Abundance and diversity of tidal marsh plants along the salinity gradient of the San Francisco Estuary: implications for global change ecology

From 2003 through 2005, tidal marsh plant species diversity and abundance on historically surveyed vegetation transects along the salinity gradient of the San Francisco Estuary were investigated to establish empirical relationships between plant distributions and environmental conditions, and furthermore to examine and predict past and future plant distribution changes. This study suggests that for most species, salinity is the primary control on plant distribution. Thus, ongoing changes in estuarine conditions (increasing sea level and salinity) are resulting in a complex mix of plant distribution changes. On the low marsh, where sediment salinity is similar to that of ambient water, halophytic species are replacing salt-intolerant taxa. However, on marsh plains, where increased tidal flooding is moderating high salinity (concentrated by evaporation), halophytic “high marsh” species are being replaced by salt-intolerant “low marsh” taxa. Thus, future changes in plant distributions will hinge on whether marsh sediment accumulation keeps pace with sea level rise.     

Effect of variation of environmental conditions on the microbial communities of deep-sea vent chimneys, cultured in a bioreactor

Both cultivation and molecular techniques were used to investigate the microbial diversity and dynamic of a deep-sea vent chimney. The enrichment cultures performed in a gas-lift bioreactor were inoculated with a black smoker chimney sample collected on TAG site on the mid-Atlantic ridge. To mimic as close as possible environmental conditions, the cultures were performed in oligotrophic medium with nitrogen, hydrogen and carbon dioxide (N₂/H₂/CO₂) gas sweeping. Also, the temperature was first settled at a temperature of 85°C and colloidal sulphur was added. Then, the temperature was lowered to 60°C and sulphur was omitted. Archaeal and bacterial diversity was studied in both culture and natural samples. Through 16S rRNA gene sequences analysis of the enrichment cultures microorganisms affiliated to Archeoglobales, Thermococcales were detected in both conditions while, Deferribacterales and Thermales were detected only at 65°C in the absence of sulphur. Single-stranded conformational polymorphism and quantitative PCR permit to study the microbial community dynamic during the two enrichment cultures. The effect of environmental changes (modification of culture conditions), i.e. temperature, medium composition, electron donors and acceptors availability were shown to affect the microbial community in culture, as this would happen in their environment. The effect of environmental changes, i.e. temperature and medium composition was shown to affect the microbial community in culture, as this could happen in their environment. The modification of culture conditions, such as temperature, organic matter concentration, electron donors and acceptors availability allowed to enrich different population of prokaryotes inhabiting hydrothermal chimneys.     

Reproduction and larval morphology of broadcasting and viviparous species in the Cryptasterina species complex

The Cryptasterina group of asterinid sea stars in Australasia comprises cryptic species with derived life histories. C. pentagona and C. hystera have planktonic and intragonadal larvae, respectively. C. pentagona has the gonochoric, free-spawning mode of reproduction with a planktonic lecithotrophic brachiolaria larva. C. hystera is hermaphroditic with an intragonadal lecithotrophic brachiolaria, and the juveniles emerge through the gonopore. Both species have large lipid-rich buoyant eggs and well-developed brachiolariae. Early juveniles are sustained by maternal nutrients for several weeks while the digestive tract develops. C. hystera was reared in vitro through metamorphosis. Its brachiolariae exhibited the benthic exploration and settlement behavior typical of planktonic larvae, and they attached to the substratum with their brachiolar complex. These behaviors are unlikely to be used in the intragonadal environment. The presence of a buoyant egg and functional brachiolaria larva would not be expected in an intragonadal brooder and indicate the potential for life-history reversal to a planktonic existence. Life-history traits of species in the Cryptasterina group are compared with those of other asterinids in the genus Patiriella with viviparous development. Modifications of life-history traits and pathways associated with evolution of viviparity in the Asterinidae are assessed, and the presence of convergent adaptations and clade-specific features associated with this unusual mode of parental care are examined.     

A role for regulated binding of p150(Glued) to microtubule plus ends in organelle transport

A subset of microtubule-associated proteins, including cytoplasmic linker protein (CLIP)-170, dynactin, EB1, adenomatous polyposis coli, cytoplasmic dynein, CLASPs, and LIS-1, has been shown recently to target to the plus ends of microtubules. The mechanisms and functions of this binding specificity are not understood, although a role in encouraging microtubule elongation has been proposed. To extend previous work on the role of dynactin in organelle transport, we analyzed p150(Glued) by live-cell imaging. Time-lapse analysis of p150(Glued) revealed targeting to the plus ends of growing microtubules, requiring the NH2-terminal cytoskeleton-associated protein-glycine rich domain, but not EB1 or CLIP-170. Effectors of protein kinase A modulated microtubule binding and suggested p150(Glued) phosphorylation as a factor in plus-end binding specificity. Using a phosphosensitive monoclonal antibody, we mapped the site of p150(Glued) phosphorylation to Ser-19. In vivo and in vitro analysis of phosphorylation site mutants revealed that p150(Glued) phosphorylation mediates dynamic binding to microtubules. To address the function of dynamic binding, we imaged GFP-p150(Glued) during the dynein-dependent transport of Golgi membranes. Live-cell analysis revealed a transient interaction between Golgi membranes and GFP-p150(Glued)-labeled microtubules just prior to transport, implicating microtubules and dynactin in a search-capture mechanism for minus-end-directed organelles.     

The role of cell-cell interactions in a two-phase model for avascular tumour growth

 A two-phase model is presented to describe avascular tumour growth. Conservation of mass equations, including oxygen-dependent cell growth and death terms, are coupled with equations of momentum conservation. The cellular phase behaves as a viscous liquid, while the viscosity of the extracellular water manifests itself as an interphase drag. It is assumed that the cells become mechanically stressed if they are too densely packed and that the tumour will try to increase its volume in order to relieve such stress. By contrast, the overlapping filopodia of sparsely populated cells create short-range attractive effects. Finally, oxygen is consumed by the cells as it diffuses through the tumour. The resulting system of equations are reduced to three, which describe the evolution of the tumour cell volume fraction, the cell speed and the oxygen tension. Numerical simulations indicate that the tumour either evolves to a travelling wave profile, in which it expands at a constant rate, or it settles to a steady state, in which the net rates of cell proliferation and death balance. The impact of varying key model parameters such as cellular viscosity, interphase drag, and cellular tension are discussed. For example, tumours consisting of well-differentiated (i.e. viscous) cells are shown to grow more slowly than those consisting of poorly-differentiated (i.e. less viscous) cells. Analytical results for the case of oxygen-independent growth are also presented, and the effects of varying the key parameters determined (the results are in line with the numerical simulations of the full problem). The key results and their biological implications are then summarised and future model refinements discussed. Received: 3 May 2001 / Revised version: 7 January 2002 / Published online: 17 July 2002     

Modelling the early growth of ductal carcinoma in situ of the breast

The growth of a tumour in a rigid walled cylindrical duct is examined in order to model the initial stages of tumour cell expansion in ductal carcinoma in situ (DCIS) of the breast. A nutrient-limited growth model is formulated, in which cell movement is described by a Stokes flow constitutive relation. The effects on the shape of the tumour boundary of the material properties (i.e. the viscosity) and the extent to which the cells adhere to the duct wall are studied using numerical and asymptotic methods. It is shown how stable, non-planar, interface configurations result and that, during these initial stages, before the duct wall has been breached, few cells die and a nutrient-rich model is usually sufficient to capture the behaviour. Finally, we discuss the relevance of this approach to DCIS and suggest possible avenues for further work. Send offprint requests to:S.J. Franks at Centre of Mathematical Medicine.     

Formate dehydrogenase--a versatile enzyme in changing environments

Several structures belonging to the large bis-molybdopterin guanine dinucleotide enzyme family have been published during the past four years. These include the structures of three formate dehydrogenases containing intrinsic selenocysteine residues - two soluble enzymes and one integral membrane protein. Together these have given detailed structural and mechanistic information about this family of enzymes.     

Self-deception in an evolutionary game

From the perspective of philosophy, the idea of humans lying to themselves seems irrational and maladaptive, if even possible. However, the paradigm of cognitive modularity admits the possibility of self-deception. Trivers argues that self-deception can increase fitness by improving the effectiveness of inter-personal deception. Ramachandran criticizes Trivers' conjecture, arguing that the costs of self-deception outweigh its benefits. We first modify a well-known cognitive modularity model of Minsky to formalize a cognitive model of self-deception. We then use Byrne's multi-dimensional dynamic character meta-model to integrate the cognitive model into an evolutionary hawk-dove game in order to investigate Trivers' and Ramachandran's conjectures. By mapping the influence of game circumstances into cognitive states, and mapping the influence of multiple cognitive modules into player decisions, our cognitive definition of self-deception is extended to a behavioral definition of self-deception. Our cognitive modules, referred to as the hunger and fear daemons, assess the benefits and the cost of competition and generate player beliefs. Daemon-assessment of encounter benefits and costs may lead to inter-daemonic conflict, that is, ambivalence, about whether or not to fight. Player-types vary in the manner by which such inter-daemonic conflict is resolved, and varieties of self-deception are modeled as type-specific conflict-resolution mechanisms. In the display phase of the game, players signal to one another and update their beliefs before finally committing to a decision (hawk or dove). Self-deception can affect player beliefs, and hence player actions, before or after signaling. In support of Trivers' conjecture, the self-deceiving types do outperform the non-self-deceiving type. We analyse the sensitivity of this result to parameters of the cognitive model, specifically the cognitive resolution of the players and the influence of player signals on co-player beliefs.