Floral Temperature and Optimal Foraging: Is Heat a Feasible Floral Reward for Pollinators?

As well as nutritional rewards, some plants also reward ectothermic pollinators with warmth. Bumble bees have some control over their temperature, but have been shown to forage at warmer flowers when given a choice, suggesting that there is some advantage to them of foraging at warm flowers (such as reducing the energy required to raise their body to flight temperature before leaving the flower). We describe a model that considers how a heat reward affects the foraging behaviour in a thermogenic central-place forager (such as a bumble bee). We show that although the pollinator should spend a longer time on individual flowers if they are warm, the increase in total visit time is likely to be small. The pollinator's net rate of energy gain will be increased by landing on warmer flowers. Therefore, if a plant provides a heat reward, it could reduce the amount of nectar it produces, whilst still providing its pollinator with the same net rate of gain. We suggest how heat rewards may link with plant life history strategies.     

The intermediate filament network in cultured human keratinocytes is remarkably extensible and resilient

The prevailing model of the mechanical function of intermediate filaments in cells assumes that these 10 nm diameter filaments make up networks that behave as entropic gels, with individual intermediate filaments never experiencing direct loading in tension. However, recent work has shown that single intermediate filaments and bundles are remarkably extensible and elastic in vitro, and therefore well-suited to bearing tensional loads. Here we tested the hypothesis that the intermediate filament network in keratinocytes is extensible and elastic as predicted by the available in vitro data. To do this, we monitored the morphology of fluorescently-tagged intermediate filament networks in cultured human keratinocytes as they were subjected to uniaxial cell strains as high as 133%. We found that keratinocytes not only survived these high strains, but their intermediate filament networks sustained only minor damage at cell strains as high as 100%. Electron microscopy of stretched cells suggests that intermediate filaments are straightened at high cell strains, and therefore likely to be loaded in tension. Furthermore, the buckling behavior of intermediate filament bundles in cells after stretching is consistent with the emerging view that intermediate filaments are far less stiff than the two other major cytoskeletal components F-actin and microtubules. These insights into the mechanical behavior of keratinocytes and the cytokeratin network provide important baseline information for current attempts to understand the biophysical basis of genetic diseases caused by mutations in intermediate filament genes.     

Relationships between soil bulk electrical conductivity and the principal component analysis of topography and soil fertility values

Measures of soil electrical conductivity (EC) and elevation are relatively inexpensive to collect and result in dense data sets which allow for mapping with limited interpolation. Conversely, soil fertility information is expensive to collect so that relatively few samples are taken and mapping requires extensive interpolation with large estimation errors, resulting in limited usefulness for site-specific applications in precision agriculture. Principal component (PC) analysis and cokriging can be applied to create meaningful field scale summaries of groups of attributes and to decrease the estimation error of maps of the summarized attributes. Deep (0–90 cm) and shallow (0–30 cm) EC, elevation, and soil fertility attributes were measured in fields under corn (Zea mays L.) and soybean (Glycine max L.) rotations, at two sites in Illinois (IL) and two sites in Missouri (MO). Soil fertility and topography attributes were summarized by PC analysis. The first topography PC (TopoPC1) contrasted flow accumulation against elevation and curvature, to describe the main topographic pattern of the fields. The first soil fertility PC (SoilPC1) consistently grouped together cation exchange capacity (CEC), Ca, Mg, and organic matter (OM). SoilPC1 was well correlated to soil EC for all sites and cokriging with EC had higher r ² in the crossvariogram models compared to ordinary kriging. The second and third soil fertility PCs (SoilPC2 and SoilPC3) were concerned with soil pH and P, and reflected historic land use patterns. Maps of SoilPC2 and SoilPC3 had little relationship to soil EC or topography and so could not be improved by cokriging.     

Fruit colour polymorphism in Acacia ligulata: seed and seedling performance, clinal patterns, and chemical variation

Fruit colour polymorphisms are widespread in nature, but their ecological and evolutionary dynamics remain poorly understood. Here we examine Acacia ligulata, a shrub of the Australian arid zone which exhibits a red/orange/yellow aril colour polymorphism. We asked whether the polymorphism had a genetic basis; whether selection acted differentially on morphs during the seed and seedling stages; whether geographic variation in morph frequencies was correlated with environmental factors; and whether morphs differed in physical or chemical characteristics that might influence selection on them. When grown to maturity in a common greenhouse environment, maternal families of seeds showed phenotypic patterns consistent with biparental genetic control of the polymorphism. In contrast to other fruit-colour polymorphic species, progeny of A. ligulata morphs did not vary in rates of seedling emergence or survival in a common garden. Sampling along a 580 km transect revealed clinal variation in morph frequencies. Frequencies of the yellow morph decreased, and frequencies of the red morph increased, across a gradient of decreasing temperature and increasing rainfall. Morphs did not differ in seed mass, aril mass, or in profiles of fatty acids and flavonoids in either arils or seeds. However, morphs showed consistent differences in carotenoid profiles' and elemental content of arils, suggesting that selection by avian and insect seed dispersers, seed predators and herbivores should be investigated. These patterns indicate that both abiotic and biotic factors may contribute to selection on the A. ligulata polymorphism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Substrate profiling of PRMT1 reveals amino acid sequences that extend beyond the "RGG" paradigm

Protein arginine methyltransferase 1 (PRMT1) catalyzes the mono- and dimethylation of certain protein arginine residues. Although this posttranslational modification has been implicated in many physiological processes, the molecular basis for PRMT1 substrate recognition is poorly understood. Most modified arginine residues in known PRMT1 substrates reside in repeating "RGG" sequences. However, PRMT1 also specifically methylates Arg3 of histone H4 in a region that is not glycine-arginine rich, suggesting that PRMT1 substrates are not limited to proteins bearing "RGG" sequences. Because a systematic evaluation of PRMT1 substrate specificity has not been performed, it is unclear if the "RGG" sequence accurately represents the consensus target for PRMT1. Using a focused peptide library based on a sequence derived from the in vivo substrate fibrillarin we observed that PRMT1 methylated substrates that had amino acid residues other than glycine in the "RX (1)" and "RX (1)X (2)" positions. Importantly, eleven additional PRMT1 substrate sequences were identified. Our results also illustrate that the two residues on the N-terminal side of the modification site are important and need not both be glycine. PRMT1 methylated the eukaryotic initiation factor 4A1 (eIF4A1) protein, which has a single "RGG" sequence. Methylation of eIF4A1 and the similar eIF4A3 could be affected using single site mutations adjacent to the modification site, demonstrating the importance of amino acid sequence in PRMT1 protein substrates. Dimethylation of the parent library peptide was shown to occur through a dissociative mechanism. In summary, PRMT1 selectively recognizes a set of amino acid sequences in substrates that extend beyond the "RGG" paradigm.     

Environment quality predicts parental provisioning decisions

Although avian parents appear to exhibit a variety of feeding strategies in nature, there currently exist no models or theories that account for this range of diversity. Here we present the results of a computer simulation designed to model inter-dependent parental decisions, where investment is meted out in small doses, and must be distributed over time to maximize return on investment at the end of the parental-care period. With this technique we show that the success of various simple, observed, parental rules of thumb varies with environmental resource level, and that increasing the complexity of parental decision rules does not necessarily result in increased fitness.