Consequences of forced silking

The forced silking of a spider to obtain major ampullate (MA) silk for experiments is a standard practice; however, this method may have profound effects on the resulting silk's properties. Experiments were performed to determine the magnitude of the difference in the forces required to draw silk from the MA gland between unrestrained spiders descending on their draglines and restrained spiders from which MA silk was drawn with a motor. The results show that freely falling spiders can spool silk with as little as 0.1 body weights of force, which generates a stress that is about 2% of the silk's tensile strength. In contrast, forcibly silked spiders apply as much as 4 body weights of force with an internal braking mechanism, and this force creates silk stresses in excess of 50% of the silk's tensile strength. The large forces observed in forced silking should strongly affect the draw alignment of the polymer network in the newly spun fibers, and this may account for the differences in material properties observed between naturally spun and forcibly spun MA silks. In addition, the heat produced by the internal friction brake during forced silking may set the upper limit of forced silking speed.     

Buoyancy effect on forced convection in the leaf boundary layer

Abstract. Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences ( T _L- T _A) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T _L- T _A. An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance ( G _A) for mixed convection, which became higher with higher T _L- T _A as compared with the conductance for pure forced convection without buoyancy. This increase in G _A appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'.     

Effect of forced convection on the skin thermal expression of breast cancer

A bioheat-transfer-based numerical model was utilized to study the energy balance in healthy and malignant breasts subjected to forced convection in a wind tunnel. Steady-state temperature distributions on the skin surface of the breasts were obtained by numerically solving the conjugate heat transfer problem. Parametric studies on the influences of the airflow on the skin thermal expression of tumors were performed. It was found that the presence of tumor may not be clearly shown due to the irregularities of the skin temperature distribution induced by the airflow field. Nevertheless, image subtraction techniques could be employed to eliminate the effects of the flow field and thermal noise and significantly improve the thermal signature of the tumor on the skin surface. Inclusion of the possible skin vascular response to cold stress caused by the airflow further enhances the signal, especially for deeply embedded tumors that otherwise may not be detectable.     

Evolutionary constraints on yeast protein size

Background  

Despite a strong evolutionary pressure to reduce genome size, proteins vary in length over a surprisingly wide range also in very compact genomes. Here we investigated the evolutionary forces that act on protein size in the yeast Saccharomyces cerevisiae utilizing a system-wide bioinformatics approach. Data on yeast protein size was compared to global experimental data on protein expression, phenotypic pleiotropy, protein-protein interactions, protein evolutionary rate and biochemical classification.     

Foliage influences forced convection heat transfer in conifer branches and buds

Conifer foliage structures affect branch and bud temperature by altering the development and convective resistance of the thermal boundary layer. This paper examines foliage effects on forced convection in branches and buds of Picea glauca (Moench) Voss and Pinus contorta Dougl. Ex. Loud., two species that represent the range of variation in foliage structure among conifers. Forced convection is characterized by a power law relating Nusselt (heat transfer) and Reynolds (boundary layer development) numbers. Data were collected in a laminar flow wind tunnel for free stream velocities of 0.16-6.95 m s(-1). Scaling parameters were compared against literature values for silver cast branch replicas, a bed of real foliage, cylinders, and tube banks. Foliage structures reduced Nusselt numbers (heat transfer) relative to cylinders, which are typically used to approximate leafless branches and buds. Significantly different scaling relationships were observed for all foliage structures considered. Forced convection scaling relationships varied with foliage structure. The scaling relationships reported here account for variation within populations of branches and buds and can be used to characterize forced convection in a forest canopy.     

Leeway and constraints in the forced evolution of a regulatory RNA helix.

The start of the coat protein gene of RNA phage MS2 adopts a well-defined hairpin structure of 12 bp (including one mismatch) in which the start codon occupies the loop position. An earlier expression study using partial MS2 cDNA clones had indicated that the stability of this hairpin is important for gene expression. For every -1.4 kcal/mol increase in stability a 10-fold reduction in coat protein was obtained. Destabilizations beyond the wild-type value did not affect expression. These results suggested that the hairpin was tuned in the sense that it has the highest stability still compatible with maximal ribosome loading. Employing an infectious MS2 cDNA clone, we have now tested the prediction that the delta G 0 of the coat protein initiator helix is set at a precise value. We have introduced stabilizing and destabilizing mutations into this hairpin in the intact phage and monitored their evolution to viable species. By compensatory mutations, both types of mutants quickly revert along various pathways to wild-type stability, but not to wild-type sequence. As a rule the second-site mutations do not change the encoded amino acids or the Shine-Dalgarno sequence. The return of too strong hairpins to wild-type stability can be understood from the need to produce adequate supplies of coat protein. The return of unstable hairpins to wild-type stability is not self-evident and is presently not understood. The revertants provide an evolutionary landscape of slightly suboptimal phages, that were stable at least for the duration of the experiment (approximately 20 infection cycles).(ABSTRACT TRUNCATED AT 250 WORDS)     

On the use of size functions for shape analysis

According to a recent mathematical theory a shape can be represented by size functions, which convey information on both the topological and metric properties of the viewed shape. In this paper the relevance of the theory of size functions to visual perception is investigated. An algorithm for the computation of the size functions is presented, and many theoretical properties of the theory are demonstrated on real images. It is shown that the representation of shape in terms of size functions (1) can be tailored to suit the invariance of the problem at hand and (2) is stable against small qualitative and quantitative changes of the viewed shape. A distance between size functions is used as a measure of similarity between the representations of two different shapes. The results obtained indicate that size functions are likely to be very useful for object recognition. In particular, they seem to be well suited for the recognition of natural and articulated objects.     

Physical constrints on the evolution of ctenophore size and shape

Summary Cilia bundled into combs or ctenes are an evolutionary innovation that allow comb jellies (animals in the phylum Ctenophora) to swim faster and grow to sizes at least two orders of magnitude larger than animals that propel themselves by beating single cilia. Ctenophore size, shape and swimming behaviors, however, may be constrained by the mechanisms that coordinate comb plate oscillations.Oscillations of comb plates onPleurobrachia bachei (a cydippid comb jelly), are coupled by fluid interactions between combs. Ctenes beat metachronously (in sequence) and the flows generated byP. bachei are retarded by the amount of time it takes a wave to pass down a group of ctenes. Our model predicts thatP. bachei size is constrained by the maximum thrust that can be produced by ctenes that beat in sequence and our flow visualization studies suggest that swimming via metachronous comb oscillations may constrainP. bachei to spherical shapes.In contrast, comb plate oscillations onMnemiopsis leidyi, a lobate comb jelly, are neurally coordinated and groups of ctenes beat in synchrony. As a result, fluid flows generated byM. leidyi are not retarded by the passage of metachronal waves down each comb row.M. leidyi reach sizes 15 times larger, but swim relatively slower (body lengths per second) thanP. bachei.We propose that propulsion via metachronous or synchronous comb plate oscillations has played an important role in the evolution of ctenophore shape and size and may have divided comb jellies into two evolutionary lineages.     

Ecological and phylogenetic constraints on body size in Indo-Pacific fishes

Synopsis Regional Indo-Pacific fish faunas were examined for broad patterns in species size composition. An analysis of the New Guinea fauna, based on data compiled by Munro (1967), revealed that (i) maximum body size for a species tended to be larger in the more advanced teleost families; (ii) intrafamilial size variation (expressed by the standard deviation of log-transformed maximum body size) was significantly lower in the suborder Percoidei than in families drawn from broader taxonomic groupings; and (iii) size variation was significantly positively correlated with mean maximum body size and, in the percoids only, with the number of species in a family. An analysis of Marshall Islands reef fish assemblages, based mainly on the data of Matt & Strasburg (1960), indicated, that (i) mean maximum body size varied significantly between habitats and feeding categories, and tended to increase with openness of habitat and with trophic level; (ii) size variation within feeding categories increased with the number of species, but not significantly so; and (iii) confamilial species generally exhibited close similarities in terms of preferred habitats, trophic levels and foraging modes. These findings indicate that interspecific body size variation is both phylogenetically and ecologically constrained. Size variation within ecological categories (especially habitats) was much greater than within families. Thus, confamilial species generally did not exhibit the range of body sizes theoretically open to members of their habitat feeding guilds. The results are also consistent with aspects of resource-partitioning theory, notably that resource-utilisation breadth should increase with the number of coexisting species.     

Family resemblance for components of craniofacial size and shape

Path analysis is used to analyze family resemblance for eight principal components extracted from 30 anthropometric measurements of the craniofacial complex. Based on likelihood ratio tests, the null hypothesis of no assortative mating is rejected for the nasal height component. The null hypothesis of no common sibling environmental effect is rejected for the cranial size, craniofacial breadth, and nasal height components. Finally, the hypothesis that transmission from both mother and father is equal to 1/2, consistent with simple autosomal polygenic inheritance, is rejected for components corresponding to craniofacial breadth and upper facial height, thus implicating some effect of familial environment. Transmissibility is higher for components related to cranial size and facial height than for those related to facial breadth or ear dimensions.     

Evolution of plant shape: Design constraints

The fossil record documents the morphological and anatomical changes that have occurred over more than 400 million years of land plant evolution. Many of these changes can be interpreted, and even predicted, on the basis of standard engineering principles pertaining to mechanical stability, hydraulics, and the rescaling of form to maintain a constant level of performance or function. Theoretically based hypotheses, constructed using modern computer technology, can be compared with the major trends seen in the evolution of land plants and so provide insights into the factors motivating morphological and anatomical changes during the Phanerozoic.     

Thermodynamic constraints shape the structure of carbon fixation pathways

Thermodynamics impose a major constraint on the structure of metabolic pathways. Here, we use carbon fixation pathways to demonstrate how thermodynamics shape the structure of pathways and determine the cellular resources they consume. We analyze the energetic profile of prototypical reactions and show that each reaction type displays a characteristic change in Gibbs energy. Specifically, although carbon fixation pathways display a considerable structural variability, they are all energetically constrained by two types of reactions: carboxylation and carboxyl reduction. In fact, all adenosine triphosphate (ATP) molecules consumed by carbon fixation pathways - with a single exception - are used, directly or indirectly, to power one of these unfavorable reactions. When an indirect coupling is employed, the energy released by ATP hydrolysis is used to establish another chemical bond with high energy of hydrolysis, e.g. a thioester. This bond is cleaved by a downstream enzyme to energize an unfavorable reaction. Notably, many pathways exhibit reduced ATP requirement as they couple unfavorable carboxylation or carboxyl reduction reactions to exergonic reactions other than ATP hydrolysis. In the most extreme example, the reductive acetyl coenzyme A (acetyl-CoA) pathway bypasses almost all ATP-consuming reactions. On the other hand, the reductive pentose phosphate pathway appears to be the least ATP-efficient because it is the only carbon fixation pathway that invests ATP in metabolic aims other than carboxylation and carboxyl reduction. Altogether, our analysis indicates that basic thermodynamic considerations accurately predict the resource investment required to support a metabolic pathway and further identifies biochemical mechanisms that can decrease this requirement.     

The effect of visibility on territory size and shape

Although several authors have suggested that visibility affectsterritorial behavior, there have been few systematic studiesof this phenomenon. Here we review five ways that visibilitymight influence the use and defense of space in territorialanimals, and then we discuss a laboratory study on the effectof visibility on territory size and shape. Juvenile lizardswere allowed to establish territories in two habitats identicalexcept for a visual obstacle that bisected one of the two habitats.Lizards in the habitat with good visibility defended compactterritories of a size comparable to those in the field. In contrast,those in the low-visibility situation avoided territories thatincluded the visibility barrier and instead chose either verysmall or highly elongated territories. Territories with goodvisibility probably cost less to defend than those with poorvisibility; territory owners may also prefer high-visibilityterritories because they reduce predation risk or increase foragingsuccess.     

Evolution of eye size and shape in primates

Strepsirrhine and haplorhine primates exhibit highly derived features of the visual system that distinguish them from most other mammals. Comparative data link the evolution of these visual specializations to the sequential acquisition of nocturnal visual predation in the primate stem lineage and diurnal visual predation in the anthropoid stem lineage. However, it is unclear to what extent these shifts in primate visual ecology were accompanied by changes in eye size and shape. Here we investigate the evolution of primate eye morphology using a comparative study of a large sample of mammalian eyes. Our analysis shows that primates differ from other mammals in having large eyes relative to body size and that anthropoids exhibit unusually small corneas relative to eye size and body size. The large eyes of basal primates probably evolved to improve visual acuity while maintaining high sensitivity in a nocturnal context. The reduced corneal sizes of anthropoids reflect reductions in the size of the dioptric apparatus as a means of increasing posterior nodal distance to improve visual acuity. These data support the conclusion that the origin of anthropoids was associated with a change in eye shape to improve visual acuity in the context of a diurnal predatory habitus.