Showing papers in "Philosophical Transactions of the Royal Society B in 1988"

The structure of 2Zn pig insulin crystals at 1.5 A resolution.

Abstract: The paper describes the arrangement of the atoms within rhombohedral crystals of 2Zn pig insulin as seen in electron density maps calculated from X-ray data extending to 1.5 A (1 A = 10(-10) m = 10(-1) nm) at room temperature and refined to R = 0.153. The unit cell contains 2 zinc ions, 6 insulin molecules and about 3 x 283 water molecules. The atoms in the protein molecules appear well defined, 7 of the 102 side chains in the asymmetric unit have been assigned alternative disordered positions. The electron density over the water molecules has been interpreted in terms of 349 sites, 217 weighted 1.0, 126 weighted 0.5, 5 at 0.33 and 1 at 0.25 giving ca. 282 molecules. The positions and contacts of all the residues belonging to the two A and B chains of the asymmetric unit are shown first and then details of their arrangement in the two insulin molecules, 1 and 2, which are different. The formation from these molecules of a compact dimer and the further aggregation of three dimers to form a hexamer around two zinc ions, follows. It appears that in the packing of the hexamers in the crystal there are conflicting influences; too-close contacts between histidine B5 residues in neighbouring hexamers are probably responsible for movements of atoms at the beginning of the A chain of one of the two molecules of the dimer that initiate movements in other parts, particularly near the end of the B chain. At every stage of the building of the protein structure, residues to chains of definite conformation, molecules, dimers, hexamers and crystals, we can trace the effect of the packing of like groups to like, aliphatic groups together, aromatic groups together, hydrogen-bonded structures, positive and negative ions. Between the protein molecules, the water is distributed in cavities and channels that are continuous throughout the crystals. More than half the water molecules appear directly hydrogen bonded to protein atoms. These are generally in contact with other water molecules in chains and rings of increasing disorder, corresponding with their movement through the crystals. Within the established crystal structure we survey next the distribution of hydrogen bonds within the protein molecules and between water and protein and water and water; all but eight of the active atoms in the protein form at least one hydrogen bond.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptive Coloration in Young Cuttlefish (Sepia Officinalis L.): The Morphology and Development of Body Patterns and Their Relation to Behaviour

Abstract: Young Sepia officinalis (0-5 months) were studied in the laboratory and in the sea, and their appearance and behaviour compared with that of adult animals Cuttlefish lay large eggs and the hatchlings are miniature replicas of the adults From the moment of hatching they show body patterns as complex as those of adults and far more elaborate than those shown by most juvenile cephalopods There are 13 body patterns: 6 of these are \`chronic' (lasting for minutes or hours) and 7 are \`acute' (lasting for seconds or minutes) The patterns are built up from no fewer than 34 chromatic, 6 textural, 8 postural and 6 locomotor components, used in varying combinations and intensities of expression Nearly all these components occur in young animals: 26 of the chromatic, all the textural and locomotor, and 6 of the postural components Nevertheless, patterning does change with age and we have recorded this and correlated the changes with behaviour The components are built up from units, which themselves comprise four elements organized in precise relation to one another: chromatophores, iridophores, leucophores and skin muscles The chromatophores are always especially important: they are muscular organs innervated directly from the brain and controlled ultimately by the highest centres (optic lobes) The areas in the Sepia brain that control patterning are already well developed at hatching, for the appearance of the skin is as much part of the brain's motor program as is the attitude of the arms or fins, or the posture of the entire animal The iridophores and leucophores develop later and are especially important constituents of many adult patterns, notably the Intense Zebra of the mature male Experiments confirm that patterning is neurally controlled and apparently mediated exclusively by the visual system Young cuttlefish use patterning primarily for concealment, utilizing such strategies as general colour resemblance, disruptive coloration, obliterative shading, shadow elimination, disguise and adaptive behaviour Older animals also conceal themselves but increasingly use patterns for signalling, both interspecifically (warning or `deimatic' displays) and intraspecifically (sexual signalling) Laboratory-reared cuttlefish were released in the sea and observed underwater They quickly and effectively concealed themselves on the substrate; it was easy for the human observer to lose them and many passing fish behaved as if they were not there One local predator, Serranus cabrilla, was observed to attack them and no fewer than 35 attacks were recorded, only six of which were successful Laboratory-reared cuttlefish apparently distinguished between these predators and other, non-predatory, fish the first time they encountered them in nature

Frequency-dependent predation, crypsis and aposematic coloration.

Abstract: Frequency-dependent predation may maintain or prevent colour pattern polymorphisms in prey, and can be caused by a variety of biological phenomena, including perceptual processes (search images), optimal foraging and learning. Most species are preyed upon by more than one predator species, which are likely to differ in foraging styles, perceptual and learning abilities. Depending upon the interaction between predator vision, background and colour pattern parameters, certain morphs may be actively maintained in some conditions and not in others, even with the same predators. More than one kind of predator will also affect stability, and only slight changes in conditions can cause a transition between polymorphism and monomorphism. Frequency-dependent selection is not a panacea for the explanation of variation in animal colour patterns, although it may be important in some systems.

The History of the Great Northwest European Rivers During the Past Three Million Years

Abstract: This paper is based on a review of the histories of the Rivers Elbe, Saale, Weser, Rhine, Meuse, Scheldt, Thames, Somme and Seine. Two further rivers no longer in existence, the Baltic and Channel rivers, are also included. The histories of these rivers illustrate how the interplay of tectonics and climate have influenced the northwest European drainage system through the late Cainozoic. The foundations of the modern drainage system were laid in the Miocene when earth movements associated with Alpine orogenesis and the opening of the North Atlantic were at their height. In general, these early rivers occupied shallow valleys and transported only chemically resistant minerals and lithologies. The Pleistocene was marked by the appearance of cold climates. These climates resulted in fluvial dissection of the landscape, which stripped first regolith, then fresh material derived by periglacial processes. This material accumulated in the river valleys as gravel and sand deposits, which make up the overwhelming bulk of Pleistocene fluvial sediments. The rivers generally adopted braided courses during cold stages. The deeply incised modern valley system has developed largely as a result of rapid climatic changes over the past 2.4 Ma or so. Throughout this period the river system has undergone repeated adjustments in response to continental glaciation. These responses are discussed. Particular attention is paid to the impact of the Anglian-Elsterian glaciation that blocked the southern North Sea to produce a vast ice-dammed lake, the overspill from which initiated the Dover Straits. By contrast, interglacial sedimentation comprises predominantly fine, often fossiliferous sediments with rivers normally adopting single-thread channels, while estuarine sediments were deposited in areas invaded by high eustatic sea levels. The impact of sea-level change on the length of rivers and their courses is considered.

The evolution of aggression: can selection generate variability?

Abstract: Three models--the war of attrition, the size game and the badges of dominance game--are described, in which natural selection can maintain genetic variability for aggression. The models differ in whether or not the traits that settle contests are costly in contexts other than fighting, and also in whether signals are used. It is concluded that contests will be settled by non-costly traits only if the value of the contested resource is small relative to the cost of fighting, and that 'honest' signalling of aggressiveness is stable only if individuals giving signals that are inconsistent with their behaviour suffer costs. The literature on 'badges of dominance' in birds is reviewed. New data on great tits, greenfinches and corn buntings show that there is plumage variability within age and sex that sometimes serves to settle contests, and that, in the first two species but not the third, the badges are uncorrelated with size, and settle contests only over trivial resources.

Frequency-dependent selection in bacterial populations.

Abstract: There are many situations in which the direction and intensity of natural selection in bacterial populations will depend on the relative frequencies of genotypes. In some cases, this selection will favour rare genotypes and result in the maintenance of genetic variability; this is termed stabilizing frequency-dependent selection. In other cases, selection will only favour genotypes when they are common. Rare types cannot invade and genetic variability will not be maintained; this is known as disruptive frequency-dependent selection. Phage-mediated selection for bacteria with novel restriction-modification systems is frequency-dependent and stabilizing. In mass culture, selection for the production of toxins and allelopathic agents is likely to be frequency-dependent but disruptive. This also occurs in selection favouring genes and transposable elements that cause mutations. Here I review the results of theoretical and experimental studies of stabilizing and disruptive frequency-dependent selection in bacterial populations, and speculate on the importance of this kind of selection in the adaptation and evolution of these organisms and their accessory elements (plasmid, phage and transposons).

Frequency-dependent selection by predators.

Abstract: Sometimes predators tend to concentrate on common varieties of prey and overlook rare ones. Within prey species, this could result in the fitness of each variety being inversely related to its frequency in the population. Such frequency-dependent or 'apostatic' selection by predators hunting by sight could maintain polymorphism for colour pattern, and much of the supporting evidence for this idea has come from work on birds and artificial prey. These and other studies have shown that the strength of the observed selection is affected by prey density, palatability, coloration and conspicuousness. When the prey density is very high, selection becomes 'anti-apostatic': predators preferentially remove rare prey. There is still much to be learned about frequency-dependent selection by predators on artificial prey: work on natural polymorphic prey has hardly begun.

Spatial and temporal aspects of cell signalling

Abstract: As new techniques are developed to measure intracellular messengers it becomes increasingly apparent that there is a remarkable spatial and temporal organization of cell signalling. Cells possess a small discrete hormone-sensitive pool of inositol lipid. In some cells such as Xenopus oocytes and Limulus photoreceptors this phosphoinositide signalling system is highly concentrated in one region of the cell, so establishing localized calcium gradients. Another example is the hydrolysis of inositol lipids in eggs at the point of sperm entry resulting in a localized increase in Ins(1,4,5) P 3 and calcium which spreads like a wave throughout the egg. In hamster eggs this burst of calcium at fertilization recurs at 1-3 min intervals for over 100 min, a particularly dramatic example of spontaneous activity. Spontaneous oscillations in intracellular calcium exist in many different cell types and are often induced by agonists that hydrolyse inositol lipids. We have made a distinction between oscillations that are approximately sinusoidal and occur at a higher frequency where free calcium is probably continuously involved in the oscillatory cycle and those where calcium falls to resting levels for many seconds between transients. In the former case, the oscillations are thought to be induced through a cytoplasmic oscillator based on the phenomenon of calcium-induced calcium release. Such oscillations can be induced in Xenopus oocytes after injection with Ins(1,4,5) P 3 . A receptor-controlled oscillator based on the periodic formation of I ns (1,4,5) P 3 is probably responsible for the generation of the widely spaced calcium transients. The function of such calcium oscillations is currently unknown. They may be a reflection of the feedback interactions that operate to control intracellular calcium. Another possibility emerged from observations that in some cells the frequency of calcium oscillations varied with agonist concentration, suggesting that cells might employ these oscillations as a way of encoding information. One advantage of using such a frequency-dependent mechanism may lie in an increase in fidelity, especially at low agonist concentrations. Whatever these functions might be, it is clear that uncovering the mechanisms responsible for such oscillatory activity will greatly enhance our understanding of the relation between the phosphoinositides and calcium signalling.

Northern Hemisphere climate regimes during the past 3 Ma: possible tectonic connections

Abstract: The 100 ka rhythm of orbital eccentricity has dominated large-amplitude climatic variations in the high-latitude North Atlantic during the Brunhes magnetic chron (0-0.735 Ma BP). Earlier, during the Matuyama chron (0.735-2.47 Ma BP), climatic variations in this region were lower in amplitude and concentrated mainly at the 41 ka rhythm of orbital obliquity. These rhythmic climatic responses to orbital forcing are evident both in stable isotopic ( $\delta^{18}$ O) indicators of ice volume or temperature and in biotic and lithologic indicators of local North Atlantic surfaceocean variability. The synchronous responses of these indicators are consistent with results from atmospheric general circulation models showing that the North American ice sheet directly controls North Atlantic surface-ocean responses via strong cold winds that are generated on the northern ice-sheet flanks and blow out across the ocean, chilling its surface. Before 2.47 Ma BP, smaller-scale quasiperiodic oscillations of the planktonic fauna and flora occurred, but the cause of these variations in the absence of significant ice sheets is unclear.

The transmission dynamics of human immunodeficiency virus (HIV).

Abstract: The paper first reviews data on HIV infections and AIDS disease among homosexual men, heterosexuals, intravenous (IV) drug abusers and children born to infected mothers, in both developed and developing countries. We survey such information as is currently available about the distribution of incubation times that elapse between HIV infection and the appearance of AIDS, about the fraction of those infected with HIV who eventually go on to develop AIDS, about time-dependent patterns of infectiousness and about distributions of rates of acquiring new sexual or needle-sharing partners. With this information, models for the transmission dynamics of HIV are developed, beginning with deliberately oversimplified models and progressing - on the basis of the understanding thus gained - to more complex ones. Where possible, estimates of the model's parameters are derived from the epidemiological data, and predictions are compared with observed trends. We also combine these epidemiological models with demographic considerations to assess the effects that heterosexually-transmitted HIV/AIDS may eventually have on rates of population growth, on age profiles and on associated economic and social indicators, in African and other countries. The degree to which sexual or other habits must change to bring the `basic reproductive rate', R $_0$ , of HIV infections below unity is discussed. We conclude by outlining some research needs, both in the refinement and development of models and in the collection of epidemiological data.

Biological control: challenges and opportunities

Abstract: Biological control, the use of living organisms as pest control agents, has enjoyed varying popularity over the past century, but today is well established as an important component of integrated pest management. We examine some current challenges to the use of biological control and particularly to classical biological control, the introduction of exotic natural enemies. These include conflicts of interest (1) with the conservation of native species and (2) between agricultural lobbies. On a scientific level, we examine two debates over the ecological and genetic basis of successful control. The challenge of Murdoch et al. (Am. Nat. 125, 344-366 (1985)) to the notion of stability in pest populations under biological control, reveals that the stabilizing mechanisms may differ between pest taxa with different patterns of spatial dynamics. With respect to the hypothesis of Hokkanen & Pimentel (Can. Ent. 116, 1109 (1984)) on the better chances of 'new associations' in biological control, we present an analysis that reaches different conclusions. Finally, we discuss future prospects for the different approaches to biological control, and suggest that longterm control methods, such as introduction and inoculation, will be used increasingly in the future.

The plant mitochondrial genome: homologous recombination as a mechanism for generating heterogeneity

Abstract: The mitochondrial genomes of higher plants are among the largest and most complex organelle genomes described. They are generally multicircular or partly linear; in some species, extrachromosomal plasmids are present. It is proposed that inter- and intramolecular homologous recombination can account for the diversity of the observed genome organizations. The ability of mitochondria to fuse establishes a panmictic mitochondrial DNA population which is in recombinational equilibrium. It is suggested that this suppresses the base mutation rate, and unequal partitioning of the cytoplasm during cell division can lead to the rapid evolution of mitochondrial genome structure. This contrasts with the observed rates of base-sequence and genome evolution in chloroplasts. This difference can be accounted for solely by the inability of chloroplasts to fuse, thereby preventing chloroplast genome panmixis.

The Organization of Mechanosensory Neuropiles in Locust Thoracic Ganglia

Abstract: This paper describes the neuropilar areas of the thoracic ganglia of locusts ( Locusta migratoria ) in which projections from mechanoreceptors terminate. Reference transverse, sagittal and parasagittal sections through a meso- and metathoracic ganglion illustrate the relation of these neuropiles to known structures such as longitudinal tracts and commissures. The ventral association centres (VAC), divided into anterior (aVAC), medial (mVAC), ventralmost (vVAC) and lateral (IVAC) parts, receive terminals from tactile hairs (H; aVAC, vVAC and IVAC) and chordotonal organs (CO; mVAC). Newly defined neuropilar areas are the lateral association centres (LAC) which are divided into an anterior (aLAC) and posterior part (pLAC). These receive projections from hair plates (HP), campaniform sensilla (CS), multipolar sensory cells (MS), strand receptors (SR) and chordotonal organs (CO). In contrast to the VACs they also contain fibres of motorneurons.

Inositol phosphates: proliferation, metabolism and function

Abstract: After the initial discovery of receptor-linked generation of inositol (1,4,5) trisphosphate (Ins(l,4,5) P 3 ) it was generally assumed that Ins(l,4,5) P 3 and its proposed breakdown products inositol(1,4)bisphosphate (Ins(1,4) P 2 ) and Ins1 P , along with cyclic inositol monophosphate, were the only inositol phosphates found in significant amounts in animal cells. Since then, three levels of complexity have been introduced. Firstly, Ins (1,4,5) P 3 can be phosphorylated to Ins (1,3,4,5) P 4 , and the subsequent metabolism of these two compounds has been found to be intricate and probably different between various tissues. The functions of Ins(1,4,5) P 3 and Ins(l,3,4,5) P 4 are almost certainly to regulate cytosolic Ca 2+ concentrations, but the reasons for the labyrinth of the metabolic pathways after their deactivation by a specific 5-phosphatase remain obscure. Secondly, inositol pentakis- and hexakisphosphates have been found in many animal cells other than avian erythrocytes. It has been shown that their synthesis pathway is entirely separate from the inositol phosphates discussed above, both in terms of many of the isomers involved and probably in the subcellular localization; some possible functions of Ins P 5 and Ins P 6 are discussed here. Thirdly, cyclic inositol polyphosphates have been reported in stimulated tissues; the evidence for their occurrence in vivo and their possible physiological significance are also discussed.

The Osteology and Relationships of Clevosaurus (Reptilia: Sphenodontida)

Abstract: The sphenodontid genus Clevosaurus from the late Triassic of south west Britain is represented by at least two different species. Exceptionally abundant remains of Clevosaurus occur as part of a diverse assemblage of microvertebrates lodged within fissure deposits at localities in Glamorgan and Avon. Although the majority of specimens occur as beautifully preserved disassociated elements, certain associated and articulated remains have been recovered. Contemporaneous vertebrates include other sphenodontians, archosaurs and procolophonids. C. hudsoni is fully restored as a lizard-like reptile approximately 25 cm long. It was essentially insectivorous, although it may have also been facultatively herbivorous, breaking down food with a well-developed precision shear bite. A smaller species, C. minor, is less widespread and its remains are generally not as well preserved. It seems probable that a third species might be distinguished on the basis of dental characteristics, and the occurrence of a fourth species cannot be discounted. A distinct suite of apomorphic characters separates the Sphenodontida from other lepidosauromorphs and these apomorphies are readily identifiable in Clevosaurus. In certain late Triassic sphenodontians the lower temporal arcade was apparently incomplete, but, contrary to Robinson's (J. geol. Soc. Lond. 129, 457 (1973)) description, the lower temporal arcade was unbroken in Clevosaurus. However, the articulation between the jugal, squamosal and quadratojugal does not conform to the configuration exhibited by all other sphenodontians in which both temporal arcades are complete. In addition, the possession of a supratemporal in Clevosaurus is unique among known sphenodontians. The parietal table in Clevosaurus is intermediate between the broad, flat plesiomorphic condition, seen in forms such as Polysphenodon and Homoeosaurus, and the derived narrow median ridge displayed by Kallimodon and Sphenodon. Consequently, within the Sphenodontida the position of Clevosaurus is unclear; however, on the basis of the prominently flanged additional teeth, some reduction in tooth numbers and at least some partial reduction in the width of the parietal table, it would appear to share certain characteristics with Sapheosaurus and Kallimodon. Palaeopleurosaurus, described by Carroll (1985) as a primitive pleurosaur, may also have affinities with this grouping. Some of the earliest sphenodontians, such as Planocephalosaurus and Diphydontosaurus, exhibit fusion of the median skull roofing elements, a short temporal region and retention of a variable degree of pleurodonty in the marginal dentition. Together with Gephyrosaurus these genera may constitute a monophyletic assemblage forming the sister group of all other sphenodontians.

Sex determination in marsupials: evidence for a marsupial-eutherian dichotomy.

Abstract: In this paper, we review briefly the current state of knowledge about sexual differentiation in eutherian mammals, and then describe the situation in detail in two marsupial species: the North American opossum and the tammar wallaby. The conventional explanation for the genesis of all male somatic sexual dimorphisms in mammals is that they are a consequence of the systemic action of testicular hormones. In the absence of testes, the embryo will develop a female phenotype. We present evidence for the tammar wallaby that calls into question the universal applicability of this hormonal theory of mammalian sexual differentiation. We have shown that extensive somatic sexual dimorphisms precede by many days the first morphological evidence of testicular formation, which does not occur until around the third day of pouch life. Male foetuses, and pouch young on the day of birth, already have a well-developed gubernaculum and processus vaginalis, paired scrotal anlagen, and a complete absence of mammary anlagen, whereas female foetuses and newborn pouch young have a poorly developed gubernaculum and processus vaginalis, no scrotal anlagen, and well-developed mammary anlagen. Because it seems unlikely that the male gonad could begin hormone secretion until after the Sertoli and Leydig cells are developed, our results strongly suggest that some sexually dimorphic somatic characteristics develop autonomously, depending on their genotype rather than the hormonal environment to which they are exposed. We have been able to confirm the hormonal independence of the scrotum, pouch and mammary gland by administering testosterone propionate daily by mouth to female pouch young from the day of birth; although the Wolffian duct was hyperstimulated, there was no sign of scrotal development, or pouch or mammary inhibition. When male pouch young were treated with oestradiol benzoate in a similar fashion, there was hyperstimulation of the Mullerian duct and inhibition of testicular migration and development, but no sign of scrotal inhibition or pouch or mammary development. Our results in the tammar wallaby are consistent with the earlier studies on the opossum, whose significance was not appreciated at the time. Further evidence in support of this hormonal independence comes from earlier studies of spontaneously occurring intersexes in several species of marsupial, including the opossum and the tammar wallaby. An XXY individual had intra-abdominal testes and complete masculinization of the male reproductive tract internally, but externally there was a pouch and mammary glands and no scrotum.(ABSTRACT TRUNCATED AT 400 WORDS)

Environmental regulation of sex determination in reptiles.

Abstract: The various patterns of environmental sex determination in squamates, chelonians and crocodilians are described. High temperatures produce males in lizards and crocodiles but females in chelonians. Original experiments on the effects of incubation at 30 degrees C (100% females) or 33 degrees C (100% males) on development in Alligator mississippiensis are described. These include an investigation of the effect of exposing embryos briefly to a different incubation temperature on the sex ratio at hatching, and a study of the effects of 30 degrees C and 33 degrees C on growth and development of alligator embryos and gonads. A 7-day pulse of one temperature on the background of another was insufficient to alter the sex ratio dramatically. Incubation at 33 degrees C increased the rate of growth and development of alligator embryos. In particular, differentiation of the gonad at 33 degrees C was enhanced compared with 30 degrees C. A hypothesis is developed to explain the mechanism of temperature-dependent sex determination (TSD) in crocodilians. The processes of primary sex differentiation are considered to involve exposure to a dose of some male-determining factor during a specific quantum of developmental time during early incubation. The gene that encodes for the male-determining factor is considered to have an optimum temperature (33 degrees C). Any change in the temperature affects the expression of this gene and affects the dose or quantum embryos are exposed to. In these cases there is production of females by default. The phylogenetic implications of TSD for crocodilians, and reptiles in particular, are related to the life history of the animal from conception to sexual maturity. Those animals that develop under optimal conditions grow fastest and largest and become male. A general association between the size of an animal and its sex is proposed for several types of vertebrate.

The Localization of Calcium Release by Inositol Trisphosphate in Limulus Photoreceptors and Its Control by Negative Feedback

Abstract: Microvillar photoreceptors of invertebrates exhibit a light-induced rise in the intracellular concentration of free calcium (Ca $_i$ ) that results in part from release of calcium from an intracellular compartment This light-induced release of calcium appears to result from a cascade of reactions that involve rhodopsin, a GTP-binding protein and a phospholipase-C which releases inositol 1,4,5-trisphosphate (Ins(1,4,5)P $_3$ ) from the plasma membrane; the Ins(1,4,5)P $_3$ acts to release calcium from smooth endoplasmic reticulum In the ventral photoreceptor of the horseshoe crab Limulus polyphemus not all of the endoplasmic reticulum is subject to calcium release by Ins(1,4,5)P $_3$ Only endoplasmic reticulum in the light-sensitive region of the cell is competent to release calcium in response to Ins(1,4,5)P $_3$ The release of calcium by Ins(1,4,5)P $_3$ in ventral photoreceptors appears to be subject to feedback inhibition through elevated Ca $_i$ We suggest that this feedback inhibition contributes to sensory adaptation in the photoreceptor and may account for oscillatory membrane responses sometimes observed with large injections of Ins(1,4,5)P $_3$

Population ecology of intestinal helminth infections in human communities.

Abstract: The distribution of worm burdens in human populations is a major determinant of both the dynamics of transmission and the level of community morbidity. The distribution exhibits convexity with host age, which appears to correlate with exposure in the young age-classes but not in adults, and may be evidence for the development of an acquired immune response. The distribution between individuals is typically overdispersed. Individuals are predisposed to high (or low) intensity of infection and to a correspondingly high (or low) rate of acquisition of infection. A major epidemiological question is whether this reflects individual differences in environmental exposure or susceptibility. Environmental studies that have observed clustering of intense infection in particular households are supportive of either mechanism. Individual host behaviours that predispose to infection have an overdispersed distribution and may alone, or as compounding factors, generate the observed distribution of infection intensity. Factors such as host nutrition and physiology may modify host immune-responsiveness and hence susceptibility. Preliminary evidence suggests correlates between infection intensity and HLA class II antigens, and tentatively implies a genetic factor in susceptibility. These findings suggest that further understanding of the relative importance of environmental factors and resistance to the acquisition of intense infection is dependent upon a multidisciplinary approach to epidemiological field study.

Inositol trisphosphate, calcium and muscle contraction.

Abstract: The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Molecular variation in vector-borne plant viruses: epidemiological significance

Abstract: Patterns of variation are examined in four groups of plant viruses, with special reference to their particle proteins and to changes in vector transmissibility and specificity. In the nepoviruses and potyviruses, non-circulative transmission, by nematodes and aphids respectively, seems dependent on structural features on the surface of the virus particles. The N-terminal part of the particle protein may play the key role in potyviruses. Similarly in the luteoviruses, and possibly in the geminiviruses, specificity of circulative transmission by aphids, whiteflies and leafhoppers is linked to the antigenic specificity of the virus particles. Among naturally occurring isolates of the same virus, variation seems often to be discontinuous, and is predominantly of two sorts. Minor variations, characterized by loss of an epitope or substitutions of a few amino acids, can be associated with loss of transmissibility in luteoviruses and potyviruses, or have no effect. Major variations are associated with differences in vector specificity and seem likely to involve radical genetic changes that have evolved over long periods. The adaptation of virus particle proteins for transmission by vectors probably results in conservation of the genes that encode them, and in greater conservation of some parts of these genes than of others.

Long-range aerial dispersal of cereal aphids as virus vectors in North America

Abstract: Cereal aphids are important as direct pests of crops and as virus vectors. Several species, including the corn-leaf aphid (Rhopalosiphum maidis) and the green bug (Schizaphisgraminum), persist and reproduce parthenogenetically throughout the year in the southern areas of the United States. They persist less readily or not at all in the northern states and Canada where winters are cold and these areas are reinvaded annually by migrants from the south, some of which are likely to be viruliferous. This paper reviews studies on the reinvasion phenomenon including the recent 'Pests and Weather' project in Illinois (1983-1985) in which radar and traps mounted on a helicopter were used to detect cereal aphids in flight at altitudes of up to 1100 m. A new back-tracking procedure, electrophoretic analyses and, fuel utilization studies were used to determine possible sources of the aphids caught. The studies on cereal aphid vectors in North America and the various difficulties encountered are discussed in a wider geographic context and in relation to studies on other types of vectors. It is concluded that long-range dispersal has received inadequate attention in relation to its biological and economic importance.

Dynamics of some simple host-parasite models with more than two genotypes in each species.

Abstract: A two-species genetic model of host-parasite interaction is used to study the dynamical consequences of varying the number of genotypes in each species, and the recombination rate in the host. With two genotypes in each species, the model's behaviour is very simple; there is either a stable interior equilibrium, a stable cycle or a smooth outward spiral toward the boundaries. But with three or more genotypes, complex cycles and apparently chaotic behaviour may arise over wide ranges of parameter values. Increasing the number of genotypes also tends to slow the rate of gene-frequency change. Recombination in the host does not affect the stability of the interior fixed point, but intermediate rates of recombination may give dynamic stability to an otherwise dynamically unstable pattern of cycling. Intermediate rates of recombination also tend to decrease the amplitudes of gene-frequency cycles in the host, which implies that they could promote the accumulation of genetic variation involved in complementary, antagonistic interactions with parasites.

Organization and expression of algal (Chlamydomonas reinhardtii) mitochondrial DNA

Abstract: The mitochondrial genome of Chlamydomonas reinhardtii, a unicellular green alga, is a linear 15.8 kilobase pair (kbp) molecule. In gene arrangement and mode of expression, as well as in size, it differs radically from the large (200-2400 kbp) mitochondrial genomes of higher plants. Heterologous hybridization experiments and nucleotide sequence analysis have revealed that C. reinhardtii mitochondrial DNA (mtDNA) is a compactly organized genome specifying at least eight proteins, a minimum of three transfer RNAs, and large subunit (LS) and small subunit (SS) ribosomal RNAs. Both strands of the mtDNA encode genetic information, with genes organized into perhaps a single transcriptional unit on each strand. Stable transcripts have been identified by Northern hybridization analysis, and transcript termini have been mapped by primer extension and S1 nuclease protection experiments. The results suggest that mature RNAs, which virtually saturate the genome, are generated by precise endonucleolytic cleavage of long precursors, with specific motifs (both primary sequence and secondary structure) implicated as processing signals. Codon usage in C. reinhardtii mitochondria is highly biased, with eight codons entirely absent from all protein-coding genes; however, even though codon usage is restricted, it appears that C. reinhardtii mtDNA cannot encode the minimum number of tRNAs needed to support mitochondrial protein synthesis. The most striking feature of C. reinhardtii mtDNA is the division of SS and LS rRNA genes into a number of separate subgenic coding segments ('modules') that are interspersed with one another and with protein-coding and tRNA genes. We have identified abundant small RNAs, transcribed from these modules, that approximate to the latter in size. This indicates that splicing of rRNA 'pieces' does not occur in this system. Rather, the mature rRNAs apparently exist and function as non-covalent complexes of small RNAs (four in SS rRNA, at least eight in LS rRNA), held together by intermolecular base pairing. These complexes contain all the conserved elements of the minimal secondary structures that define the functional core of conventional LS and SS rRNAs.

Population dynamics and biological control

Abstract: Using simple models for host-parasitoid and host-pathogen interactions, we present a basic framework for examining the outcome of releasing natural enemies against a target pest population in a classical biological control programme. In particular, we examine the conditions for the initial invasion and establishment of a natural enemy species, for the maximum depression of the host population, and for the persistence of the populations in a stable interaction. In these conditions there are close parallels between parasitoids and pathogens. The practice of augmenting an existing natural enemy population by regular mass releases has been widely practised, especially with parasitoids. The conditions for eradication of the pest are very similar in host-parasitoid and host-pathogen models, namely that releases must be greater than the equilibrium production of natural enemies in the absence of releases. Any additional density dependence acting on the host population after the stage attacked by parasitoids can influence the effectiveness of augmentative releases. This is particularly the case with over-compensating density dependence when additional releases can actually lead to an increase in the host population. A theoretical basis for biological control cannot be properly developed simply by considering the dynamics of releasing single natural enemy species. Biological control often involves the interplay among different types of natural enemies affecting the same host population. As a step in the direction of producing more complex, multispecies models, we examine the dynamics of three situations: (1) where the host is attacked by two parasitoid species; (2) by a generalist predator and a specialist parasitoid; or (3) by a parasitoid and a pathogen. The dynamics of these three-species systems can be complex, and with properties not easily foreseen from the separate pairwise interactions. These results caution us against formulating biological control strategies purely in terms of two-species systems. For the main part we examine host-parasitoid interactions with discrete, synchronized generations. These would appear to be less suitable to tropical insects where continuous generations and life cycles of the host and parasitoid of different length are to be expected. We show, however, that cycles (with a period of one host generation) can be obtained from an age-structured simulation model, and that these are promoted by the parasitoids having a life cycle half as long as that of the host. Some implications for biological control are discussed. Finally, we turn briefly to the dynamics of host-parasitoid and host-pathogen interactions where pesticides are also applied, and we discuss the evolution of pesticide resistance within the context of these models.

The Glacial History of Iceland During the Past Three Million Years

Abstract: Iceland is built up of volcanic rocks with sedimentary interbeds, which have been piled up continuously since Miocene times. In the Pleistocene rock series, sediments of fluvial, lacustrine, marine and glacial origin and soils are very common and frequently thick. A sudden climatic deterioration took place at about 3 Ma BP. The Pliocene lusitanic marine fauna was replaced by a boreal fauna. Conifers and deciduous forest vanished and the flora became similar to the present one. From 3 to 2 Ma BP, inland ice caps were common during cold spells. From then on ice sheets reaching down to sea level have covered most of the country at least 12 times during glacials.

Vegetational Evidence for Late Quaternary Climatic Changes in Southwest Europe in Relation to the Influence of the North Atlantic Ocean

Abstract: During the period 20-8 ka BP, movements of the polar front in the North Atlantic Ocean between the latitudes of Iceland and the Iberian peninsula greatly affected the climate of western Europe. During the Lateglacial, sea-surface temperature changes were particularly marked in the Bay of Biscay. Such migrations of the polar front, which have been shown to be time-transgressive, have been used to explain Lateglacial climatic events in northwestern Europe. A comparative study of Lateglacial and early Holocene records from lacustrine sites in northern and northwestern Spain and the Pyrenees confirms that the Lateglacial climatic amelioration was time-transgressive along the seaboard of western Europe, beginning 500-1000 years earlier in northwestern Spain than in the British Isles. This time-lag is further exaggerated in the vegetational response by migrational lags and edaphic factors. There are marked differences in the nature and chronology of Lateglacial plant successions, not only between southwest and northwest Europe, but particularly between sites in northwestern Spain, the coastal lowlands of the Pays Basque and the Pyrenees. Sites in northwestern Spain, including that of Sanabria Marsh, here published in detail for the first time, show the moderating climatic influence of the Atlantic Ocean throughout the Lateglacial. There, the climatic amelioration began early, perhaps before 14 ka BP. Deciduous oak forest had already begun to develop during Lateglacial times; this observation suggests that the perglacial refugia for these trees lay close to the maritime Atlantic coasts of Spain and Portugal, and not in the Pyrenees as some authors have proposed. After the onset of the Lateglacial climatic amelioration, pine and birch forest became widespread in the Pyrenees but oaks were very sparse or absent. Oak forest only developed there after 10 ka BP in the early Holocene. The Younger Dryas episode of cooling can be detected, but only by a small expansion of herbaceous plant communities in some areas and with almost no lowering of the treeline. In contrast, Lateglacial conditions in the Pays Basque appear to have been cold and bleak. Even birch and pine forest was poorly developed and may have disappeared with the onset of the Younger Dryas cooling. Acid heathland with Empetrum and ericaceous plants then developed, to be replaced by oak-hazel forest in the early Holocene. Here, clearly, the influence of cold polar water conditions in the Bay of Biscay was very strong. Pollen diagrams from marine cores in the Bay of Biscay are also reviewed, but low sedimentation rates, bioturbation and differential transport and preservation of pollen make comparison with continental pollen diagrams difficult and correlation only possible in broad terms. Accurate vegetational interpretations are impossible. Palynologists working on archaeological cave and rock shelter sequences in southwest France and northern Spain have claimed to recognize, between 32 and 14 ka BP, a series of interstadial intervals with expansions of temperate trees. Careful consideration of pollen diagrams covering the purported Laugerie and Lascaux interstadials, said to occur between 16 and 20 ka BP (conventionally the maximum period of glacial advance of the last glacial stage), suggests that temperate pollen has percolated down through overlying deposits and been preserved in certain sedimentologically favourable beds. Although widely accepted by archaeologists, these interstadials appear to have no reality and must be rejected. There is no trace of them in the long lacustrine records of Les Echets (Beaulieu & Reille 1984) and Grande Pile (Woillard 1975, 1978). There is thus no good palynological record for 30-16 ka BP from south-west Europe, other than the long pollen sequence from Padul in southern Spain (Pons & Reille 1986).

Climatic Variability During the Past Three Million Years, as Indicated by Vegetational Evolution in Northwest Europe and with Emphasis on Data from The Netherlands

Abstract: Temperate and cold stages comparable to those of the last interglacial-glacial have alternated for ca. 2.4 Ma, a time-level regarded as the base of the Quaternary. A curve showing climatic fluctuations according to a number of glacials, interglacials, and temperate oscillations of small amplitude or short duration (interstadials) is given, and the value of pollen records in this context is discussed. Because the position of the individual intervals with known vegetational development on the total timescale is controlled by superposition, the lithostratigraphic position of some of them is reviewed. Basic differences in vegetational evolution between the Tiglian, Waalian, and later interglacials, as well as the extinction of certain trees at around the time of the transition of the Early-Middle Pleistocene, probably indicate lower temperatures during the glacials which have occurred in the past million years. Although the established record shows some resemblance to the oxygen-isotope curve of the deep sea, precise correlation is not yet possible. A tentative correlation is discussed.

The sedimentary record of climatic variation in the southern North Sea

Abstract: The sedimentary sequence on the shelf of the southern North Sea records Quaternary climatic changes in two ways. They are indicated directly by moraine and glaciofluvial deposits from the Elsterian, Saalian and Weichselian glacial periods when the British and the Scandinavian ice sheets covered parts of the area. An indirect response to the climate is indicated by sea-level changes. Phases of cooling are characterized by regressions and low sea-level stands; phases of warming are indicated by marine transgressions and high sea levels during the Holsteinian, Eemian and Holocene periods. The seismic characteristics of the different lithological units, the sedimentary sequences and their fossil content are described for the offshore area and the adjacent coastal zone. This provides a record of the interaction of sedimentary processes and the palaeogeographic development as a response to climatic changes.