Showing papers in "Philosophical Transactions of the Royal Society B in 1989"

The Phylogenetic Regression

Abstract: A new statistical method called the phylogenetic regression is proposed that applies multiple regression techniques to cross-species data. It allows continuous and categorical variables to be tested for and controlled for. The new method is valid despite the problem that phylogenetically close species tend to be similar, and is designed to be used when information about the phylogeny is incomplete. Information about the phylogeny of the species is assumed to be available in the form of a working phylogeny, which contains multiple nodes representing ignorance about the order of splitting of taxa. The non-independence between species is divided into that due to recognized phylogeny, that is, to phylogenetic associations represented in the working phylogeny; and that due to unrecognized phylogeny. The new method uses one linear contrast for each higher node in the working phylogeny, thus applying the ‘radiation principle’. For binary phylogenies the method is similar to an existing method. A criterion is suggested in the form of a simulation test for deciding on the acceptability of proposed statistical methods for analysing cross-species data with a continuous y-variable. This criterion is applied to the phylogenetic regression and to some other methods. The phylogenetic regression passes this test; the other methods tested fail it. Arbitrary choices have to be made about the covariance structure of the error in order to implement the method. It is argued that error results from omitted but relevant variables, and the implications for those arbitrary choices are discussed. One conclusion is that the dates of splits between taxa, even supplemented by rates of neutral gene evolution, do not provide the ‘ true ’ covariance structure. A pragmatic approach is adopted. Several analytical results about the phylogenetic regression are given, without proof, in a mathematical appendix. A computer program has been written in GLIM to implement the phylogenetic regression, and readers are informed how to obtain a copy.

The micrometeorology of the urban forest

Abstract: Urban trees occupy a wide variety of habitats, from a single specimen competing in the urban jungle to extensive remnant or planted forest stands. Each is shown to produce distinct micro- to local scale climates contributing to the larger urban climate mosaic. These effects are discussed in relation to the radiative, aerodynamic, thermal and moisture properties of trees that so clearly set them apart from other urban materials and surfaces in terms of their exchanges of heat, mass and momentum with the atmosphere. Their resulting ability to produce shade, coolness, shelter, moisture and air filtration makes them flexible tools for environmental design.

The cell division cycle of Trypanosoma brucei brucei: timing of event markers and cytoskeletal modulations.

Abstract: We have analysed the timing and order of events occurring within the cell division cycle of Trypanosoma brucei. Cells in the earliest stages of the cell cycle possess a single copy of three major organelles: the nucleus, the kinetoplast and the flagellum. The first indication of progress through the cell cycle is the elongation of the pro-basal body lying adjacent to the mature basal body subtending the flagellum. This newly elongated basal body occupies a posterior position within the cell when it initiates growth of the new daughter flagellum. Genesis of two new pro-basal bodies occurs only after growth of the new daughter flagellum has been initiated. Extension of the new flagellum, together with the paraflagellar rod, then continues throughout a major portion of the cell cycle. During this period of flagellum elongation, kinetoplast division occurs and the two kinetoplasts, together with the two flagellar basal bodies, then move apart within the cell. Mitosis is then initiated and a complex pattern of organelle positions is achieved whereby a division plane runs longitudinally through the cell such that each daughter ultimately receives a single nucleus, kinetoplast and flagellum. These events have been described from observations of whole cytoskeletons by transmission electron microscopy together with detection of particular organelles by fluorescence microscopy. The order and timing of events within the cell cycle has been derived from analyses of the proportion of a given cell type occurring witin an exponentially growing culture.

Deposition of atmospheric pollutants on forests.

Abstract: Forests have long been considered as efficient sinks for atmospheric pollutants. The potential for large rates of deposition is provided by the turbulent structure of air above and within forest canopies. Large rates of deposition of pollutant gases, however, are only found for the very reactive gases HNO $\_3$ , HC $\_1$ and NH $\_3$ . In contrast, the pollutants SO $\_2$ and O $_3$ are deposited on forests and short vegetation at similar rates under the control of stomatal resistance. Deposition of sub-micrometre aerosol particles on forests appears to be inefficient but at high elevations in the United Kingdom (up to 500 m) these aerosols are frequently activated into cloud droplets in the size range 5-10 $\mu$ m (radius). These droplets are efficiently captured by forest canopies and this deposition pathway may make a large contribution to annual inputs at high elevation sites. The effects of afforestation on inputs of pollutants to catchments are illustrated by model calculations for inputs of sulphur and nitrogen to Kielder forest in northern England. Inputs of sulphur and nitrogen to this area as moorland are estimated at 17.5 kg ha $^{-1}$ (1 hectare = 10 $^4$ m $^2$ ) and 12.4 kg ha $^{-1}$ annually, respectively. Afforestation of the moorland increases sulphur and nitrogen inputs by 30 % and 90%, respectively.

Micrometeorology of Temperate and Tropical Forest

Abstract: The literature on the micrometeorology of temperate and tropical forests is reviewed to determine whether structural or species difference between these biomes alters their interaction with the atmosphere. Considerable consistency is found in the value of those whole-canopy features of most importance to this interaction, namely solarreflection coefficient, through-canopy radiation absorption, aerodynamic roughness, the symptoms of near-surface K-theory failure, the canopy store for rainfall interception and the magnitude and environmental response of their bulk stomatal (surface) resistance. Typical values of these parameters and functions are given with a view to their potential use in climate simulation models. Attention is drawn to the fact that this similar micrometeorological response can generate different timeaverage surface-energy partitions when interacting with different climates and, in particular, alters between the edge and the middle of continents. This is of considerable significance, implying tropical deforestation is likely to have most effect on river flow (though not climate) at continental edge and island locations. The similar micrometeorological response of forests is interpreted as the necessary consequence of energy and mass (water) conservation acting as an area average on vegetation that is, by definition, dense and extensive, to reconcile a characteristically tall growth habit with a perennial nature.

The present, past and future of human-caused extinctions.

Abstract: This paper re-evaluates whether we are really at the start of a mass extinction caused by humans. I consider the present, past and future of human-caused extinctions. As regards the present, estimates of extinction rates based on Red Data Books underestimate real values by a large factor, because the books evaluate only those species that have attracted specific attention and searches. Especially in tropical areas with few resident biologists, many poorly known species go extinct without having been the object of specific attention, and others disappear even before being described. A 'green list' of species known to be secure is needed to complement 'red books' of species known to be extinct. As regards the past, it is now clear that the first arrival of humans at any oceanic island with no previous human inhabitants has always precipitated a mass extinction in the island biota. Well-known victims include New Zealand's moas, Madagascar's giant lemurs, and scores of bird species on Hawaii and other tropical Pacific islands. Late-Pleistocene or Holocene extinctions of large mammals after the first arrival of humans in North America, South America and Australia may also have been caused by humans. Hence human-caused mass extinction is not a hypothesis for the future but an event that has been underway for thousands of years. As regards the future, consideration of the main mechanisms of human-caused extinctions (overhunting, effects of introduced species, habitat destruction, and secondary ripple effects) indicates that the rate of extinction is accelerating. The basic reason is that there are now more humans than ever before, armed with more potent destructive technology, and encroaching on the world's most species-rich habitats: the continental tropical rainforests.

Models of Integrated Photosynthesis of Cells and Leaves

Abstract: The kinetics of ribulose bisphosphate carboxylase-oxygenase (Rubisco) are integrated with the stoichiometry of NADPH consumption and production in a simplified model of C $\_3$ chloroplast photosynthesis. The extension to a leaf is discussed with reference to the gradient of irradiance that is always present within the leaf. The optimal arrangement of photosyntheic capacity is discussed in this context. Attention is then given to the effects of gradients of CO $\_2$ concentration that sometimes occur when stomata close in a heterogeneous fashion.

Phylogenetic relations between microbats, megabats and primates (Mammalia: Chiroptera and Primates).

Abstract: We examine the paraphylectic hypothesis of bat origins, both in the light of previous discussions, and in the light of new evidence from our analyses of neurological traits and wing morphology. Megabats share with primates a variety of complex details in the organization of neural pathways that have not been found in any other mammalian group, particularly not in microbats. The features previously used to link microbats and megabats have been examined and found to be questionable bases for support of a monophyletic origin. In particular, morphological analyses of the musculoskeletal adaptations associated with the flight apparatus are consistent with two separate origins of the mammalian wing. Taken together, these analyses suggest that megabats evolved from an early branch of the primate lineage. This branch was comprised of moderate-sized, phytophagous gliders, of which the other living descendants are the dermopterans. Microbats, in contrast, probably evolved much earlier from small, agile insectivores whose forelimbs had long metacarpals in relation to their phalanges.

The case for sea-level change as a dominant causal factor in mass extinction of marine invertebrates

Abstract: A correlation between global marine regressions and mass extinctions has been recognized since the last century and received explicit formulation, in a model involving habitat-area restriction, by Newell in the 1960s. Since that time attempts to apply the species-area relation to the subject have proved somewhat controversial and promoters of other extinction models have called the generality of the regression-extinction relation into question. Here, a strong relation is shown to exist between times of global or regional sea-level change inferred from stratigraphic analysis, and times of high turnover of Phanerozoic marine invertebrates, involving both extinction and radiation; this is valid on a small and large scale. In many cases the most significant factor promoting extinction was apparently not regression but spreads of anoxic bottom water associated with the subsequent transgression. The sea-level-extinction relation cannot be properly understood without an adequate ecological model, and an attempt is made to formulate one in outline.

Atmospheric carbon dioxide and forests

Abstract: Knowledge about the effects of the rise in atmospheric CO $\_2$ concentration on trees and forest is assessed and, the converse, the possible impact of forests on the atmospheric CO $\_2$ concentration is discussed. At the cellular scale, much is known about the role of CO $\_2$ as a substrate in photosynthesis, but only little about its role as an activator and regulator. At the leaf scale, the response of $CO\_2$ assimilation to $CO\_2$ concentration has been described often and is well represented by biochemically based models, but there is inadequate information to parametrize the models of $CO\_2$ -acclimated leaves. Growth and partitioning to the roots of seedlings and young trees generally increases in response to a doubling in atmospheric $CO\_2$ concentration. Experimental results are very variable, because of the differing length of the experiments, the artificial conditions and the artefactual constraints. At larger scales, direct measurements of responses to increase in atmospheric CO $\_2$ are impractical but models of canopy processes suggest that significant increases in CO2 assimilation will result from the rise in atmospheric concentration. Inferences from the increase in amplitude of the seasonal oscillation in the global atmospheric CO2 concentration at different latitudes suggest that forest is having a significant impact on the global atmospheric concentration, but it seems unlikely that expansion of the forest resource could effectively reduce the increase in atmospheric CO $_2$ .

The Comparative Morphology, Phylogeny and Evolution of the Gastropod Family Littorinidae

Abstract: An account is given of the comparative morphology of the family Littorinidae, based on examination of 122 species, grouped into 32 subgenera. The shell, operculum and principal organ systems are described, and their phylogenetic significance assessed. A total of 53 characters, coded as 131 character states, were chosen for inclusion in a cladistic analysis of the phylogenetic relationships of the subgenera. This was performed by the program PAUP, using the principle of maximum parsimony. The outgroup for the analysis comprised representatives of the Pomatiasidae and Skeneopsidae. A consensus tree was obtained from cladograms with consistency indices of 0.408 (autapomorphies excluded). The analysis supports the monophyly of the Littorinidae, and the family can be formally defined by the two synapomorphies of a spiral pallial oviduct and an anterior bursa copulatrix. Three principal clades are identified and given subfamilial rank. The Lacuninae and Laevilitorininae show more plesiomorphic character states, are specialized for life in temperate and polar waters, and occupy the low eulittoral zone and continental shelf. In contrast, the Littorininae occur mainly on tropical and temperate shores, and their synapomorphies of pelagic egg capsules, complex penial glands, paraspermatic nurse cells and sculptured shells can be interpreted as adaptations for their typical habitat in the high eulittoral zone and littoral fringe. The reconstruction of character states on the cladogram provides hypotheses about the evolution of individual characters. Primitively, the male reproductive tract appears to have been an entirely closed duct, opening at the penial tip. Progressive opening of the anterior part of the tract occurred, and was correlated with the appearance of paraspermatic nurse cells to prevent premature dispersal of euspermatozoa. The littorinid capsule gland, responsible for the production of pelagic egg capsules, is believed to be a new structure, not homologous with that of related families. In three cases there is evideiice, from both parsimony and protoconch morphology, of reversion from non-planktotrophic to planktotrophic development. The cladogram is used as a basis for a new classification of the Littorinidae, in which three subfamilies and 14 monophyletic genera are recognized. This is summarized in an appendix, with diagnoses of supraspecific taxa, including descriptions of one new subfamily and four new subgenera, and a list of the 173 recognized Recent species. The poor fossil record of the family is reviewed, and its biogeography discussed in the light of the phylogenetic hypothesis. Of particular interest is the bipolar distribution of the marine Lacuninae, the possible origin in Gondwanaland of the Indian freshwater genus Cremnoconchus, the presence of several relict taxa of Littorininae in the tropical and temperate Atlantic and the probable dispersal of the genus Littorina from the Tethys Sea to the northwestern Pacific and thence to the northern Atlantic in the late Pliocene. Some ecological implications of the phylogenetic hypothesis are considered, with special reference to the diverse types of spawn and life-history strategies. The primitive benthic gelatinous spawn can be viewed as a phylogenetic constraint on the range of habitat and latitudinal distribution of the Lacuninae and Laevilitorininae. The pelagic egg capsules of the Littorininae may have been an important adaptation permitting their exploitation of the littoral fringe and tropical regions, but preventing radiation into terrestrial and freshwater habitats. Non-planktotrophic, non-planktonic development in benthic egg masses is found only at high latitudes, and has appeared independently in Lacuninae, Laevilitorininae and Littorina. The only other non-planktotrophic littorinids are two ovoviviparous tropical species of restricted distribution and probably recent origin.

Phosphofructokinase: structure and control.

Abstract: Phosphofructokinase from Bacillus stearothermophilus shows cooperative kinetics with respect to the substrate fructose-6-phosphate (F6P), allosteric activation by ADP, and inhibition by phosphoenolpyruvate. The crystal structure of the active conformation of the enzyme has been solved to 2.4 A resolution, and three ligand-binding sites have been located. Two of these form the active site and bind the substrates F6P and ATP. The third site binds both allosteric activator and inhibitor. The complex of the enzyme with F6P and ADP has been partly refined at 2.4 A resolution, and a model of ATP has been built into the active site by using the refined model of ADP and a 6 A resolution map of bound 5'-adenylylimidodiphosphate (AMPPNP). The $\gamma$ -phosphate of ATP is close to the 1-hydroxyl of F6P, in a suitable position for in-line phosphoryl transfer. The binding of the phosphate of F6P involves two arginines from a neighbouring subunit in the tetramer, which suggests that a rearrangement of the subunits could explain the cooperativity of substrate binding. The activator ADP is also bound by residues from two subunits.

The pachypleurosaurids (Reptilia: Nothosauria) from the middle triassic of Monte San Giorgio (Switzerland) with the description of a new species.

Abstract: The largest and most diverse collection of Pachypleurosauridae (Nothosauria, Reptilia) comes from Monte San Giorgio, Switzerland. Several hundred complete skeletons were collected from four distinct horizons of bituminous limestones and shales of Anisian-Ladinian boundary to early Ladinian age (Middle Triassic). Serpianosaurus mirigiolensis comes from the oldest strata, the Grenzbitumenzone Beds. The three younger strata, all in the Lower Meride Limestone, yield three species of Neusticosaurus. Neusticosaurus pusillus comes from the Cava Inferiore horizon, Neusticosaurus peyeri, new species, from the Cava Superiore horizon, and Neusticosaurus edwardsii, new combination, from the Alla Cascina horizon. Neusticosaurus pusillus is biostratigraphically important because it is one of the rare species reported from both the Germanic and the Alpine Triassic. Neusticosaurus pusillus and N. peyeri are small and very similar in their anatomy. Neusticosaurus species are easiest separated by their number of presacral vertebrae. Ornamentation of the bone surface is distinctive for all four pachypleurosaurids. Soft parts are rarely preserved, except for one partial squamation. The biological age of Neusticosaurus individuals can be determined by skeletochronology (aging by bone annuli). Small species of Neusticosaurus were sexually mature after three to four years and lived for six to nine years. Taphonomic analysis of the small species indicates attritional mortality and suggests weak bottom currents in the Monte San Giorgio basin during early Ladinian times. Morphometric comparison of all four pachypleurosaurids indicates that the changing vertebral numbers between species are largely due to a change in number of segments. All Monte San Giorgio pachypleurosaurids are sexually dimorphic in forelimb development. Sex x has poorly differentiated and relatively short humeri whereas sex y has well differentiated and relatively long humeri. The sexes are of about the same size and represented in roughly equal numbers. Identification of gender was not possible. Good growth series, especially of Neusticosaurus peyeri, from embryo to large adult permitted qualitative and quantitative study of ontogeny. The skull grows with negative allometry; the humerus grows isometrically or with positive allometry, depending on sex and species; the femur grows isometrically. The adult size range in N. peyeri is considerably larger than in modern reptiles. The Monte San Giorgio pachypleurosaurids are a monophyletic group. The phylogeny of this group is congruent with the stratigraphic distribution of its members.

The postcranial skeletal anatomy of the carboniferous tetrapod Greererpeton burkemorani Romer, 1969

Abstract: The discovery of well-preserved skeletal remains of Greererpeton burkemorani Romer, 1969, from the Upper Mississippian at Greer, West Virginia, has prompted a redescription of its postcranial skeletal anatomy. The vertebrae are rhachitomous, more specifically schizomerous. The presacral count is approximately 41. Neural arch elements of the atlas-axis complex consist of paired pro- and atlas arches and a relatively massive axis arch. All presacral vertebrae appear to have borne short, gently curved ribs, most of which developed a flange or stiletto-like uncinate process. The sacral rib is long. In adults, the scapulocoracoid and the dermal pectoral girdle are large and well ossified. The ascending iliac blade is undivided and the pubis is poorly ossified. A cladistic analysis indicates that Greererpeton burkemorani shares a more recent common ancestor with the anthracosaur Proterogyrinus scheelei than it does with either Crassigyrinus scoticus or Ichthyostega sp.

Photosynthetic Light Gradients and Spectral Regime within Leaves of Medicago sativa

Abstract: By using a fibre-optic probe, light gradients were measured at 450, 550 and 680 nm in sun leaves, 125 $\mu$ m thick, of Medicago sativa L. cv. Armor. The space irradiances immediately beneath the leaf surface were 1.5-2.0 times greater than the incident light for these wavelengths, indicating that M. sativa leaves are efficient light traps. Although the palisade was only 60 $\mu$ m thick, each light gradient declined steeply within this layer. More light appeared to be scattered in forward rather than backward directions and the spectral regime of the light fluxes depended upon their direction of travel within the leaf. Spectra for transmitted light were dependent upon depth within the leaf, whereas back-scattered light consisted of mostly green and farred light at all depths. PAR (photosynthetically active radiation, 400-700 nm) within both the palisade and spongy mesophyll consisted mostly of green and far-red light, and the spongy mesophyll received only 0.11 of the PAR compared with the midregion of the palisade. Anomalous measurements within the palisade were traced to the epidermis, which was found to act as a mosaic of microlenses that focused light within the palisade layer. In M. sativa leaves, the light microenvironment, leaf anatomy and photosynthesis seem to be strongly interrelated.

Processes Controlling Understorey Evapotranspiration

Abstract: The understorey often accounts for a significant proportion of forest evapotranspiration. In this paper we discuss the role of the understorey radiation regime, and the aerodynamic and stomatal conductance characteristics of the understorey in understorey evapotranspiration. Values of the McNaughton-Jarvis parameter 12 for the understorey in two mid-rotation Douglas-fir stands indicate considerable coupling between the understorey and the atmosphere above the overstorey. However, the stronger coupling between the oveistorey and the atmosphere accounts for the observation that the fraction of stand evapotranspiration originating at the understorey increases as the water vapour pressure deficit increases and the soil dries. We also discuss the approaches to describing the process of evaporation from the forest floor and the results of understorey removal experiments. These show small decreases in stand evapotranspiration and root-zone soil water content, but significant increases in the transpiration and growth of the trees.

The causes of extinction

Abstract: A species may go extinct either because it is unable to evolve rapidly enough to meet changing circumstances, or because its niche disappears and no capacity for rapid evolution could have saved it. Although recent extinctions can usually be interpreted as resulting from niche disappearance, the taxonomic distribution of parthenogens suggests that inability to evolve may also be important. A second distinction is between physical and biotic causes of extinction. Fossil evidence for constant taxonomic diversity, combined with species turnover, implies that biotic factors have been important. A similar conclusion emerges from studies of recent introductions of predators, competitors and parasites into new areas. The term 'species selection' should be confined to cases in which the outcome of selection is determined by properties of the population as a whole, rather than of individuals. The process has been of only trivial importance in producing complex adaptations, but of major importance in determining the distribution of different types of organisms. An adequate interpretation of the fossil record requires a theory of the coevolution of many interacting species. Such a theory is at present lacking, but various approaches to it are discussed.

An Allometric Study of Pulmonary Morphometric Parameters in Birds, with Mammalian Comparisons

Abstract: Comprehensive pulmonary morphometric data from 42 species of birds representing ten orders were compared with those of other vertebrates, especially mammals, relating the comparisons to the varying biological needs of these avian taxa. The total lung volume was strongly correlated with body mass. The volume density of the exchange tissue was lowest in the charadriiform and anseriform species and highest in the piciform, cuculiform and passeriform species. The surface area of the blood-gas (tissue) barrier, the volume of the pulmonary capillary blood and the total morphometric pulmonary diffusing capacity were all strongly correlated with body mass. The harmonic mean thickness of both the blood-gas (tissue) barrier and the plasma layer were weakly correlated with body mass. The mass-specific surface area of the blood-gas (tissue) barrier (surface area per gram body mass) and the surface density of the blood-gas (tissue) barrier (i.e. its surface area per unit volume of exchange tissue) were inversely correlated (though weakly) with body mass. The passeriform species exhibited outstanding pulmonary morphometric adaptations leading to a high specific total diffusing capacity per gram body mass, consistent with the comparatively small size and energetic mode of life which typify passeriform birds. The relatively inactive, ground-dwelling domestic fowl (Gallus gallus) had the lowest pulmonary diffusing capacity per gram body mass. The specific total lung volume is about 27% smaller in birds than in mammals but the specific surface area of the blood-gas (tissue) barrier is about 15% greater in birds. The ratio of the surface area of the tissue barrier to the volume of the exchange tissue was also much greater in the birds (170-305%). The harmonic mean thickness of the tissue barrier was 56-67% less in the birds, but that of the plasma layer was about 66% greater in the birds. The pulmonary capillary blood volume was also greater (22%) in the birds. Except for the thickness of the plasma layer, these morphometric parameters all favour the gas exchange capacity of birds. Consequently, the total specific mean morphometric pulmonary diffusing capacity for oxygen was estimated to be about 22% greater in birds than in mammals of similar body mass. This estimate was obtained by employing oxygen permeation constants for mammalian tissue, plasma and erythrocytes, as avian constants were not then available. Recalculations using recent values for the rate of oxygen uptake by avian whole blood indicate that the superiority of the avian pulmonary diffusing capacity for oxygen is even greater, the value for birds exceeding that of mammals by about 82%. However, because of the small numbers of some of the avian species investigated and the lack of representatives of many important groups of birds, our allometric computations should be regarded as essentially a preliminary basis for comparing the pulmonary morphometric characteristics of birds and mammals. It is suggested that the greater physiological efficiency of the avian pulmonary system compared with that of mammals can be attributed partly to the pulmonary morphometric differences between these two vertebrate classes. Other major factors are the cross-current relation of parabronchial gas and blood, the auxiliary counter-current relation of air capillary gas and blood, and the bellows action of the air sacs.

Spectroscopic methods in photosynthesis: photosystem stoichiometry and chlorophyll antenna size

Abstract: Light-induced absorbance change and fluorescence measurements were employed in the quantitation of photosystem stoichiometry and in the measurement of the chlorophyll (Chl) antenna size in thylakoid membranes. Results with thylakoid membranes from diverse photosynthetic tissues indicated a PSII/PSI reaction-centre stoichiometry that deviates from unity. Cyanobacteria and red algae have a PSII/PSI ratio in the range of 0.3 to 0.7. Chloroplasts from spinach and other vascular-plant species grown under direct sunlight have PSII/PSI = 1.8±0.3. Chlorophyll b -deficient and Chi b -lacking mutants have PSII/PSI > 2. The observation that PSII/PSI ratios are not unity and show a large variation among different photosynthetic membranes appears to be contrary to the conventional assumption derived from the Z-scheme. However, the photosystem stoichiometry is not the only factor that needs to be taken into account to explain the coordination of the two photosystems in the process of linear electron transport. The light-harvesting capacity of each photosystem must also be considered. In cyanobacterial thylakoids (from Synechococcus 6301, PSII/PSI = 0.5±0.2), the phycobilisome-PSII complexes collectively harvest as much light as the PSI complexes. In vascular plant chloroplasts, the light-harvesting capacity of a PSI I complex (250 molecules, Chi a/Chi b = 1.7) is lower than that of a PSI complex (230 Chl, Chl a /Chl b = 8.0) because Chi b has a lower extinction coefficient than Chi a . A differential attenuation of light intensity through the grana further reduces the light absorbed by PSII. Hence, a PSII/PSI ratio greater than one in vascular-plant chloroplasts compensates for the lower absorption of light by individual PSII complexes and ensures that, on average, PSII will harvest about as much light as PSI. In conclusion, distinct light-harvesting strategies among diverse plant species complement widely different photosystem stoichiometries to ensure a balanced absorption of light and a balanced electron flow between the two photoreactions, thereby satisfying the requirement set forth upon the formulation of the Z-scheme by Hill & Bendall ( Nature, Lond. 186, 136-137 (1960)) and by Duysens, Amesz & Kamp ( Nature, Lond . 190, 510-511 (1961)).

A New Pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland

Abstract: Pachypleurosaur material from the Middle Triassic `Grenzbitumen'-horizon (Anis-Ladin boundary) of Monte San Giorgio, Kanton Tessin, Switzerland, is described as a new genus and species, Serpianosaurus mirigiolensis. A detailed morphological description is followed by a quantitative analysis. The taxon differs from other pachypleurosaurids primarily by its relatively large skull and its straight lower jaw. Rib pachyostosis is absent. In most characters the taxon remains plesiomorphous with respect to other pachypleurosaurs from the Middle Triassic deposits of Monte San Giorgio, which accords well with its early stratigraphic occurrence at that locality. Sexual dimorphism is expressed by the size and shape of the humerus. A cladistic analysis shows the Pachypleurosauridae to constitute the sistergroup of all other Sauropterygia. The Sauropterygia and the Placodontia together form a monophyletic group, the Euryapsida, which is subordinated to the Diapsida and to the Neodiapsida, but which is classifed outside the archosauromorph-lepidosauromorph dichotomy within the Neodiapsida. A concluding paragraph discusses the status of the pachypleurosaurs within the Sauropterygia. Skull structure, and jaw mechanics in particular, suggest a relatively plesiomorph position of pachypleurosaurs within the Sauropterygia. Their adaptation to an aquatic mode of life was not carried to the degree observed in other sauropterygians, and the ability for rapid and deep dives seems to have been limited by middle-ear structure.

The transmission dynamics of the human immunodeficiency virus type 1 in the male homosexual community in the United Kingdom: the influence of changes in sexual behaviour

Abstract: This paper examines the transmission dynamics of human immune deficiency virus type 1 (HIV-1) in the male homosexual population in the U.K. via numerical studies employing a mathematical model representing the principal epidemiological process. The model is based on an assumption of proportionate mixing between different sexual-activity classes (defined by the rate of sexual partner change per unit of time) and incorporates heterogeneity in sexual activity, distributed infection and incubation periods and the recruitment of susceptibles to the sexually active population. The sensitivity of model predictions to various assumptions and parameter assignments is examined. Numerical studies of model behaviour focus on the influence of changes in the magnitudes of the transmission parameters, associated with three periods of infectiousness during the incubation period of acquired immune deficiency syndrome (AIDS), on the magnitude and duration of the epidemic and on the level of the endemic equilibrium state. Predicted temporal trends in the incidence of AIDS are shown to be particularly sensitive to changes in the intensities and durations of the stages of infectiousness. Most of the paper addresses the influence of changes in sexual behaviour on the magnitude and duration of the epidemic. Numerical simulations show that the manner in which behavioural changes occur and who is influenced by such changes (i.e. infecteds or susceptibles, the sexually active population or new recruits to this population) have a major impact on the future timecourse of the epidemic. The greatest reduction in the incidence of AIDS over the coming decades is induced by changes in the rate of sexual-partner change among the sexually active population, particularly those currently infected. The time periods at which changes in behaviour occur, in relation to the starting point of the epidemic (assumed to be 1979), are also of particular significance to the future pattern of the incidence of disease and infection. Changes in behaviour early on in the timecourse of the epidemic have a much greater impact than equivalent changes at latter time points. On the basis of limited data on the pattern of change in sexual behaviour among the male homosexual community in the U.K., numerical studies of model behaviour tentatively suggest that the epidemic is at, or near to, a period of peak incidence of the disease AIDS. Analyses suggest that, following the peak in incidence, there will be a period of slow decline over many decades provided recent changes in behaviour are maintained in the coming years.(ABSTRACT TRUNCATED AT 400 WORDS)

The Biology of Mass Extinction: A Palaeontological View

Abstract: Extinctions are not biologically random: certain taxa or functional/ecological groups are more extinction-prone than others. Analysis of molluscan survivorship patterns for the end-Cretaceous mass extinctions suggests that some traits that tend to confer extinction resistance during times of normal ('background') levels of extinction are ineffectual during mass extinction. For genera, high species-richness and possession of widespread individual species imparted extinction-resistance during background times but not during the mass extinction, when overall distribution of the genus was an important factor. Reanalysis of Hoffman's (1986) data (Neues Jb. Geol. Palaont. Abh. 172, 219) on European bivalves, and preliminary analysis of a new northern European data set, reveals a similar change in survivorship rules, as do data scattered among other taxa and extinction events. Thus taxa and adaptations can be lost not because they were poorly adapted by the standards of the background processes that constitute the bulk of geological time, but because they lacked--or were not linked to--the organismic, species-level or clade-level traits favoured under mass-extinction conditions. Mass extinctions can break the hegemony of species-rich, well-adapted clades and thereby permit radiation of taxa that had previously been minor faunal elements; no net increase in the adaptation of the biota need ensue. Although some large-scale evolutionary trends transcend mass extinctions, post extinction evolutionary pathways are often channelled in directions not predictable from evolutionary patters during background times.

Devices and Methods for Room-Temperature Fluorescence Analysis

Abstract: An overview of the various types of photochemical and non-photochemical fluorescence quenching in vivo is given. Devices and methods are outlined that allow specific information to be obtained from complex fluorescence responses. The importance of correlated measurements of other, independent photosynthesis signals is emphasized. It is shown that a recently introduced pulse-amplitude modulation fluorometer (PAM fluorometer) can also be used with modified emitter-detector units to measure absorbance changes. Examples are given for absorbance changes of the Hill reagent methyl purple, induced by single turnover flashes, and for P $\_{700}$ absorbance changes measured simultaneously with fluorescence. Correlated P $\_700$ and fluorescence measurements give deeper insights into the control of electron transfer from PQH $2$ to cytochrome (cyt) b/f and into the intersystem acceptor-pool size of sun and shade leaves. Possible explanations for differences in pool sizes determined by P $\_{700}$ and fluorescence measurements are discussed. By using P $\_{700}$ reduction as an indicator, it is shown that in saturating light the plastoquinone (PQ) pool is already reduced within 50 ms, whereas the last phase of the fluorescence rise (I $\_2$ -P) takes about 300 ms and is paralleled by the re-reduction of P $\_{700}$ . It is concluded that I $_2$ -P reflects removal of photochemical quenching at PSI and that 50 ms saturation pulses are appropriate to eliminate the relevant photochemical PSII quenching.

Automated Measurement of Leaf Photosynthetic O $_2$ Evolution as a Function of Photon Flux Density [and Discussion]

Abstract: An automated procedure is described that allows the rate of photosynthesis, as a function of photon flux density (PFD), to be determined and plotted within 30 minutes. The method is based on polarographic measurement of O 2 evolution from a piece of leaf enclosed in a chamber and illuminated from above by an array of light-emitting diodes. The light emitted from these diodes is altered by a computer which also facilitates analyses of the data so derived. Applications of the procedure to leaves of shade and sun plants, to studies of photoinhibition and to analysis of the Kok effect, are described.

Fluorescence as a tool in photosynthesis research: application in studies of photoinhibition, cold acclimation and freezing stress

Abstract: Chlorophyll fluorescence induction (at 20 $^\circ$ C and 77 K) and quenching were analysed in relation to effects of environmental stresses imposed by chilling in high light and by freezing and thawing of spinach (Spinacia oleracea L.) leaves. The data indicate that cold acclimation of spinach plants, which leads to increased frost tolerance of the leaves, results in decreased susceptibility to photoinhibition of photosynthesis at chilling temperatures. When plants acclimated to 18 $^\circ$ C and 260-300 $\mu$ mol quanta $m^{-2} s^{-1}$ were exposed to higher light (550 $\mu$ mol quanta $m^{-2} s^{-1}$ ) at 4 $\circ$ C, they developed strong photoinhibition, as characterized by decreased quantum yield of $O\_2$ evolution and decreased ratio of variable: maximum fluorescence ( $F\_v$ / $F\_M$ ) of photosystem II. The decrease in $F\_v$ / $F\_M$ resulted from a decline in $F\_v$ and an increase in $F\_0$ . The $F\_v$ / $F\_M$ ratio was lowered to a significantly greater extent when induction was recorded at 20 $\circ$ C, as compared with 77 K. The effects related to photoinhibition were fully reversible at 18 $\circ$ C in dim light. Plants that had been cold-acclimated for 10 days exhibited slightly decreased quantum yield and lowered $F\_v$ / $F\_M$ ratio. However, they did not show further photoinhibition on exposure to 550 $\mu$ mol quanta $m{-2} s{-1}$ at 4 $\circ$ C. The reversible photoinhibition is discussed as a protective pathway serving for thermal dissipation of excessive light energy. It is hypothesized that such a mechanism prevents destruction of the photosynthetic apparatus, until other means of protection become effective during long-term acclimation to high light. Inhibition of photosynthetic carbon assimilation caused by freezing and thawing of leaves in the dark was closely correlated with inhibition of photochemical fluorescence quenching ( $q\_Q$ ). As a sensitive response of the thylakoid membranes to freezing stress, the energy-dependent quenching, $q\_E$ , was inhibited. Only more severe impact of freezing caused a significant decline in the $F\_v$ / $F\_M$ ratio. It is concluded that measurements of fluorescence induction signals ( $F\_v$ / $F\_M$ ratios) provide a sensitive tool with which to investigate photoinhibition, whereas freezing damage to the photosynthetic system can be detected more readily by the quenching coefficients $q\_Q$ and $q\_E$ than by $F\_v$ / $F_M$ ratios.

Three-dimensional structures of influenza virus neuraminidase-antibody complexes

Abstract: X-ray diffraction analysis of crystals of a monoclonal Fab fragment NC41 bound to a viral antigen, influenza virus neuraminidase, shows an epitope involving five surface loops of the antigen. In addition it reveals an unusual pairing pattern between the domains of light and heavy chains in the variable module of the antibody. We interpret this result to imply that association with antigen can induce changes in the quaternary structure of the Fab, through a sliding of domains at the variable light/ variable heavy chains (V L - V H ) interface. In addition, Fab binding has altered the conformation of some of the surface loops of the antigen. The structure of the NC10 Fab-neuram inidase complex has now also been solved. It binds an epitope that overlaps the NC41 epitope. In this structure, there is no electron density for the C-module of the Fab fragment, implying it is disordered in the crystal lattice. The implications of these, and other antibody-antigen structures, for immune recognition are discussed.

Stand overstorey processes

Abstract: Typical turbulent eddies in a plant canopy are coherent over much of the canopy depth, making gradient-diffusion theory inappropriate for describing the vertical transfer of scalars and momentum. A better description of scalar transfer is obtained by regarding the canopy as an assembly of source elements (individual leaves), each releasing a plume of scalar material into the turbulent flow. The contrasting behaviours of the plume from each leaf in the near and far fields (or small and large travel times, in comparison with the turbulence time scale) account naturally for observed phenomena such as counter-gradient fluxes within the canopy. With two hypotheses, this paradigm yields a simple, analytic expression for the scalar concentration profile corresponding to a given source density in the canopy, effectively replacing gradient-diffusion theory.

Fluorescence analysis during steady-state photosynthesis

Abstract: Photosynthetic gas exchange of attached leaves has been measured under steady-state conditions at different light intensities and correlated with simultaneous measurements of chlorophyll fluorescence and oxidized P $\_{700}$ (by absorbance changes at 820 nm). The data suggest that during light-saturated assimilation, photosystem II (PSII) photochemistry is mainly controlled by non-photochemical and non-radiative dissipation of excitation energy, rather than by accumulation of reduced acceptor, Q $\_A$ , and this could be related to 'high-energy quenching' of fluorescence. The occurrence of oxidized P $\_{700}$ at saturating light and low concentration of CO $\_2$ suggests that in the steady state PSI photochemistry is controlled by a shortage of electron donation from the plastoquinone pool (photosynthetic control), rather than by excess electrons at the acceptor side. The significance of the oxidized form of P $\_{700}$ as a 'quencher' of excitation energy is discussed. This control of photosystems I and II, both related to the proton gradient across the thylakoid. membrane, may serve to match the potential rate of net photochemistry to the demand by the biochemical reactions. However, when light-saturated assimilation is not limited by CO $\_2$ , PSI activity is controlled by accumulation of reduced electron acceptors, rather than by photosynthetic mechanisms. Photosynthetic control has been found to determine the redox state of the ferredoxin-thioredoxin system.

Ammonoid extinction events

Abstract: The ammonoid cephalopods range from the early Devonian to the late Cretaceous, a period of some 320 Ma. Because of their importance for biostratigraphic discrimination and their use in practical age dating for this period they have been intensively studied. Major extinctions at the close of the Devonian, end Permian, end Triassic and end Cretaceous have long been recognized and linked with regressional palaeogeographical events. The recognition of smaller-scale extinction events is relatively new and is especially well shown in the Palaeozoic, when there was a simpler distribution of land and sea pathways than in later periods when the influence of latitudinal distributions and local provinces was more severe. Extinction events in the Devonian show the nature of the process. Usually a gradual decline in diversity is followed by extinction; then there is a period of low diversity but often individual abundance. Then novelty appears and is seen in new characters of the early stages; elaboration and diversification follow. These fluctuations can often be correlated with changes in other groups and also with sedimentological and palaeogeographical changes. Usually a regression-transgression couplet is involved with evidence of ocean turnover indicated by anoxic or low-oxygen events. A new family, Sobolewiidae, is diagnosed. A new analysis of diversity, appearances and extinctions is made at the family level for 2 Ma time units throughout the history of the Ammonoidea. This record is compared with modern attempts to portray sea-level fluctuations and onlap and offlap movements of marine seas. The correlation, even in detail, is impressive and gives support for the species/area theory. But it is argued that temperature, as well as sea-level factors, is important. The evidence, on both large and small scales, shows an association of evolutionary change with palaeogeographical change. The new evidence does not suggest a role for periodicity above the Milankovitch Band level. Whether or not periodicity is involved, such factors seem more readily explained in endogenic earth causations and for the present these provide the most parsimonious explanations.

Anatomical plasticity of synapses in the lamina of the optic lobe of the fly.

Abstract: Insects are frequently assumed to have hard-wired nervous systems that fail to demonstrate functional plasticity. We have produced changes in synaptic frequency, and analysed their developmental time course, dynamics and reversibility, in the lamina underlying the compound eye of the fly, by exposing young adults to different visual stimuli. The class of synapse examined feeds back from L2, one of the monopolar cells found in each lamina cartridge, to photoreceptor terminals; each site is a synaptic dyad marked by the presence of a few, round vesicles surrounding a T-shaped presynaptic ribbon and, in the photoreceptor, by a subsynaptic vacuole. In control adult flies reared in normal room lighting, the frequency of synaptic profiles scored in micrographs of single sections initially increased until one day post-eclosion (E + 1), but declined thereafter. Frequencies measured in left and right eyes of the same control animals were closely matched. Experimental flies were put for one to two days into an integrating sphere illuminated continuously with square-wave, 25 Hz green light. They had one eye occluded, so providing control comparisons between flicker-reared (FR) and occluded (dark-reared, DR) eyes within the same animal. The DR eyes invariably (n greater than 22) had higher frequencies of synaptic profiles than those seeing light, regardless of age or the period of light exposure, although the detailed relative effects of FR and DR depend upon the age of the animal. The evidence suggests that exposure to light actively depresses synaptic frequency and increases its variability. The greatest difference (30%) achieved was at two to four days after eclosion and there was no difference beyond six days, so demarcating a prospective sensitive period. Rearing in DC light was equally effective as FR, so visual contrasts per se are apparently inessential. Frequency values can change rapidly. During the first 24 h post-eclosion, DR resulted in new synapses adding to L2's complement of 25-35 at a maximum rate of 4 per 6 h, whereas light exposure caused a frequency decrease after as little as 6 h. Alternating 24 h periods of light and dark during the first two days produced reversible synaptic frequency changes. Individual synaptic contacts enlarge with age but not significantly with different visual experiences. The decrease in frequency of synaptic profiles with age thus actually underestimates the true decrease in synaptic number, whereas the altered synaptic frequencies seen after differential exposure represent true differences in synaptic number.(ABSTRACT TRUNCATED AT 400 WORDS)