Showing papers in "Biological Reviews in 1988"

The effect of added nitrogen on the rate of decomposition of organic matter

Abstract: Summary (1) N added to decomposing organic matter often has no effect or a negative effect on microbial activity, at least in the long term. More than 60 papers are cited in support of this statement. (2) The negative effect of N is mainly found with recalcitrant organic matter with a high C/N ratio (straw, wood, etc.), whereas a positive effect of N is common for easily degradable organic material with low C/N ratio. (3) The negative effect of N could be explained by: (i) N disturbs the outcome of competition between potent and less potent decomposers; (ii) through ‘ammonia metabolite repression’, N blocks production of certain enzymes, at least in basidiomycetes, and enhances breakdown of the most available cellulose, whereby recalcitrant lignocellulose accumulates; (iii) amino compounds condense with polyphenols and other decomposition products, forming ‘browning precursors’ which are toxic or inhibitory. (4) The effect of adding N may depend on the microflora present. (5) There are indications that some microorganisms have a ‘luxury uptake’ of N when it is present in sufficient amounts, thereby delaying N mineralization. (6) The addition of N seems to increase the formation of water-soluble, brown, recalcitrant compounds, but to decrease the amount of humus formed.

Mating frequency and fecundity in insects.

Abstract: Summary 1. The paper summarizes the published evidence on the relation between mating frequency and fecundity in insects. There is experimental evidence of varying quality for 63 species and non-experimental evidence for about 60. 2. Repeated mating may be universally necessary for full fecundity and fertility in female insects (in species in which the females normally mate more than once). 3. The evidence is remarkably poor. We need more properly designed experiments (and not just observations of natural variation), with sufficient sample sizes and statistics, and measurements of the fecundities and fertilities of singly and multiply mated females, when the multiple matings are separated by many days or weeks. Most of the existing experiments of this sort are defective in some way. 4. In species with greater total fecundity and longevity, multiple mating may be more likely to enhance fertility than in species with small fecundity and short life span. 5. Females in naturally monandrous species do not show increased fecundity or fertility with repeated mating, whereas females of polyandrous species do. 6. There is no obvious connexion between paternal investment, in so far as we know about it, and the increase of fecundity by repeated mating. 7. There is a small tendency for females to breed more quickly and be shorter lived if they mate repeatedly.

On the means whereby mammals achieve increased functional durability of their dentitions, with special reference to limiting factors

Abstract: ( I ) Types of solutions available . . . . . . . . . (2) Increased wear resistance of dental tissues . . . . . . (3) Increased tooth size . . . . . . . . . . . (4) Additional teeth, and a discussion of bilophodonty . . , . , (5) Increased tooth height . . . . . . . . . . (6) Combinations of methods . . . . . . . . . . IV. Conclusion . . . . . . . . . . . . . V. Acknowledgements. . . . . . . . . . . . VI. References . . . . . . . . . . . . . (2) Functional demands, with a note on dental wear

Cold tolerance of microarthropods

Abstract: Summary 1. Microarthropods (Acari and Collembola) are dominant components of the terrestrial fauna in the Antarctic. Their cold tolerance, which forms the mainspring of their adaptational strategy, is reviewed against a background of their structure and function, and by comparison with other arthropods. 2. Two species, the isotomid collembolan Cryptopygus antarcticus Willem and the oribatid mite Alaskozetes antarcticus (Michael), are examined in detail, and afford a comparative approach to the mechanisms underlying cold tolerance in insect and arachnid types. 3. All microarthropods appear to be freezing-susceptible (unable to tolerate tissue ice), and they utilize varying levels of supercooling to avoid freezing. Gut contents are considered to be the prime nucleation site in most arthropods when supercooled, particularly for Antarctic species. Moulting also increases individual supercooling ability especially in Collembola, and the activity of ice-nucleating bacteria in cold-hardy arthropods may be important. 4. Sources of ice nucleators are many and varied, originating externally (motes) or internally (ice-nucleating agents). They act either extracellularly (mainly in the haemolymph) to promote freezing in ice-tolerant life stages, or intracellularly in freezing-susceptible forms. Thermal hysteresis proteins, acting colligatively, occur in many arthropods including Collembola; they depress both the freezing point of body fluids and the whole-body supercooling point of freezing- susceptible and freezing-tolerant species. 5. Bimodal supercooling point distributions are a feature of microarthropods and water droplets. Samples of field populations of Antarctic mites and springtails show significant seasonal changes in these distributions, which in some respects are analogous to purely physical systems of water droplets. Supercooling points are confirmed as accurate measures of cold-hardiness and survival for Antarctic species, but not necessarily for other arthropods. The effects of constant sub-zero temperatures approaching the limit of the supercooling ability of arthropods require study. 6. Desiccation and dehydration influence microarthropod physiology in several ways; in Alaskozetes it triggers glycerol synthesis. Glycerol may aid binding of water in severely dehydrated insects, but the relationship of such ‘bound’ water to cold-hardiness is unclear. 7. Sugar alcohols (polyols) and sugars are accumulated as potential cryoprotectants in many arthropods at low temperatures, and antifreeze systems may be single or multi-component in structure. Cryoprotectant synthesis and regulation have been studied principally in insects, and fresh weight concentrations of 0–3-5 M of polyols have been found. Trehalose accumulation may also influence cold-hardiness. 8. Microarthropods fall within the spectrum of cold tolerance observed for arthropods and other invertebrates. No special adaptations are found in Antarctic species, and similar strategies and mechanisms are present in both insects and arachnids. The colonization and maintenance of microarthropod populations of polar land habitats seem not to have required the evolution of any novel features with respect to cold tolerance.

The functional organization of filtration nephridia

Abstract: Summary (1) Based on the classical studies of Goodrich, protonephridia are believed to be phylogenetic antecedents of metanephridia. It is argued here that the primary factor determining the type of nephridium expressed is body size rather than phylogenetic status. (2) The proposed model defines a nephridium functionally and predicts two general configurations for filtration nephridia in animals. (3) Application of the model to metanephridial and protonephridial systems indicates differences in the sites of ultrafiltration and mechanisms of pressure generation. (4) Metanephridial systems function by muscle-mediated filtration of vascular fluid into a coelomic space before modification by an excretory duct. (5) Protonephridial systems function by cilia-mediated filtration of extracellular fluid into the lumen of a protonephridial terminal cell before modification in an adjoining duct. (6) The model predicts a correlation between animals with blood vessels and metanephridia, and animals without blood vessels and protonephridia. The correlation is shown to be nearly perfect. (7) Exceptions to the model are discussed. (8) Original experimental evidence is given for the permeability of the protonephridial terminal cell to iron dextran and its reabsorption by the protonephridial duct in the polychaete, Glycera dibranchiata. (9) Experimental data for proto- and metanephridial systems are summarized and shown to support the proposed model. (10) The ultrastructure of the exceptional amphioxus ‘protonephridium’ is reviewed and original data are presented. Its organization is structurally and perhaps functionally intermediate between proto- and metanephridial systems. (11) An original ultrastructural comparison is made of monociliated nitration cells in a size range of larval invertebrates from five phyla. Filtration cells that are structurally intermediate between protonephridial solenocytes and metanephridial podocytes are noted in larvae intermediate in body size between the two extremes. The comparative data suggest that (i) podocytes and solenocytes are homologous cells and (ii) that body size is correlated with which of the two designs is expressed. (12) The fates of larval podocytes are followed through metamorphosis in three species. The results confirm the equivalence of podocytes and solenocytes as suggested by the comparative analysis. They further indicate that which morph is expressed is a function of body design factors discussed in the model. (13) Protonephridia are believed to be primitive to metanephridia because they occur in presumably primitive animals and in ontogenetic stages of many animals with metanephridia as adults. It is suggested here that the distribution of protonephridia is related to small body size and the lack of blood vessels, regardless of phylogenetic status. The occurrence of protonephridia in the larvae of species with metanephridia as adults is explained similarly as a function of the small larval size and lack of blood vessels.

On the reconstruction of pterosaurs and their manner of flight, with notes on vortex wakes

Abstract: Summary Classical pterosaur reconstructions are variants on a ‘bat-analogy’, whereby the wing is conceived as a simple membrane with no inherent bending strength, stretched between the arm and leg skeletons. The legs are considered to be splayed out to the sides, as in bats, so that the animal would have to adopt a quadrupedal stance on the ground, supported on its feet and the metacarpo-phalangeal joints. In recent years an alternative ‘bird-analogy’ has come to be generally accepted. This hypothesis, most elements of which are due to Padian (1983 a, b) calls for the animal to stand upright on its legs like a bird. The wings are independent of the legs, as in birds, are stiffened by skeletal fibres in the membrane, and have a very narrow, sharply pointed shape. There are difficulties in reconciling the bird-analogy with the evidence. The long-tailed rhamphorhynchs might conceivably have balanced their weight about their hip joints but this would not have been possible for the short-tailed pterodactyls. The bird pelvis shows modifications which permit bipedal standing in spite of the reduction of the tail, but no equivalent adaptations are seen in pterodactyls. Besides, all known pterosaur pelvises, except that of the giant pterodactyl Pteranodon were open ventrally, which would have precluded the legs from being brought to a parasagittal position, as required for bipedal walking. The notion that the wing was not attached to the legs is based on negative evidence, in that no clear impressions of the inner end of the wing membrane are preserved in the fossils. However one pterodactyl fossil shows a membrane edge approaching the ankle joint. In fossils that are preserved with the wings forward, the legs have been pulled forwards by the ankles. A tendon connecting the ankle to the wing tip is consistent with the evidence. The ‘fibres’ in the wing membranes are actually impressions of surface ridges, with no internal structure, and are better interpreted as surface wrinkles in the skin, caused by contraction of elastic fibres within the membrane. The bird analogy also results in a very unsatisfactory wing from an aerodynamic point of view. The structure of an animal wing is best understood in terms of the type of vortex wake it is adapted to generate. Hummingbirds, and insects capable of economical hovering, have wings that can be inverted on the upstroke, and when hovering, generate a wake consisting of two vortex rings per wingbeat cycle. The span of such wings is fixed, which implies that they create a ‘ladder wake’ in cruising flight, consisting of a pair of undulating wing-tip vortices, joined by a transverse vortex at each transition from downstroke to upstroke and back. Normal birds cannot invert their wings, and so are less efficient in hovering, but they can shorten the wing during the upstroke in cruising flight. This creates a ‘concertina wake’, with no transverse vortices. Hummingbirds show very limited migration performance, compared with normal birds, with the implication that a wing capable of creating a concertina wake is more economical in cruising flight than one creating a ladder wake, and is an essential adaptation for long-distance migration. A revised reconstruction of the pterosaur wing starts from the observations that, contrary to the currently popular bird-analogy, pterosaurs were not bipedal, their wings did not contain stiffening fibres but did contain elastic fibres, and the trailing edge of the membrane was supported by a tendon joining the tip of the wing finger to the ankle. A hypothetical arrangement of elastic fibres, that accounts well for the observed pattern of wrinkles in contracted wings, also allows the planform shape of the wing to be adjusted in much the same way as seen in birds, although using a completely different mechanism. It opens the possibility that pterosaurs could fly with a concertina wake, and thus could have been long-distance migrators like modern birds. Although this hypothetical wing is mechanically somewhat bat-like, it is not a return to the classical bat-analogy. It would not have the high degree of control over profile shape, which gives bats their outstanding manoeuvrability. On the other hand bats do not have the degree of control over their wingspan that is suggested here for pterosaurs, and consequently are not notable for migration performance.

Molecular biology of the extracellular matrix proteins.

Abstract: Summary 1. Extracellular matrices are organized networks of diverse macromolecules, secreted and deposited in the vicinity of cells. They not only play structural roles but are also involved in dynamic processes such as cell migration and differentiation, embryo development, wound healing and cancerous transformation. They are composed, mainly, of collagens, adhesive glycoproteins and proteoglycans, which interact with each other and with cell-surface receptors through specific binding sites. 2. Collagens are a multigenic family whose proteins have triple-helical domains which contain repeats of the Gly-X-Y sequence. They aggregate to form fibrils, networks or filamentous structures. Gene organization reveals that fibril-forming collagens might have originated from an ancestral 54 bp exon encoding 6 units of the Gly-X-Y triplet. Non-fibrillar collagens, on the contrary, have evolved through different pathways which are not closely related to this mechanism. 3. Fibronectins are dimers made up of three types of internal repeats: I, II and III. The first two are encoded by one exon each and have homologous counterparts in other proteins. Most of the type three repeats are encoded by two exons each. Cell-specific alternative splicing in three different regions of the primary transcript generates, in humans, up to 20 polypeptide variants and explains structural differences between cellular and plasma fibronectins. Fibronectin interacts with its cell receptors through the sequence Arg-Gly-Asp. 4. Laminin is a cross-shaped molecule, characteristics of basement membranes, formed by three distinct polypeptides. Primary structure of one of its subunits reveals a repetitive organization with regions homologous to other proteins like myosin and epidermal growth factor. Laminin has a cell-binding site, different from the Arg-Gly-Asp tripeptide, which is constituted by the sequence Tyr-Ile-Gly-Ser-Arg. 5. Von Willebrand factor is a high-molecular-weight glycoprotein stored in specialized structures of platelets and endothelial cells. It participates in haemostatic mechanisms favouring the formation of the platelet plug. This protein has a particularly long propeptide and four types of internal homologies. It binds to two different platelet surface receptors, one of which interacts with an Arg-Gly-Asp sequence present in the von Willebrand polypeptide. 6. Thrombospondin is an adhesive glycoprotein formed by three identical subunits which show striking homologies with Plasmodium proteins, epidermal growth factor and procollagen I. It also contains multiple calcium-binding sites similar to those of calmodulin. An Arg-Gly-Asp sequence is also present, but its surface receptor has not yet been identified. 7. Vitronectin is a glycoprotein, presumably involved in the process of blood coagulation, which is related to the extracellular matrix through binding to various of its components. It also binds to cell surfaces via an Arg-Gly-Asp sequence which is disrupted by a proteolytic cleavage that, concomitantly, originates somatomedin B, a peptide of unknown function. 8. Proteoglycans are formed by glycosaminoglycan chains covalently bound to core proteins. They show a wide tissue distribution and structural variations. Most or all core proteins could possibly be synthesized as pre-propolypeptides, and contain Ser-Gly or Thr-Gly repeats, which represent attachment sites for the glycosaminoglycans. 9. A superfamily of cell-surface receptors that recognizes RGD-containing proteins is described. These receptors are intrinsic membrane proteins with large extracellular domains and an α/β heterodimeric structure. They are grouped in four families, each of them characterized by dimers which share a common β subunit and different α chains. Other receptors for extracellular matrix proteins that do not fit in the RGD superfamily are also reported.

Osmoregulation in crocodilians

Abstract: Summary 1. The osmoregulatory strategies of crocodilians in both saline and fresh-water environments are discussed and dissected into their separate components. 2. Contrasts between members of the Alligatoridae and the Crocodylinae emerge repeatedly in aspects such as integumental permeabilities, functioning of the renal/cloacal system, and the presence of lingual salt-secreting glands. 3. These contrasts contribute to the view that the alligatorid and crocodyline stocks are more divergent than has been suspected previously. In particular, there is cogent evidence of a significant marine phase in the evolution of the Crocodylinae but not of the Alligatoridae. 4. The physiological evidence to support this view of a very basic dichotomy among the eusuchians is reviewed in detail and avenues which would contribute most to its critical evaluation are identified.

Structural and ultrastructural features of cortical cells in motor organs of sensitive plants

Abstract: Summary (1) The movements are only expressed in motor cells, regardless of the nature of the stimulation or its point of application. Therefore, these cells have structures capable of traducing the different stimulation-induced messages which are received in parts incapable of movement. (2) K+, Cl- and Ca2+ are the major ions. Their fluxes have been followed during nyctinastic movements as well as during stimuli-induced movements. At the moment, the location and the role of these ions are being studied. (3) The movement results from the integrated activity of all (n) motor cells in the pulvini (i.e. n > 350 × 103 in primary pulvini 3 mm long and 1·9 mm thick). (4) The motor cell is a full-grown cell whose osmotic activity induces turgor variations allowing foliar movements. (5) The motor cell is a highly differentiated cell, which, up to now, has never been able to dedifferentiate in order to produce callus. (6) The motor cell has original features in its apoplastic compartment (large meatuses, wall foldings, large periplasm with membranes) and in its symplastic compartment (double vacuolar apparatus, morphological polarity given by the tannin vacuole location near the nucleus, abundant mitochondria). (7) Its cytoskeleton includes microtubules, cytoplasmic and vacuolar fibrils (in particular in the tannin vacuole), and a wall with special properties. (8) The motor cell is supposed to contain contractile proteins, whose nature and location are being investigated. (9) The shape change of the motor cell is obvious after pulvinar bending. This change is probably associated with a volume change in several intracellular compartments (vacuole, mitochondria, vesicles). (10) At the cellular and subcellular level the same general features are observed in motor cells of non-seismonastic and of seismonastic species. Probably, functional differences depend upon differences occurring at the molecular level. (11) The motor cell is an interesting model for the study of the osmoregulation mechanism in plant cells, to test the effect of toxic products, in particular to find their optimal efficiency in the circadian cycle.

History of aridland vegetation and climate: a global perspective

Abstract: Summary 1. The origin and history of aridlands and their vegetational cover is closely related to geological history, especially in relation to plate tectonics, mountain building, land-and sea-level changes and ice ages, and the arrival of modern humans and their subsequent development. 2. A close relationship exists between world temperatures and precipitation. Therefore, the development of present-day zonal, aridland vegetation and climate cannot be divorced from the history of the latest ice age. 3. The combined geological and palaeobotanical evidence demonstrate that whilst the origin of modern openland and aridland vegetation went back to the time of the origin of angiosperms during the Cretaceous, its subsequent expansion went hand in hand with the lowering of world temperatures during the Palaeogene and Neogene, when a series of angiospermous families having dominantly herbaceous and openland taxa, as members, appeared successively in the stratigraphic record. 4. The successive lowering of world temperatures had the overall effect of reducing precipitation levels all over the globe. Consequently, high rainfall areas, bearing closed forests, became progressively smaller and smaller and the decreased rainfall promoted the evolution and expansion of low biomass, openland and aridland vegetation. 5. The break-up of regional closed forests had started from the middle Miocene but the main expansion of zonal, aridland vegetation, in low and middle latitudes, appears to have taken place, together with the expansion of tundra vegetation, at high latitudes, from the late Pliocene. Glaciations of a magnitude of at least two-thirds that of the late Pleistocene glacial maxima started to occur about that time, 25 Ma ago. 6. The alternations between cold, glacial and warm, interglacial periods, during the late Neogene, especially with increased amplitude during the last 0.4 Ma, allowed less and less time for forest vegetation to expand and stabilize during the warm intervals, with the result that openland and aridland vegetation was able to expand to unprecedented levels. 7. Further, as man increased fire frequencies during the late Pleistocene, the relatively fire-sensitive and mesophytic taxa were selectively eliminated and more and more forests were opened to invasion by openland taxa in low and middle latitudes. Later on, with the clearance of forests for agriculture, the overall effect on vegetation was to create open landscapes which favoured the expansion of openland taxa at low, middle and high latitudes, during an interglacial, that is, the Holocene, a feature that is unprecedented in the entire earlier geological record.

Sperm thrusts and the problem of penetration

Abstract: Summary An equation of Lighthill's is used to calculate sperm thrusts. They have values in the range 5–350 pN, depending on species. The limitations of this approach are discussed and comparison is made with the measured thrust for human sperm. The effect of sperm thrusts of this magnitude on covalent bonds and reversible bonds is discussed. Sperm cannot break covalent bonds, but can reduce the lifetime of reversible bonds. The structure and physical properties of the zona pellucida are examined in relation to sperm penetration. The evidence suggests that sperm cannot penetrate it solely by force. A model for sperm penetration is elaborated in which the conjunctive application of thrust and a soluble enzyme leads to strain-induced proteolysis and the formation of the penetration slit. The potential mechanism of the zona block is discussed, as is the site of the acrosome reaction. The effects of other mechanical inputs into fertilization such as stirring and swimming are examined briefly. Evidence suggests that sperm penetration of the cumulus oophorus and cervical mucus is mechanical, but that in the case of cervical mucus, it is affected by changes in the physical properties of the mucus.

Second messengers and the regulation of ca 2+ fluxes by ca 2+ ‐mobilizing agonists in rat liver

Abstract: Summary Knowledge of the mechanism of action of Ca2+-mobilizing agonists in liver has progressed considerably following the discovery that their interaction with specific receptors on the plasma membrane is accompanied by the hydrolysis of PIP2 and the generation of the second messengers diacylglycerol and IP3, for the activation of protein kinase C and the mobilization of intracellular Ca2+, respectively. Although the second messenger functions of diacylglycerol and IP3 in these actions seem well established, it is not yet clear how the agonists are able to regulate Ca2+ influx across the plasma membrane, an event which is crucial for those actions of the agonists which are dependent on the maintenance of an elevated level of cytosolic Ca2+, Whilst there is evidence for the existence of more than one pathway for Ca2+ influx in liver, it appears that in each instance the Ca2+ influx process is regulated differently to the Ca2+ influx through the volage-sensitive Ca2+ channels that is known to occur in excitable tissues. At present it is not clear whether any of the Ca2+ influx pathways in liver is regulated by direct coupling to the agonist receptor mechanism on the outer surface of the plasma membrane, or whether the regulation involves the production of some second messenger(s). However, indirect evidence from a number of tissues appears to favour the involvement of both IP3 and IP4 in the regulation of Ca2+ influx. The mechanism by which IP3 and IP4 may regulate Ca2+ influx remains to be established, but it has been proposed that Ca2+ entry into the cell occurs through a pathway connecting the plasma membrane and the endoplasmic reticulum, following the release of intracellular Ca2+ from the lumen of the endoplasmic reticulum. Although it is not yet known whether glucagon (or cyclic AMP) activates the same pathway for Ca2+ influx as Ca2+-mobilizing agonists, the marked potentiation by cyclic AMP of the Ca2+ influx induced by Ca2+-mobilizing agonists has provided a powerful system with which to study the regulation of Ca2+ influx in liver. Whether this Ca2+ influx process occurs through some ion exchange mechanism (such as Ca2+/Na+ exchange) remains to be determined. Results from this study suggests that the Ca2+ influx is inhibited by neomycin, acidic pH, and a depolarization of the plasma membrane. The observation that cyclic AMP synergistically potentiates the influx of Ca2+ induced by Ca2+-mobilizing agonists, that this influx appears to correlate with the reported ability of these agonists to induce PIP2 hydrolysis and accumulation of IP3, and that cyclic AMP synergistically potentiates the production of IP4 by vasopressin, are all consistent with the notion that IP3 and IP4 are involved in regulating Ca2+ influx. Whilst little is known about the Ca2+ transport process itself, these studies coupled with the recent finding that Ca2+ influx into the liver cell can occur through different pathways, seem set to lead to a better understanding of this important process in the near future.

Operation of the purine nucleotide cycle in animal tissues.

Abstract: Summary 1. The operation of the purine nucleotide cycle, consisting of the enzymes adenylate deaminase (E.C. 3.5.4.6), adenylosuccinate synthetase (E.C. 6.3.4.4) and adenylosuccinate lyase (E.C. 4.3.2.2), has been reviewed with reference to its metabolic function in animal tissues. 2. Abundant evidence, both from in vitro and in vivo studies, suggests that the purine nucleotide cycle serves to stabilize the adenylate ‘energy charge’ (or ‘phosphorylation potential’) in the cytoplasm of vertebrate cells during a temporary imbalance between ATP-consumption and ATP-production. This stabilization, however, is absent or much less efficient in tissues of invertebrates. 3. The hypothesis that AMP-deaminase is involved in the regulation of glycolysis is not supported by recent work. In a variety of cell types, including skeletal muscle and blood platelets, blocking of AMP-deaminase activity (due to a genetic defect or to pharmacological inhibition) is without effect on the glycolytic rate. Detailed kinetic and histochemical analysis of energy metabolism shows lack of correlation between AMP-deaminase activity and glycolysis in skeletal muscle during exercise. 4. The purine nucleotide cycle appears to control the level of citric acid cycle intermediates in skeletal muscle. Pharmacological inhibition of adenylosuccinate lyase or adenylosuccinate synthetase leads to a reduced availability of four-carbon ‘sparker’ molecules to the Krebs cycle with a concomitant impairment of aerobic energy production during muscular work. 5. The cycle appears to be a major pathway for amino acid deamination in skeletal muscle and brain of vertebrates, but not in kidney or liver.

An account of the hatching strategies of birds

Abstract: Summary 1. Three basic hatching methods are described together with one subsidiary method. The symmetrical method is characterized by rotation of the chick in the egg during hatching climax; a line of damage around the circumference of the egg is evident at the beginning of a pushing phase which causes a fairly symmetrical cap to be broken from the shell. The asymmetrical method is used by a few long-billed species; it involves little or no rotation of the chick in the egg, and produces asymmetrical shell remains. The megapodes have developed a unique hatching method in response to their unusual incubation conditions. Parental assistance has been observed in some species, but only as an auxiliary to either the symmetrical or asymmetrical method. Approximately 150 species have been categorized according to the hatching method(s) they use. 2. Among those species adopting the symmetrical method, there is considerable variability as to how far the chick turns in the egg during hatching climax. On this basis, a spectrum of behaviour, expressed in terms of angle of rotation (θ), has been proposed. At one end lie species such as the bobwhite quail and little owl (θ≥ 360°), and at the other, the ostrich and rhea (θ≤ 90°). 3. The theory that, with the exception of the megapodes, there is only one basic hatching method is examined. The tenets of this theory are found to be inconsistent with recent observations of species differences in hatching behaviour. It is concluded that hatching behaviour is governed by an intrinsic species-specific programme, in turn influenced by mechanical or other external factors. 4. The literature contains several suggestions as to the external factors that might influence hatching technique, but only one detailed investigation. It is proposed that interspecific differences in the mechanical properties of the egg integument (the shell and its underlying membranes) can be regarded as forming a spectrum from very brittle to comparatively tough. The amount of climax rotation by the chick is seen as an adaptive response to brittleness or toughness of the egg integument. The hatching technique of the chick is further modified as a response to the effect of moisture content on the integument. 5. The selective pressures leading to the evolution of the symmetrical and asymmetrical hatching methods are discussed. The previous model for the development of the asymmetrical method is amended to account for those species of gull which may adopt either method.

Histological staining of lipids for the light and electron microscope

Abstract: Summary 1. It is generally agreed that the blackening of osmium tetroxide by unsaturated lipid is too unpredictable to demonstrate lipid in tissues. 2. At neutral pH osmium tetroxide combines with the double bonds in the lipoproteins of cellular membranes (mitochondria, etc.) and the deep colour reaction of ethyl gallate with this osmium provides good staining of lipid for the light microscope. 3. Osmium taken up by tissue proteins at neutral pH is only a small fraction of that taken up by the lipid. (After acid fixatives osmium tetroxide is a general protein stain.) 4. The uptake of Sudan black B by partition from dilute solution is a specific test for lipid, but in normally fixed tissue most of the structural lipid is ‘bound’ and is not accessible to the dye. 5. Cautious treatment of fixed tissue with dilute sodium hypochlorite will unmask this lipid for viewing by the light microscope. 6. Direct fixation with neutral osmium tetroxide is an effective method for visualizing lipid for the electron microscope (as in the ethyl gallate method for the light microscope). But the poor penetration of osmium limits its use in this way. 7. After formol/glutaraldehyde fixation much of the lipid in the tissues is ‘bound’ and does not take up osmium. It can be unmasked by a saturated aqueous solution of thymol. 8. The unmasked lipid can then be rendered more osmiophil by partition in a solution of the highly unsaturated terpene farnesol, thus increasing the uptake of osmium in a renewed application. 9. Some of the novel observations on tissue lipids made by these methods are reviewed.

Sex determination and sex differentiation in malaria parasites

Abstract: Summary 1. The most coherent body of information on the malarial life-cycles comes from studies on P. berghei and P. falciparum. For both species there is an extensive and accurate description of the life-cycle available based on synchronized in-vitro and in-vivo infections (the latter only for P. berghei). 2. The trophozoites preceding the young gamonts are already sexually differentiated, that is, they are micro- and macrogamontoblasts. They are, however, morphologically indistinguishable from the asexual trophozoites which will develop into meronts. This is in contrast with the gamontoblasts of the closely related eimerian species, which are already morphologically differentiated at this stage of their sexual development. Experiments with synchronized P. berghei infections suggest that the induction towards asexual or sexual development of intra-erythrocytic parasites occurs between 8–12 h after erythrocyte invasion. 3. DNA measurements by direct fluorometry of individual Feulgen-stained stages of P. berghei and P. falciparum, has shown that all stages are haploid, except for the zygote. Hence, meiosis occurs within the zygote. The latter has been confirmed in EM studies of ookinete (zygote) development of P. berghei within its mosquito vector. 4. EM studies also provided evidence for the fact that the malarial genome is organized in 10–14 chromosomes, which is in line with the estimates based on PFG analysis of molecular karyotypes of different malarial species. 5. Infections with clones of the asexual intra-erythrocytic stages resulted in the formation of both micro- and macrogamonts. Since these asexual stages are haploid, gamonts develop from genetically identical cells and their development must be due to selective gene expression. How many genes are involved in this process, how they are induced and regulate differentiation is still an open question. 6. Macrogamonts contain an extra amount of nuclear DNA in excess of the haploid value. Roughly the same amount of extra DNA is still present in the zygote, indicating that it is probably due to a selective amplification of certain genes or repetitive sequences. No indications have been found for an amplification of rRNA genes in macrogamonts or in cloned lines of Plasmodium parasites producing high numbers of infective macrogamonts. 7. Mature microgamonts possess DNA values between 1C and 2C and are not octaploid as has been generally accepted. DNA replication in microgamonts, necessary for the production of the 8 microgametes, starts at the activation of the microgamont. During this process, microgamonts perform three mitotic divisions in which they replicate their entire genome in about 3 min. It is assumed that microgamonts activate simultaneously approximately 1300 replication origins to enable such a fast genome replication. Direct evidence for this hypothesis in the form of direct ultrastructural evidence of DNA molecules of replicating microgamonts is, however, lacking. 8. In-vitro fertilization of macrogametes of P. berghei takes place within 1 h of gametogony. The formed zygote (2C) immediately starts to synthesize DNA and within 3 h a tetraploid value is reached. Thereafter, DNA synthesis ceases for at least 21 h. EM studies of this process in vivo showed that synaptonemal complexes are formed at 25 h after fertilization. Taken together, these results strongly indicate that genome duplication precedes the first meiotic division, i.e. meiosis in Plasmodium follows the normal eukaryotic pattern. Thus, as expected Plasmodium shows recombination between genes determining characters such as enzymes, antigens, drug resistance and virulence. 9. DNA synthesis in malaria parasites is regulated by a DNA polymerase-a like enzyme that differs in some respect in the corresponding enzyme of the host and this could become a target for chemotherapeutic control of the parasite life-cycle.

The role of metabolism and calcium in the control of mitosis and ooplasmic movements in insect eggs: a working hypothesis

Abstract: Summary Patterns of mitosis and ooplasmic movements in the plasmodial phase of insect embryogenesis and their supramolecular basis are reviewed. Evidence is provided for both the relative independence and the precise correlation of the nuclear cycle and various cycling movements of the ooplasm. I suggest that the timing of these cycles is controlled by a metabolic cycle. The latter may act via a cyclic rise and fall either of the level of free calcium or of the sensitivity of contractile cytoplasmic proteins to a constant level of free calcium. Thus mitotic patterns may reflect metabolic patterns, which in turn may reflect the distribution and activity of mitochondria and may also be related to egg size and shape by a gradient of surface-to-volume ratios. The total number of cycles may depend on a limiting cytoplasmic factor, which together with the number of nuclei in a given cycle defines the nucleo-cytoplasmic ratio. I also propose that both natural and experimental activation of insect eggs trigger the metabolic cycle either directly, by supplying oxygen or water, or indirectly, via a calcium pulse. Possible molecular mechanisms of control are discussed and applied to mitosis and ooplasmic movements. A brief discussion of various cell cycle models in light of data from insect embryogenesis is included.