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Showing papers in "Philosophical Transactions of the Royal Society B in 1977"


Journal ArticleDOI
TL;DR: The efficiency of crop production is defined in thermodynamic terms as the ratio of energy output (carbohydrate) to energy input (solar radiation). Temperature and water supply are the main climatic constraints on efficiency as mentioned in this paper.
Abstract: The efficiency of crop production is defined in thermodynamic terms as the ratio of energy output (carbohydrate) to energy input (solar radiation). Temperature and water supply are the main climatic constraints on efficiency. Over most of Britain, the radiation and thermal climates are uniform and rainfall is the main discriminant of yield between regions. Total production of dry matter by barley, potatoes, sugar beet, and apples is strongly correlated with intercepted radiation and these crops form carbohydrate at about 1.4 g per MJ solar energy, equivalent to 2.4% efficiency. Crop growth in Britain may therefore be analysed in terms of ( a ) the amount of light intercepted during the growing season and ( b ) the efficiency with which intercepted light is used. The amount intercepted depends on the seasonal distribution of leaf area which, in turn, depends on temperature and soil water supply. These variables are discussed in terms of the rate and duration of development phases. A factorial analysis of efficiency shows that the major arable crops in Britain intercept only about 40 % of annual solar radiation and their efficiency for supplying energy through economic yield is only about 0.3%. Some of the factors responsible for this figure are well understood and some are immutable. More work is needed to identify the factors responsible for the large differences between average commercial and record yields.

3,304 citations


Journal ArticleDOI
TL;DR: Preliminary experiments suggest that the layer IVC columns in juvenile macaque monkeys are not fully developed until some weeks after birth, which explains the critical period for deprivation effects in the layerIV columns.
Abstract: Ocular dominance columns were examined by a variety of techniques in juvenile macaque monkeys in which one eye had been removed or sutured closed soon after birth. In two monkeys the removal was done at 2 weeks and the cortex studied at 1\frac{1}{2} years. Physiological recordings showed continuous responses as an electrode advanced along layer IVC in a direction parallel to the surface. Examination of the cortex with the Fink-Heimer modification of the Nauta method after lesions confined to single lateral-geniculate layers showed a marked increase, in layer IVC, in the widths of columns belonging to the surviving eye, and a corresponding shrinkage of those belonging to the removed eye. Monocular lid closures were made in one monkey at 2 weeks of age, for a period of 18 months, in another at 3 weeks for 7 months, and in a third at 2 days for 7 weeks. Recordings from the lateral geniculate body showed brisk activity from the deprived layers and the usual abrupt eye transitions at the boundaries between layers. Cell shrinkage in the deprived layers was moderate - far less severe than that following eye removal, more marked ipsilaterally than contralaterally, and more marked the earlier the onset of the deprivation. In autoradiographs following eye injection with a mixture of tritiated proline and tritiated fucose the labelling of terminals was confined to geniculate layers corresponding to the injected eye. Animals in which the open eye was injected showed no hint of invasion of terminals into the deprived layers. Similarly in the tectum there was no indication of any change in the distribution of terminals from the two eyes. The autoradiographs of the lateral geniculates provide evidence for several previously undescribed zones of optic nerve terminals, in addition to the six classical subdivisions. In the cortex four independent methods, physiological recording, transneuronal autoradiography, Nauta degeneration, and a reduced-silver stain for normal fibres, all agreed in showing a marked shrinkage of deprived-eye columns and expansion of those of the normal eye, with preservation of the normal repeat distance (left-eye column plus right-eye column). There was a suggestion that changes in the columns were more severe when closure was done at 2 weeks as opposed to 3, and more severe on the side ipsilateral to the closure. The temporal crescent representation in layer IVC of the hemisphere opposite the closure showed no obvious adverse effects. Cell size and packing density in the shrunken IVth layer columns seemed normal. In one normal monkey in which an eye was injected the day after birth, autoradiographs of the cortex at 1 week indicated only a very mild degree of segregation of input from the two eyes; this had the form of parallel bands. Tangential recordings in layer IVC at 8 days likewise showed considerable overlap of inputs, though some segregation was clearly present; at 30 days the segregation was much more advanced. These preliminary experiments thus suggest that the layer IVC columns are not fully developed until some weeks after birth. Two alternate possibilities are considered to account for the changes in the ocular dominance columns in layer IVC following deprivation. If one ignores the above evidence in the newborn and assumes that the columns are fully formed at birth, then after eye closure the afferents from the normal eye must extend their territory, invading the deprived-eye columns perhaps by a process of sprouting of terminals. On the other hand, if at birth the fibres from each eye indeed occupy all of lay IVC, retracting to form the columns only during the first 6 weeks or so, perhaps by a process of competition, then closure of one eye may result in a competitive disadvantage of the terminals from that eye, so that they retract more than they would normally. This second possibility has the advantage that it explains the critical period for deprivation effects in the layer IV columns, this being the time after birth during which retraction is completed. It would also explain the greater severity of the changes in the earlier closures, and would provide an interpretation of both cortical and geniculate effects in terms of of competition of terminals in layer IVC for territory on postsynaptic cells.

1,567 citations


Journal ArticleDOI
Pasko Rakic1
TL;DR: Results show that all neurons in the primate visual system have been generated, reached their final positions and formed their basic connections subserving ocular dominance before birth, i.e. before visual experience.
Abstract: Autoradiographic evidence from juvenile rhesus monkeys that had been exposed to a pulse of [3H]thymidine at different embryonic (E) and early postnatal (P) days indicates that all neurons which compose the visual system of this primate have been generated two months before birth. The first retinal ganglion cells (RGC) are generated around E30 preceding by a few days the onset of genesis of neurones destined for the dorsal lateral geniculate body (LGd) and superior colliculus (SC) both of which begin at E36. Production of neurons destined for the primary visual cortex (area 17) begins at approximately E43 and ends by E102. Neurons destined for layer IV, the major target of axons from the LGd, are generated between E70 and E85. The prenatal development of visual connections was studied by the autoradiographic method of anterograde axoplasmic transport in foetuses killed 14 days after unilateral eye injection of a mixture of [3H]proline and [3H]fucose. Initially, in the LGd and in the SC projections from both eyes overlap. Segregation of the axons and/or terminals from the two eyes occurs in the LGd and SC during the middle period of gestation. Transneuronal transport of tritium shows that although LGd axons form the optic radiation before E78, these fibres do not yet enter the developing cortical plate at this foetal age. During the second half of gestation, geniculocortical axons carrying input from each eye invade the cortex but are not yet segregated into ocular dominance columns. Rather, grains are distributed uniformly over the entire layer IV at E124. Three weeks before birth, at E144, segregation of afferents into sublayers IVA and IVC is apparent, and the first hint of ocular dominance columns is displayed by slight differences in grain counts in alternating areas of layer IV. These results show that all neurons in the primate visual system have been generated, reached their final positions and formed their basic connections subserving ocular dominance before birth, i.e. before visual experience. In the SC and LGD, monocular segregation is well established during the middle period of gestation, whereas in the cortex it has begun, but is not fully developed at birth.

719 citations


Journal ArticleDOI
TL;DR: The reeler cerebellum offers a model to study to what extent local cellular interactions are required to achieve the planar organization of the Purkinje cell dendrites and the normal synaptic investment of these cells.
Abstract: The cerebellum of the homozygous reeler mouse shows a marked reduction in size and in the number of fissures, its dry mass and DNA content are respectively $\frac{1}{3}$ - $\frac{1}{4}$ and $\frac{1}{7}$ - $\frac{1}{8}$ of those of normal animals. Its high content in the P $_{400}$ protein, which is abundant in the Purkinje cell, indicates that the decrease in cell number associated with the fall in DNA affects primarily the granular cells. The anatomy of the reeler cerebellum is rather unique: a thin cortex with almost normal molecular, granular and Purkinje cell layers embracing a central mass of closely packed large neurons, mostly Purkinje cells. Purkinje cells may therefore be found in four different cellular environments: (1) at their normal position in the superficial cortical structure; (2) within the granular layer; (3) intermingled with white matter in the central mass; (4) overlapping with neurons of the deep cerebellar nuclei. The reeler cerebellum therefore offers a model to study to what extent local cellular interactions are required to achieve the planar organization of the Purkinje cell dendrites and the normal synaptic investment of these cells. Concerning the three-dimensional shape of Purkinje cells, only the rare ones located at their normal position and receiving a normal ratio of all their synaptic afferences succeed to develop a characteristic dendritic pattern. Purkinje cells within the granular layer show three distinct patterns of dendritic arrangements. The variation in shape of the Purkinje cell dendrites located in the central agranular mass mimics that described in other agranular cerebella: in particular they show randomly oriented dendrites devoid of spiny branchlets. Concerning the cerebellar circuitry, the specificity of most of the synaptic connections is preserved, despite important disorders in Purkinje cell distribution. Several important differences with the normal cerebellum have, however, been observed at the level of the Purkinje cell: (i) The density of climbing fibre varicosities increases in the central cerebellar mass, where Purkinje cells are deprived of parallel fibre afferences. In addition, electrophysiological studies reveal that, at this position, the response of the Purkinje cells to climbing fibre stimulation is graded by steps as a function of stimulus intensity instead of being all-or-none as found in the superficial cortex or in normal cerebellum. These deep Purkinje cells receive therefore several climbing fibres instead of only one as in normal adult cerebellum. (ii) Ectopic synapses (somato-dendritic and dendro-dendritic) between the soma and/or the dendrite of the granule cell as presynaptic element and mainly the Purkinje cell dendrites as postsynaptic element may form. (iii) Heterologous synapses between mossy fibres and Purkinje cell spines are found in the granular layer and within the central mass. The electrophysiological studies show that these synapses are functional.

394 citations


Journal ArticleDOI
TL;DR: The presence of ineffective synapses in the adult offers an alternative to sprouting or the opening up of polysynaptic pathways as a possible mechanism to explain plasticity of connections in adult brains.
Abstract: In this paper, we shall show that there are substantial numbers of nerve terminals which are normally ineffective. In the intact animal, occasional signs of the postsynaptic effectiveness of these fibres can be seen under conditions of optimal spatial summation or increased excitability or decreased inhibition. If the normally functioning afferent nerve fibres are blocked or cut, some of the previously ineffective fibres immediately establish an effective drive of cells. If the normal afferents are cut and allowed to degenerate, large numbers of cells begin to respond to new inputs. The presence of ineffective synapses in the adult offers an alternative to sprouting or the opening up of polysynaptic pathways as a possible mechanism to explain plasticity of connections in adult brains.

327 citations


Journal ArticleDOI
TL;DR: Ecological interactions of the consumers, principally in relation to krill Euphausia superba, are discussed and attention drawn to some of the ways in which ecological separation is achieved, both within and between species.
Abstract: There are fewer species of marine mammals in the Antarctic than in the Arctic, probably because of the wide deep ocean with no geographical barriers to promote speciation. The stocks are substantially larger in the Antarctic and the body sizes of individual species are larger, probably owing to a more abundant food supply. Seasonal changes in the environment in the Southern Ocean are marked and food available to baleen whales is very much greater in summer. Ecological interactions of the consumers, principally in relation to krill Euphausia superba , are discussed and attention drawn to some of the ways in which ecological separation is achieved, both within and between species. Estimates of abundances, biomasses and food requirements are given for the seals and large whales. The original numbers of whales in the Antarctic were far greater than in other oceans, but the stocks have been severely reduced by whaling. This may have increased the availability of krill to other consumers by as much as 150 million tonnes annually. Increased growth rates, earlier maturity and higher pregnancy rates have been demonstrated for baleen whale species, and earlier maturity for the crabeater seal. While it has not been possible to demonstrate increases in the populations of any of these species, the stocks of fur seals and penguins have been monitored and show significant population increases. A key question is whether the original balance of this ecosystem can be regained with appropriate management.

293 citations


Journal ArticleDOI
TL;DR: The findings on modle systems aid the identification of the kinetic and thermodynamic factors responsible for the freezing-thawing survival of living cells.
Abstract: The tendencies to non-equilibrium freezing behaviour commonly noted in representative aqueous systems derive from bulk and surface properties according to the circumstances. Supercooling and supersaturation are limited by heterogeneous nucleation in the presence of solid impurities. Homogeneous nucleation has been observed in aqueous systems freed from interfering solids. Once initiated, crystal growth is often slowed and, very frequently, terminated with increasing viscosity. Nor does ice first formed always succeed in assuming its most stable crystalline form. Many of the more significant measurements on a given system can be combined and displayed in the form of a 'supplemented phase diagram', the latter permitting the simultaneous representation of thermodynamic and non-equilibrium properties. The diagram incorporates equilibrium melting points, heterogeneous nucleation temperatures, homogeneous nucleation temperatures, glass transition and devitrification temperatures, recrystallization temperatures, and, where appropriate, solute solubilities and eutectic temperatures. Taken together, the findings on model systems aid the identification of the kinetic and thermodynamic factors responsible for the freezing - thawing survival of living cells.

278 citations


Journal ArticleDOI
TL;DR: Oxygen isotope measurements have been made in foraminifera from over 60 deep-sea sediment cores as discussed by the authors, which comprises a unique body of stratigraphic data covering most of the important areas of calcareous sediment over the whole world ocean.
Abstract: Oxygen isotope measurements have been made in foraminifera from over 60 deep-sea sediment cores. Taken together with the oxygen isotope measurements published by Emiliani from Caribbean and Equatorial Atlantic cores, this comprises a unique body of stratigraphic data covering most of the important areas of calcareous sediment over the whole world ocean. The oxygen isotopic composition of foraminifera from cores of Late Pleistocene sediment varies in a similar manner in nearly all areas; the variations reflect changes in the oxygen isotopic composition of the ocean. The oceans are mixed in about 1 ka so that ocean isotopic changes, resulting from fluctuations in the quantity of ice stored on the continents, must have occurred almost synchronously in all regions. Thus the oxygen isotope record provides an excellent means of stratigraphic correlation. Cores accumulated at rates of over about 5 cm/ka provide records of oxygen isotopic composition change that are almost unaffected by post-depositional mixing of the sediment. Thus they preserve a detailed record of the advance and retreat of the ice masses in the northern hemisphere, and provide a unique source of information for the study of ice-sheet dynamics.

251 citations


Journal ArticleDOI
TL;DR: In this article, the effects of soil compaction on the potential ability of the soil to produce crops is uncertain, because the relations between soil physical conditions and crop growth are poorly understood.
Abstract: Intensive arable production normally involves an increasing frequency of cultivation, and hence an increasing probability of soil compaction, particularly if the soil is cultivated when it is wet. The effects on the potential ability of the soil to produce crops is uncertain, because the relations between soil physical conditions and crop growth are poorly understood. To measure damage requires improved techniques for measuring pore size distribution, as well as pore continuity and stability. Most damage takes the form of a reduction in the number of transmission pores (those greater than 0.05 mm, equivalent cylindrical diameter) although storage pores may also be lost in intensively cultivated soils. Where damage is primarily in the surface soil it can usually be repaired by appropriate cultivations. Where compaction of subsurface soil occurs, or blockage of subsurface pores by dispersion of clay from disrupted aggregates, the damage is less easily repaired. Soils differ considerably in their ability to withstand intensive cultivation. The behaviour of soil aggregates on immersion in water provides a useful guide to those likely to suffer more permanent damage, and those where increasingly intensive production can be safely practised.

238 citations


Journal ArticleDOI
TL;DR: Prediction of the population structure for both plants and herbivores may be a more attainable objective of theory and more practically important than prediction of total biomass at each trophic level.
Abstract: Interactions of herbivorous copepods with their phytoplankton food depend on the size composition of organisms in both trophic levels. A simulation model is used to analyse these size-dependent relations with the following conclusions. 1. Relative size structure of herbivores and their food is more important than total biomass of each trophic level in determining modes of transfer from plants to herbivores. In nearly all cases, in the model, food limitation affects reproduction or the first feeding stage of the nauplii. 2. No single factor emerges as predominant in determining the size structure of both populations. 3. The nature of predation on the herbivores is at least as important in determining both phytoplankton and herbivore size composition as physical or nutrient parameters. 4. The magnitude of the population of the larger herbivores such as Calanus, important as food for fish, depends on their coexistence with the smaller copepod species which control the smaller phytoplankton. 5. Stress on the system, if it affects adversely the smaller herbivores, can lead to the breakdown of the Calanus-diatom component. 6. Prediction of the population structure for both plants and herbivores may be a more attainable objective of theory and more practically important than prediction of total biomass at each trophic level.

224 citations


Journal ArticleDOI
TL;DR: Age determinations from the teeth of 195 breeding cows reveal a low mean age, early first breeding and a predominance of the younger age groups relative to the age structure of a stable population of northern fur seals Callorhinus ursinus.
Abstract: The Antarctic fur seal Arctocephalus gazella is a polygynous Otarid in which a single pup is produced annually by cows over the age of 2 years. Following exploitation to the verge of extinction, a small breeding colony was discovered at Bird Island, South Georgia, in the 1930s. Up to 10 000 pups a year were produced in the early 1960s and by 1975 the figure had reached an estimated 90 000. The rapid population increase has resulted in the colonization of extensive breeding areas on the adjacent mainland of South Georgia with incipient colonies springing up on more distant parts of the island, and also in the South Sandwich, South Orkney and South Shetland Islands. Age determinations from the teeth of 195 breeding cows reveal a low mean age, early first breeding and a predominance of the younger age groups relative to the age structure of a stable population of northern fur seals Callorhinus ursinus. Annual adult cow survival is estimated at 89.8%, while that of first-year animals is about 64.5%. A decline in the rate of population increase is forecast within ten years and an outline for investigating the most likely factors influencing such a change is suggested.

Journal ArticleDOI
TL;DR: In this paper, the authors used stable isotopic ratios to determine the former climate of ice sheets and found that changes of isotopic $delta$ ratios have been proportional to changes of surface temperatures; this relationship is discussed in terms of the physical processes involved.
Abstract: The paper deals primarily with the use of stable isotopic ratios to determine the former climate of ice sheets. Studies of temperature profiles throughout ice sheets have shown that for at least several thousand years, changes of isotopic $\delta$ ratios have been proportional to changes of surface temperatures; this relationship is discussed in terms of the physical processes involved. It is considered reasonable to use a similar relation for earlier periods in Antarctica, but in Greenland the relation may have varied with time. When determining past climates from the isotopic record, allowances have to be made for changes in the flow and thickness of ice sheets during major glacial periods. These factors are considered in relation to major ice cores from Vostok and Byrd stations in Antarctica and from Camp Century in Greenland. Vostok is the simplest case glaciologically, Camp Century the most complex. On purely glaciological grounds it appears that the ice age gave way to present-day climates some 10 000 $\pm$ 1000 a B.P., the coldest period being 20 000 $\pm$ 3000 a B.P., when the climate in Antarctica was 6-8 $^\circ$ C colder than at present. Glaciological data suggest a duration of 50 000 to 100 000 years for the last ice age. Before this period, climates in Greenland and Antarctica appear to have been around 2-3 $^\circ$ C warmer than at present.

Journal ArticleDOI
TL;DR: In this article, the rates of response of northeast Atlantic surface waters to large-scale palaeoclimatic changes have been reconstructed in two forms: (1) as changes through time of estimated temperature at selected points in space, and (2) as movements through space of the polar front during particular intervals in time.
Abstract: Estimated rates of response of northeast Atlantic surface waters to large-scale palaeoclimatic changes have been reconstructed in two forms: (1) as changes through time of estimated temperature at selected points in space, and (2) as movements through space of the polar front during particular intervals in time. Three periods of rapid change between the glacial and interglacial climatic extremes were examined: a short cold episode during isotopic stage 7 (the next-to-last interglaciation); the deglacial warming into the last interglaciation at the isotopic stage 6/5 boundary; and the most recent deglacial warming into the present interglaciation at the isotopic stage 2/1 boundary. Changes in sea-surface temperature of 7-11 °G (estimated from transfer function analysis) are characteristically registered in these cores in a few thousand years. The corresponding temporal rates of cooling or warming recorded usually average 1-5 °C/1000 years (a) for the complete climatic shift. During local passage of the polar front, these rates are even higher. Regional advance and retreat rates of the polar front along a NW/SE axis from Cape Farewell, Greenland, to Spain characteristically range from 200 to 1600 m/a during these intervals. These estimates represent the rates of change presently recorded in the sediments. The actual (faster) rates of oalaeo-oceanographic change in the overlying North Atlantic surface waters will only be determined once the smoothing effects of vertical mixing can be removed.

Journal ArticleDOI
TL;DR: For the period ca. 15 000-10 000 years ago, which spans the interval between the latest presence of lowland ice and the final disappearance of mountain glaciers from Britain, so many botanical data are now available that it is possible to analyse plant distributions and vegetation composition in detail not yet possible for earlier periods as mentioned in this paper.
Abstract: For the period ca. 15 000-10 000 years ago, which spans the interval between the latest presence of lowland ice and the final disappearance of mountain glaciers from Britain, so many botanical data are now available that it is possible to analyse plant distributions and vegetation composition in detail not yet possible for earlier periods. Species lists, provided by identification of pollen and of macroscopic remains, show a combination of diverse phytogeographic elements into an assemblage characteristic of the period - an admixture of apparently thermophilous plants with those now found in northern and arctic situations, and of oceanic and steppe elements. Ordination of data on floras has revealed much similarity between the regions of Britain, but pollen analysis continues to emphasize how strong must have been the regional differentiation of vegetation. A comparison of pollen diagrams published since 1970, from sites lying on a broad north-south transect of western Britain, shows much variety in the pollen assemblage zones which have been distinguished, and in the vegetation interpreted from the pollen data by comparison with surface samples and modern vegetation. Sufficient $^{14}$ C dating is now available, however, to permit chronostratigraphic correlation of these pollen zones, and to show that there is consistent evidence for climatic amelioration at about 13 000 B.P., and rapid deterioration at about 11 000 B.P., leading to conditions of incomplete vegetation cover and universal soil disturbance which can be correlated with geomorphological evidence for the recrudescence of mountain glaciation in western Scotland, the Lake District, and North Wales, in the period called Younger Dryas on the continental mainland. Between 13 000 and 11 000 there is in western Britain evidence for a woodland biozone, or palaeobotanical interstadial, equivalent to Bolling plus Allerod of continental stratigraphers (Mangerud, Andersen, Berglund & Donner 1974), and divided by a very minor regression of vegetation during Older Dryas time (ca. 12 000-11 800). The now closely $^{14}$ C-dated site at Low Wray Bay, Windermere, is described in detail, and suggested as a reference site for this interstadial.

Journal ArticleDOI
TL;DR: The three major features of meiosis are first synapsis, then exchange, and finally, disjunction of homologous chromosomes; these phenomena occur before pachytene, during pachyTene, and after pachy tene respectively.
Abstract: The three major features of meiosis are first synapsis, then exchange, and finally, disjunction of homologous chromosomes; these phenomena occur before pachytene, during pachytene, and after pachytene respectively. The effects of meiotic mutants, or other perturbations, either endogenous or exogenous, on the meiotic process may be assigned tentatively to one of these intervals, based on the earliest discernible abnormality. Thus mutants exhibiting abnormal disjunction and normal exchange affect post-pachytene functions; mutants exhibiting abnormal disjunction and exchange but with ultrastructurally normal appearing synaptonemal complex affect pachytene functions; and mutants with abnormal disjunction, exchange, and synaptonemal complex affect prepachytene functions. This rationale is applied to the temporal seriation of effects of meiotic mutants and chromosomal abnormalities on the meiotic programme.

Journal ArticleDOI
TL;DR: The sensitivity of the blue-sensitive mechanism of the eye appears to be controlled not only by quanta absorbed by the blue receptors but also by a mechanism with a different spectral sensitivity.
Abstract: Adaptation of the human eye to long-wavelength light leaves it insensitive to short-wavelengths: a blue flash that is visible in the presence of a yellow adapting field may remain invisible for several seconds after the field has been turned off (see experiment 1 and Appendix). This ‘transient tritanopia’ occurs for a large range of adapting intensities, but is abolished if the adapting field is very bright (experiment 2). The loss of sensitivity is primarily confined to the blue-sensitive cone mechanism (experiments 2 a , 3 and 4 ; and Appendix) and can be produced by small attenuations of the adapting field (experiment 5). It occurs in both foveal and parafoveal vision (experiment 6) but is absent when adapting and test stimuli are presented to opposite eyes (experiment 7). It was found in a protanope (experiment 9 a ) and, in a modified form, in a deuteranope (experiment 9 b ). No differences in sensitivity were found for blue flashes presented in the light and dark phases of a field flickering at a rate above the fusion frequency (Appendix). The sensitivity of the blue-sensitive mechanism of the eye appears to be controlled not only by quanta absorbed by the blue receptors but also by a mechanism with a different spectral sensitivity

Journal ArticleDOI
TL;DR: The observed duration of meiosis is the result of a complex genotype-nucleotype-environment interaction and has been suggested that DNA influences the rate of meiotic development in two ways: first through its informational content (the genotype), and second indirectly by the physical and mechanical effects of its mass independently of its informationalcontent.
Abstract: Ever since meiosis was recognized as a process there has been a continuing interest in its temporal aspects. Two main types of meiotic timing experiments have been conducted: first, experiments to estimate the duration of meiosis (and sometimes its stages); second, experiments to locate the sensitive stage(s) when exposure of meiocytes to various treatments can affect meiotic chromosome behaviour (e.g. pairing or recombination). Such experiments have played an important role in increasing our under-standing of the meiotic process. The duration of meiosis has been estimated in about 70 organisms, including two prokaryotes (yeast and Chlamydomonas) and the following eukaryotes: 1 Basidiomycete (Coprinus lagopus), 2 Gymnosperms (Larix decidua and Thuja plicata gracilis), at least 39 angiosperms, and at least 26 animal species. The duration of female meiosis has been estimated in far fewer species than male meiosis. However, estimates of the duration of female meiosis are available for 6 angiosperms, Drosophila melanogaster, Xenopus laevis, and several mammals. Comparison of these data shows that the duration of meiosis is one of the most variable aspects of the meiotic process, ranging from less than 6 h in yeast to more than 40 years in the human female. Developmental holds at different stages of meiosis are common in plants and animals, and inevitably prolong the meiotic division. However, even among species without developmental holds, the duration of meiosis is very variable. For instance, in animals it ranges from about 1-2 days in male Drosophila melanogaster to more than 24 days in male Homo sapiens and several Orthopterans. Despite the large variation in the duration of meiosis three generalizations can be made: (i) first prophase is always very long compared with the remaining meiotic stages, (ii) the rate of meiotic development is very slow compared with the rate of development in dividing somatic meristem cells of the same organisms under the same conditions, (iii) the duration of meiosis is characteristic of the genotype and species. Four main factors have been recognized which effect or determine the duration of meiosis, namely (1) environmental factors (e.g. temperature); (2) nuclear DNA content; (3) ploidy level of the organism; and, (4) the genotype. Because nuclear DNA content plays a major role in determining the duration of meiosis, it has been suggested that DNA influences the rate of meiotic development in two ways: first through its informational content (the genotype), and second indirectly by the physical and mechanical effects of its mass independently of its informational content (i.e. the nucleotype). Thus, the observed duration of meiosis is the result of a complex genotype-nucleotype-environment interaction. With the obvious exception of variation caused by developmental holds, changes in the duration of meiosis usually involve proportional changes in the durations of all its stages. This is true irrespective of whether the variation in meiotic time is associated with changes in temperature, nuclear DNA amount, ploidy level, or sex difference. While results for animal species show some evidence of a similar phenomenon, the relative proportions of meiosis taken by individual meiotic stages is clearly much more variable between animal species than between plants. The duration of meiosis often has much wider implications for the organism than those affecting the meiotic process per se. Examples are given from which it is concluded, first, that the duration of meiosis can limit the type of life cycle which a species can display; and second, that in many species the duration of the meiosis is an essential adaptive feature suited to its life cycle type in its normal environment.

Journal ArticleDOI
TL;DR: A chronic cat preparation with long-term optical reversal of vision has now been developed and shows similar adaptive and recovery changes at low test stimulus amplitudes, but different patterns of adaptive response at high amplitudes.
Abstract: Human subjects with maintained reversal of their horizontal field of vision exhibit very substantial adaptive changes in their 'horizontal' vestibulo-ocular reflex (v.o.r.). Short durations (8 min) of vision reversal during natural head movement led to 20% v.o.r. attenuation while long periods (4 weeks) eventually led to approximate reversal of the reflex. The reversed condition is approached by a complex, but highly systematic, series of changes in gain and phase of the reflex response relative to normal. Recovery after return to normal vision exhibits a similar duration, but different pattern, to that of the original adaptation. A chronic cat preparation with long-term optical reversal of vision has now been developed and shows similar adaptive and recovery changes at low test stimulus amplitudes, but different patterns of adaptive response at high amplitudes. An adaptive neural model employing known vestibulo-ocular pathways is proposed to account for these experimentally observed plastic changes. The model is used to predict the adapted response to patterns of stimulation extending beyond the range of experimental investigation.

Journal ArticleDOI
TL;DR: The association between extrafoveal cone outer segments and pigment epithelial cells was studied by transmission electron microscopy in three human retinas and Digestion of phagosomes appears to begin in the apical processes.
Abstract: The association between extrafoveal cone outer segments and pigment epithelial cells was studied by transmission electron microscopy in three human retinas; ages 5,45 and 60. The pigment epithelial apical surface from a fourth human retina, age 38,was viewed in the scanning electron microscope. Multiple villous-like apical processes protrude from the pigment epithelium into the space above each cone. Sometimes one or more of these processes is sheet-like in form and contains a wealth of intracellular organelles, including mitochondria. One or more of the villous-like procesess reaches the cone and expands to ensheath the upper one-third of the outer segment. Llike vertebrate rods, extrafoveal human cones shed their terminal disks in packets and these packets are phagocytosed by the ensheathing apical processes. The phagosomes then ascend in the processes toward the pigment epithelia soma. Digestion of phagosomes appears to begin in the apical processes.

Journal ArticleDOI
TL;DR: In this paper, radio echo sounding methods are used to provide reconnaissance mapping of the Antarctic ice sheet on a continental scale, which yields significant information on the internal deformation of the ice sheet, on the flow of ice shelves and the nature of the bottom surface of the sea ice.
Abstract: The main Antarctic ice sheet is so vast that large-scale international programmes are necessary for an adequate study of its dynamics, thermodynamics and past history. Radio echo sounding methods are providing reconnaissance mapping of the ice sheet on a continental scale. The soundings also yield significant information on the internal deformation of the ice sheet, on the flow of ice shelves and the nature of the bottom surface of the ice. This information is being used to amplify and extrapolate detailed results from other groups who are studying the mass balance, ice movement, and past history by deep drilling; it is also of value in isolating critical sites for future studies. Reconnaissance mapping has now shown the presence of sub-ice lakes larger than those reported in 1973. Studies of radio wave reflexions from internal layers which are believed to be former depositional surfaces, have shown that the conformity between these layers and the bedrock relief decreases as ice movement increases. Statistical analyses of bedrock relief and radio wave reflexion parameters are being used to delimit major geological provinces beneath the ice and investigate the structural and lithological transition between the Transantarctic Mountains and East Antarctica. Such studies have recently indicated the presence and structure of two large intracratonic sedimentary basins in eastern Antarctica.

Journal ArticleDOI
TL;DR: In this paper, features of the global cycle of fixed nitrogen are reviewed with an emphasis on perturbations due to man, and it is argued that agricultural practices and combustion may lead to an increase in the concentration of atmospheric N2_O with consequent effects on O_3.
Abstract: Features of the global cycle of fixed nitrogen are reviewed with an emphasis on perturbations due to man. It is argued that agricultural practices and combustion may lead to an increase in the concentration of atmospheric N2_O with consequent effects on O_3. The level of O_3 may drop by about 20% over the next 100 years if world population and the demand for food should continue to grow at anything like rates which prevailed in the recent past. Uncertainties in the model are highlighted and note is taken of areas where there is need for additional data.

Journal ArticleDOI
TL;DR: The behaviour of human and frog red cells, platelets and rigid spheres were studied in the annular vortex formed in steady or pulsatile flow at the sudden concentric expansion of a 151 $\mu$ m into 504 $\mu $ m diameter glass tube.
Abstract: The behaviour of human and frog red cells, platelets and rigid spheres were studied in the annular vortex formed in steady or pulsatile flow at the sudden concentric expansion of a 151 $\mu$ m into 504 $\mu$ m diameter glass tube. During a single orbit the measured particle velocities and paths in steady flow were in good agreement with those calculated for the fluid, predicted by theory to circulate in closed orbits. Over longer periods, however, single blood cells and latex spheres $\mu$ m diameter migrated across the streamlines out of the vortex at a rate depending on the Reynolds number whereas spheres and aggregates of red cells > 30 $\mu$ m diameter remained in the vortex at all Reynolds numbers. Similar behaviour was noted in pulsatile flow when the vortex moved in phase with upstream fluid velocity and particles described spiral orbits of continually changing diameter. With red cell suspensions of 15-45% haematocrit in steady flow, migration of the corpuscles was also observed and resulted in the formation of a particle-free vortex. In pulsatile flow, cells were always present in the vortex, but their concentration which varied periodically was lower than that in the mainstream. The formation of aggregates of latex spheres and human platelets through collisions occurring in orbit, and their migration to the vortex centre was also observed.

Journal ArticleDOI
TL;DR: The vegetation of the Late Devensian period in Ireland is reviewed in this paper, where a sequence of phases of vegetation development for the Dublin region is described and regional variation elsewhere in Ireland discussed.
Abstract: The vegetation of the Late Devensian period in Ireland is reviewed. New investigations at Ballybetagh, Co. Dublin, Dunshaughlin, Co. Meath, Glenveagh, Co. Donegal and Poulroe, Co. Clare, are reported. A sequence of phases of vegetation development for the Dublin region is described and regional variation elsewhere in Ireland discussed. Pollen influx values for two Late Devensian sites in southeastern Ireland are reported. A case is made that the Juniperus-Empetrum phase between 12 400 and 12 000 B.P. was the warmest phase of the Late Devensian. The reason for absence of birch woodland in late-glacial Ireland is discussed. Evidence for widespread soil erosion at the end of the Juniperus-Empetrum phase is presented. The occurrence of a corrie glaciation at Lough Nahanagan in the Wicklow Mountains in the Artemisia phase is documented. A radiocarbon chronology for events in the Late Devensian of Ireland is proposed.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the stratigraphy of the Pleistocene sequence in the area of the Vale of St Albans and the results used to link the sequences in East Anglia and the Middle Thames region.
Abstract: The stratigraphy of the Pleistocene sequence in the area of the Vale of St Albans has been investigated and the results used to link the sequences in East Anglia and the Middle Thames region. Lithological units have been characterized using various methods and thirteen formal lithostratigraphical units are proposed. The bulk of the deposits are included in the Anglian glacial stage during which two major ice advances occurred. Both advances dammed the eastward-flowing river in the Vale causing, respectively, a minor and a more extensive proglacial lake. The river was finally diverted and after the ice retreat a westward-flowing river occupied the Vale. Organic remains fill kettle holes resting on the glacial deposits. A new site at London Colney shows a late Anglian to Hoxnian Ho1 pollen sequence. Wolstonian soliflucted gravels overlie the hollow fills. It is probable, since pre-Ipswichian and Ipswichian deposits occur near the present river level, that the modern river valleys were initiated during this stage. Overlying the Ipswichian deposits in river valleys and on interfluves is brown silt, containing a substantial loess component. This is considered to represent early Devensian river aggradation. A period of downcutting then occurred, before the accumulation of valley gravels which contain organic horizons radiocarbon-dated to the Late Devensian. The sequence is extended north to Stevenage and Hitchin and east into Essex. The Middle Thames Terrace sequence is equated with the Vale using relative heights and lithology, showing that the Thames flowed through the Vale until late Anglian times, when it was diverted southwards and took up a course close to that which it follows today.

Journal ArticleDOI
TL;DR: It is concluded that water in these systems is highly mobile, that water molecules affected directly by a macromolecule tumble anisotropically about all axes relative to the macromolescule with correlation times in the region of 10 -9 s at 260 K and that these molecules exchange with water molecules free of the influence of the macROMolecule with a lifetime in the dynamically oriented state of the order of 10-6 s at temperatures around 300 K.
Abstract: The basic principles of nuclear spin relaxation, dielectric relaxation and quasielastic neutron scattering and their use in studying the motions of water molecules are outlined. A summary is given of the time scales associated with the translational and rotational motions of water molecules and of intermolecular proton exchange in pure liquid water. A model is then proposed for the dynamics of water molecules in heterogeneous systems involving regions having differing compositions, water molecules within each region existing in environments both affected by interaction with the macromolecular components and free of their influence and including exchange of water molecules between different environments and regions. The lifetime of the interaction of water molecules with the macromolecular components is assumed long compared with the time for rotation of such bound molecules. Exchange of protons between water molecules and between water molecules and macromolecules is also considered. The ways in which such processes would be expected to affect the observed nuclear magnetic resonance, dielectric and neutron scattering behaviour are outlined. Particular emphasis is placed on nuclear spin relaxation phenomena and the existence and observation of residual dipolar and quadrupolar splittings in the n.m.r. spectra of 1 H and 2 H (D) nuclei in water molecules in such systems, these splittings arising from water molecules dynamically oriented at water/macromolecule interfaces. Details are then given of particular studies of water molecule dynamics in heterogeneous systems using n.m.r., dielectric and neutron scattering techniques. The systems discussed include moist protein powders, protein solutions, phospholipid/ water and soap/water mesophases, clay/water systems and biological polymers and tissues. It is concluded that water in these systems is highly mobile, that water molecules affected directly by a macromolecule tumble anisotropically about all axes relative to the macromolecule with correlation times in the region of 10 -9 s at 260 K and that these molecules exchange with water molecules free of the influence of the macromolecule with a lifetime in the dynamically oriented state of the order of 10 -6 s at temperatures around 300 K. The ability of nuclear magnetic relaxation studies to distinguish water in different regions of a tissue is discussed and examples are given of the study of the rate of water transport across membranes using these techniques.

Journal ArticleDOI
TL;DR: Six species of atyid prawns, representing five genera, occur in streams on the West Indian island of Dominica and the ecology and habits of each are described and the relation of features of gross morphology to ways of life noted.
Abstract: Six species of atyid prawns, representing five genera, occur in streams on the West Indian island of Dominica (figures 1-6). The ecology and habits of each are described and the relation of features of gross morphology to ways of life noted. Xiphocaris elonga t, the most primitive living atyid, is a lightly built prawn whose adult habits are related to fife in quiet pools in streams. An agile species and an excellent swimmer, it picks up individual small food particles with specialized chelipeds (figures 18 an d 19) that differ from those of all other atyids and manipulates them with mouth parts (figure 77) which, while highly complex, are more primitive than those described for any other member of the family. Atya innocous and A. scabra , representing perhaps the most specialized atyid genus, are very similar in gross morphology and are robustly built ambulatory species. A. innocous is common in a variety of situations: A. scabra is rare and has been found only in fast-flowing water. Both have chelipeds whose three distal segments are extremely specialized (figure 36) and whose propus and dactylus are armed with an exceedingly complex array of long, slender bristles. These can be used either as brushes for collecting finely particulate detritus (figures 58-60) or as filtering fans (figures 68 and 69) which, held passively in flowing water, extract suspended particles. The Atyidae is unique among the Malacostraca in having representatives that filter passively by means of the chelipeds. The bristles (figure 40) are extended (figure 49), not by muscles, of which there are none in the distal parts of the propus and none anywhere in the dactylus, but by hydraulic forces. The return of the bristles to rest is by means of a cuticular spring. Some of the bristles of A. innocous are armed distally with minute denticles (figures 41 and 42) that facilitate scraping and sweeping: no such are present in A. scabra . The difference is related to the relative importance of scraping in the two species: A. innocous scrapes frequently, A. scabra seldom. Finely particulate food is transferred and manipulated by the extremely complex oral machinery (figure 78). One of the most elaborate parts of this is a teaselling device in which components of the maxillae and first maxillipeds participate (figures 80 and 81). The feeding mechanism is described. Morphologically and functionally Micratya poeyi can be regarded as a miniature version of Atya . It can both sweep and filter. Potimirim glabra is rare in Dominica and its habits but little known. Morphologically it is similar to, but more primitive than, Micratya . Its cheliped bristles are clearly specialized for sweeping and show few signs of being used for passive filtration. Jonga serrei occupies a separate and well-defined niche in the quieter parts of streams. For this it shows many morphological specializations and lacks such attributes as stout claws and robust walking legs that are the hallmark of its relatives living in fast-flowing waters. Its chelipeds are armed with distal scrapers that bear a remarkable similarity to those already described for African species of Caridina , to which animal it bears a general overall similarity. These are used for collecting food from substrates. It is incapable of passive filtration. Atyid mandibles, while specialized- greatly so in the case of Atya (figure 83) - retain primitive features both of the skeleton and musculature such as are found even in the Branchiopoda. Retention of a large molar process is clearly associated with microphagous habits. Specialization for such habits has led to end points very different from those of advanced decapods such as crabs and crayfishes whose mandibles often slice material from large food masses. Specialization has been achieved by additions to and refinements of the primitive crustacean mandible whose salient features are still retained. Comparison of Xiphovaris (primitive) and Atya (advanced) reveals many functional trends in the evolution of food manipulation. These two genera do not, however, simply represent primitive and derived conditions respectively but also end points of divergent specialisation. Likewise, Jonga, Potimirim and Micratya , while to some extent indicating stages on the route to an Atya -like condition, also indicate divergent specialization along that route. All Dominican atyids have a complex two-chambered fore-gut. The most specialized, that of Atya (figures 91 and 92), is described in detail. The complex systems of ossicles developed in its walls are specialized, not for the crushing and grinding of large items of food as they are in many 9higher9 decapods but for propelling fine particles posteriorly. Elaborately guarded channels (figures 114-119) from the gland filter that run along each side of the cardio-pyloric valve are presumed to be the route along which enzymes from the hepatopancreas pass into the cardiac chamber. Within the pyloric chamber a system of tubules made up from a delicate convoluted membrane (figures 111 and 112) ensures that there are wide spaces between the several strings of food particles thus separated (figures 105 and 106), thereby facilitating the digestive processes. The membrane is analogous to, and perhaps homologous with, a peritrophic membrane, but is permanent and not continuously renewed. Jonga, Potimirim and Micratya have a fore-gut similar to that of Atya . That of Xiphocaris is different. Apart from a somewhat different arrangement of ossicles the method of spreading food particles so as to expose a large surface area for efficient digestion and absorption is not by means of a reticulated membrane but by a spinule-covered projection that confines particles to a narrow tunnel, crescentic in transverse section (figures 93 and 94). A decapod fore-gut lacking heavy teeth and grinding ossicles is not, as has been suggested, less efficient than one that employs such structures, nor is a large molar region of the mandible necessarily indicative of a crushing function, and non-crushing mandibles are certainly not inefficient. Different foods require different treatments by both mouthparts and fore-gut, both of which show appropriate specializations. Atyids, of which there are no known marine species, have probably had a long history as freshwater animals. Fossils are known from freshwater deposits of Cretaceous age in Brazil. The occurrence of some species in brackish waters is not necessarily indicative of colonization of such habitats from the sea. Many continental species certainly reproduce in freshwater and it is likely that some of those living on islands also do so. Dispersal by sea as adults is virtually impossible and such physiological evidence as is available gives little confidence in the ability of larvae to serve as agents of dispersal across large tracts of ocean. Paradoxically atyids have colonized several remote oceanic islands, some of which have not been reached by truly freshwater fishes. Thus, while dispersal appears to have presented few problems in the past, how it was, or is, effected is uncertain. In the West Indies there is no correlation- such as holds good for example in birds-between island size and number of epigean atyids. To these prawns each stream is the analogue of an island and it seems that a small island like Dominica, with many streams, may present a similar array of habitats to a large island such as Cuba. When ecological opportunities are increased by the provision of additional habitats, such as caves, the number of species of atyids is increased.

Journal ArticleDOI
TL;DR: The spire index is a fairly adequate measure of the shape of the coiled shell of most terrestrial and freshwater gastropod shells but less so in complex marine shells with thorns, flanges and spouts, and it is suggested tentatively that they may be related to the positions in which different species normally walk and hence to their preferred feeding places.
Abstract: The spire index (height/maximum diameter of shell) is a fairly adequate measure of the shape of the coiled shell of most terrestrial and freshwater gastropod shells but less so in complex marine shells with thorns, flanges and spouts. In this study, only adult free-crawling forms with several whorls, able to retract completely into the shell, are considered. In the Stylommatophora of the Western European terrestrial fauna the distribution of the spire index is markedly bimodal, the modes, with values of about 3 and about 0.5, corresponding respectively to shells with a high to very high spire (and small spire angle) and those varying from more or less globular or trochoid to very flattened and disk-like (spire angle from 60 $^\circ$ to 180 $^\circ$ ). The same two modes are found in the taxonomically different terrestrial stylommatophorans of the U.S.A., and in the faunas of Puerto Rico (Caribbean) and New Caledonia (southwest Pacific). Basommatophorans also show two, rather different, modes. North American marine archaeogastropods are mainly equidimensional but with a few disk-like forms and a very few high-spired ones, marine mesogastropods are mainly high-spired but with disk-like forms, neogastropods high-spired, and relevant euthyneurans sharply bimodal, like the stylommatophorans. Fossil archaeogastropods of the Palaeozoic were much more various at first than modern forms. There is some indication that they became restricted in variety as caenogastropods became abundant, but also that the proportion of marine disk-like shells has decreased markedly since the Palaeozoic. Modes of h/d are characteristic of large taxonomic groups but not taxonomically restricted since given values may appear as specific, generic or subfamilial variants from a mode, and appear sporadically in unrelated forms. There is also no broad association between modal value and broad ecological characters. Since nearly all values do occur in some group or other, no mechanical requirement can be invoked to explain such variation. In the land Stylommatophora enough is known of the broad ecology to suggest that in extreme habitats species with very different size or shell-shape may occur together, and that generalized feeders with similar shells may show separation, ecological or geographical (but in that case, also ecological). Since different shapes of shell will have different mechanical characteristics when considered as burdens to be carried, it is suggested tentatively that they may be related to the positions in which different species normally walk and hence to their preferred feeding places. This would explain an apparent tendency for different taxonomic groups to occupy the same part of the scatter of h/d in different regions of the world, for many groups in the same region to occupy different portions of the scatter, and perhaps the apparent exclusion by caenogastropods of archaeogastropods from part of the scatter since the Palaeozoic. It is argued that the distributions discovered are explicable only by natural selection.

Journal ArticleDOI
TL;DR: It is concluded that the antorbital vacuity, which characterizes the Loxommatoidea, evolved as a bulging hole for a large pterygoideus muscle associated with a piscivorous habit and a kinetic inertial system of jaw closure.
Abstract: The labyrinthodont superfamily Loxommatoidea is now divided into two families. The highly aberrant Spathicephalus is placed in a new family, the Spathicephalidae, to be described elsewhere. The family Loxommatidae is retained for the remaining genera, Loxomma, Megalocephalus and Baphetes. Additional material and further preparation has made possible a redescription of the three loxommatid genera and new skull restorations have been produced for most species. In particular the loxommatid braincase and palatoquadrate are reconstructed for the first time; in many features their structure is more primitive than that hitherto described for any temnospondyl. Since an intertemporal bone is found to be a feature of Baphetes as well as Loxomma, these two genera have been separated on the basis of skull shape and on stratigraphical grounds. A specimen from the Communis zone, Westphalian A, is attributed to Loxomma, as L. rankini sp.nov., while 'Loxomma bohemicum' has been transferred to the genus Baphetes as B. bohemicus (Fritsch). A further specimen, originally associated with Macrerpeton, has also been referred to this genus as B. lintonensis sp.nov. The skull of Megalocephalus pachycephalus can be described in greater detail than that of any other loxommatid species and thus forms the basis for discussion of the functional morphology. Jaw muscles are reconstructed for this species and it is concluded that the antorbital vacuity, which characterizes the Loxommatoidea, evolved as a bulging hole for a large pterygoideus muscle associated with a piscivorous habit and a kinetic inertial system of jaw closure. Consideration of the mechanics of jaw closure sheds light on a further enigma, i.e. the function of the basipterygoid articulation in the primitive temnospondyl skull. The loxommatid skull is considered divisible into two units. The presence of a specialized cranial joint between the quadrate and quadratojugal allows potential for any movement at the basal articulation to be accommodated in the main at this site. It is suggested that the system described for loxommatids represents an alternative design to the mobile cheek region of anthracosaurs and that the articulations represent zones of elasticity, which accommodate the stresses on the skull caused by a powerful jaw musculature.

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TL;DR: It has been proposed that one component of the synaptonemal complex (s.c.) is a filamentous pairing protein with DNA binding sites which are base sequence specific and suggested that interference may depend on the presence of a limited amount of another DNA binding protein which is specifically located within the s.c.
Abstract: Although exchanges between sister chromatids are common in mitotic cells, those involving homologous chromosomes are rare. Since recombination between homologues is one of the functions of meiosis, it follows that one aspect of the differentiation of the meiocyte involves the synthesis of proteins or enzymes which facilitate synapsis and exchange. Mutants are known which seem to have constitutive levels of mitotic recombination between homologues, and these may be defective in the mechanism which normally represses mitotic recombination. It has been proposed that one component of the synaptonemal complex (s.c.) is a filamentous pairing protein with DNA binding sites which are base sequence specific. Synapsis occurs because the distribution of these sequences is the same in homologues. When only non-homologous chromsomes are present, as in haploid meiosis, only weak pairing can occur, since the base sequences are largely out of register. Although certain features of recombination at the molecular level are known, none of the models so far proposed suggest an explanation for interference between crossovers. It is suggested that interference may depend on the presence of a limited amount of another DNA binding protein which is specifically located within the s.c. A crossover between naked DNA molecules is initially a weak structure, which must be later converted into a visible and mechanically strong chiasma. It is assumed that this stabilization of a crossover is achieved by the DNA binding protein, which can diffuse freely within the s.c. and bind cooperatively to any recombinant DNA molecules within it. Depletion of the binding protein within the vicinity of a crossover makes it unlikely that the second crossover can be formed nearby.

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TL;DR: It is suggested that bilateral ear asymmetry in owls serves to make the vertical directional sensitivity patterns different between the two ears for high frequencies, thus making possible vertical localization based on binaural comparison of intensity and spectral composition of sound.
Abstract: On the basis of the literature and my own examination of living and/or dead but fresh owls of 16 species, bilateral asymmetry of external ears in owls is surveyed and ear structure briefly described. Consideration of the probability of origin of various structural similarities and dissimilarities in the ear leads to the conclusion that ear asymmetry has evolved independently in at least five lines, represented by the respective genera (1) Tyto , (2) Phodilus , (3) Bubo , Ciccaba , Strix , (4) Rhinoptynx , Asio , Pseudoscops , and (5) Aegolius . Bubo , Ciccaba , and Strix probably represent more than one line of origin of ear asymmetry. Available evidence suggests that bilateral ear asymmetry in owls serves to make the vertical directional sensitivity patterns different between the two ears for high frequencies, thus making possible vertical localization based on binaural comparison of intensity and spectral composition of sound. When an owl localizes prey by hearing, the direction of the source usually forms a shallow angle with the ground. Therefore, a certain angle of error usually converts into a longer distance along the ground for a vertical error than for a horizontal error. This is a crucial factor that calls for good vertical localization ability of owls which rely on hearing for localization of food. Selection pressure for improvement of the ability of vertical localization of sound is believed to lie behind the evolution of all types of bilateral ear asymmetry in owls. On the basis of comparative ear structure the current subdivision of family Strigidae into subfamily Buboninae and Striginae is rejected. The external ears of Rhinoptynx and Pseudoscops are described for the first time and shown to be very similar to those of Asio otus , demonstrating affinity between these three genera.