebrate community, and provide nesting sites and grazing for birds. Changed political and economic factors may lead to future diminution of these communities through intensified drainage for arable cultivation.

Some aspects of the physiology and biochemistry of marine fungi

Abstract: Summary 1. This review deals with certain aspects of the physiology and biochemistry of marine fungi. Though it is not easy to define what is meant by a marine fungus, the information presented here relates to those species that by consensus can be accepted as being truly marine. The material is in two parts, that relating to the higher fungi (Ascomycotina, Basidiomycotina and Deuteromycotina) and that relating to the lower fungi, particularly those zoosporic fungi that require sodium for growth. 2. Higher marine fungi appear to have a similar carbon, nitrogen and vitamin nutrition to their terrestrial counterparts. 3. Growth of higher marine fungi can be optimal in 100% sea water, but more frequently optimal growth is at a lower percentage. The percentage giving optimal growth may be determined by the rate of ion uptake required to generate the necessary turgor for growth. The tolerance of the vegetative phase of many terrestrial fungi to salinity appears little different from that of marine fungi. In general, members of the Basidiomycotina are particularly sensitive to salinity, while those of the Ascomycotina and Deuteromycotina are much more tolerant. 4. The degree of tolerance to salinity may be dependent upon temperature and whether or not adaptation by the fungus has occurred. 5. Maintenance of a suitable internal potassium concentration by a higher marine fungus is important for growth in a saline medium. Calcium appears to be necessary for the retention of potassium and organic solutes within the hyphae. 6. It appears that higher marine fungi are able to maintain a ratio of potassium: sodium higher than that of sea water by the presence of a plasma membrane ATPase which may have a higher pH optimum than the equivalent enzyme from terrestrial counterparts. 7. Higher fungi produce more glycerol as the salinity of the external medium is increased. Whether or not the compound is involved in osmoregulation has yet to be determined. 8. Turgor is thought to be generated in higher marine fungi growing in sea water by organic solutes (predominantly glycerol and arabitol) and by ions, with the latter playing the major role. Though interpretation of the data depends on several assumptions, the high concentrations of sodium that seem likely to be present in hyphae or cells have implications for the activity of enzymes, if they have similar properties to those of higher plants. There is a need for information on the effects of high concentrations of ions on enzymes located in the cytoplasm of higher fungi. 9. In spite of some experimental uncertainties, it seems that reproduction and spore germination of higher marine fungi are very much less affected by salinity than are the same processes of terrestrial counterparts. 10. The zoosporic marine fungi require sodium for growth. Though the ion is required at high concentration for growth, sodium cannot be replaced by potassium. Evidence indicates that sodium is involved in the transport of solutes across the plasma membrane. 11. The carbon and nitrogen requirements for the growth of the zoosporic marine fungi demand further investigation, particularly at the biochemical level. There is evidence that the respiration of these fungi possesses many interesting features. 12. Vitamin requirements of the zoosporic marine fungi depend on the isolate under investigation. Vitamin B., does not now seem to be an obligatory requirement. The ability of phospholipids and sterols to stimulate growth requires further investigation. 13. Further studies on marine fungi in the laboratory should focus particularly on growth in continuous culture. Physiological and biochemical studies would be helped by more precise guidance from those concerned with the ecology of these fungi.

Bacterial Blight of Cotton

Abstract: Summary 1. Bacterial blight of cotton is potentially one of the most damaging diseases of cotton. 2. Its symptoms and adverse effect on yield are described. 3. Control measures include: (i) cultural techniques of crop sanitation, and the destruction of crop residues, as well as close seasons and crop rotation; (ii) chemical seed treatments and foliar sprays; (iii) the breeding of resistant varieties. 4. Techniques are described for the breeding of resistant varieties, and sources of useful genetic plant resistance identified. 5. The complex nature of the interactions between host genetic resistance, variation in the pathogen and environment is emphasized. 6. Successful resistance-breeding programmes in Africa and the USA are reviewed, and attention is drawn to the pressing need for resistant varieties in India and Pakistan. 7. Continued success in the control of bacterial blight will require integrated programmes that include cultural methods, the use of chemicals (particularly as seed treatments) and the cultivation of resistant varieties with durable resistance.

Insecticides: their route of entry, mechanism of transport and mode of action.

Abstract: Summary (1) The assumption that the circulatory system of the insect is instrumental in transporting insecticides to their site of action appeared not to be based on good evidence. On the contrary, experiments specifically designed to test the hypothesis provided ample evidence to refute the idea. Salient points to disprove the haemolymph route of entry can be summed up as follows: (a) A topically applied dose of insecticide does not readily penetrate the insect and the minor fraction that does so is largely retained in the body wall. The small amount that actually passes into the blood is too small to cause symptoms of toxicity if injected into the haemocoel. (b) The amount of insecticide present in the haemolymph (and CNS) does not appear to have any bearing on toxicity - internally introduced insecticides are, in fact, basically very much less toxic than those externally applied. Where injected doses appear to be more toxic than equal amounts topically applied, this is due to the boosting effect of organic solvent carriers. Tests with parabiotically joined insects provide support for the view that haemolymph-borne insecticide is of no consequence. (c) The topographical proximity of the locus of external application with the site of action (the thoracic ganglia) seems to be important irrespective of the general direction of the blood flow, and this should not be so if the circulatory system was instrumental in the transport of insecticide. (d) The introduction into the haemocoel of material such as olive oil, which is an excellent absorbent for insecticides, does not affect the toxicity (speed of action) of externally applied compounds to a significant extent. It should have a pronounced effect if haemolymph-borne insecticide were an essential element in the process of poisoning. (e) As judged by speed of action, blocking of the blood circulation does not hamper the insecticide's movement to the site of action. (2) Only two other feasible routes remain. (a) The insecticide might reach the CNS via peripheral nerves and nerve cord, but the results of histochemical assays of cholinesterase inhibition in the insect's CNS make the idea improbable for organo-phosphates. The nerve route is also incompatible with the observation that a wax barrier blocked the movement in and over the body wall so as to delay the onset of symptoms of toxicity, as such a barrier would not hinder movement into lateral nerves near the locus of application. (6) The only other feasible alternative, i.e. entry by means of lateral transport via the integument of the body wall and tracheae, is supported by autoradiographic and other evidence which showed the insecticide to accumulate in the tracheal system. It is further supported by the fact that inter-tracheal introduction is faster acting than topical treatment, indicating that the tracheal system offers a very effective pathway to the internal organs. (3) Regarding the mechanism of entry, earlier reviews and text books maintain this to be associated exclusively with penetration into and through the integument by a physicochemical process. However, there is good evidence to show that an active process (requiring metabolic activity as the driving force) plays an essential part in lateral movement in the integument - the epidermis, being the only living tissue continuous throughout the general integument, must perform this function. (4) As to the mode of action, the new hypothesis expounded here implies a single mode based on the fact that insecticides cause the extrusion of fluid from the epidermis into the cuticle and beyond, fluid lost from the epidermal cell layer being replaced from haemolymph and internal tissues. The precise mechanism is not clear but could conceivably involve an as yet hypothetical local endocrine system designed to keep the water content in the integument within certain limits. It is suggested that water extrusion affects the integument's permeability to respiratory gases, resulting in a rate of respiration not commensurate with metabolic need, and that the insecticide's arrival in the trachea (tracheoles) of the CNS leads to excitation and knockdown. Death is thought most likely to be due to dehydration of the CNS and subsequent histological degeneration.

Cytochrome c oxidase: two models.

Abstract: Cytochrome c oxidase is an enzyme present in the inner membrane of the mitochondrion where it functions as the terminal carrier in the electron transferring sequence known as the respiratory chain. Cytochrome c oxidase oxidizes ferrocytochrome c and is itself oxidized by dioxygen. Following the addition of four protons, the products of the reduction of one molecule of dioxygen are two molecules of water. Cytochrome c oxidase (EC I . 9 . 3 . I ) is the name given to the complete enzyme complex. The two-enzyme hypothesis proposed by Keilin & Hartree (1939) supports the concept that oxidase consists of cytochrome a and cytochrome a3. Cytochrome a3 is considered to be the dioxygenand carbon monoxide-reactive part of the molecule, and Cu, its associated copper. Cu, should, therefore, be associated with cytochrome a. Recently, Saraste, Penttila & Wikstrom (1981) have reported that the minimum molecular weight (per haem or per copper) of cytochrome c oxidase is about 50000 daltons. From this result it appears that the monomer of the enzyme, i.e. that protein unit which contains one each of six or seven protein subunits and has a size of I 10000

Epigenetic mechanisms in the early amphibian embryo: cell differentiation and morphogenetic elements.

Abstract: I . Introduction . . . . . . . . 11. Cell differentiation . . . . . . . (a) Cell classification . . . . . . (6) Patterns of differentiation . . . . . ( I ) Spontaneous differentiation . . . (2) Induced differentiation . . . . (3) Proteoglycans and differentiation . . (4) Twice-transformed cells . . . . (5) Fate maps . . . . . . . (c) Mechanisms of differentiation . . . . ( I ) Filocyte activation . . . . . (2) Colligocyte induction. . . . .

The fusion of sexual nuclei.

Abstract: Summary A classification scheme is proposed for the types of sexual nuclear fusion that occur in eukaryotes. The two main classes are envelope fusion and envelope vesiculation and each is further divided into subclasses. The formation of sexual nuclei (pronuclei) has been detailed in representatives from various phyla, but is best understood in animals, in which the development of male and female pronuclei differs in some respects. The only characterized cytoplasmic mediator of pronuclear movement are microtubules. Groups of eukaryotes can be classified according to the type of nuclear fusion they reveal. Envelope fusion occurs in animals whose eggs are fertilized at the pronuclear stage, and in all plants, fungi, protozoa and algae studied to date. Ultrastructural details of envelope fusion have shown variations that are classified in our scheme as direct and indirect, the latter being restricted to the plant kingdom. Envelope vesiculation only occurs in animals, in which it is the most common means of nuclear fusion. Four subclasses can be defined according to the timing of the vesiculation of the nuclear envelopes, and the extent of envelope surface projections prior to fusion. The amount of work reported on the controlling mechanisms of nuclear fusion has been limited, but some evidence of genetic control has been provided, particularly in fungi. Evidence is presented to indicate that the control of the fusion competence of nuclei is a negative one. This review of the information available on nuclear fusion points out the need for extensive future comparative studies if this important process is to be better understood.