Proximity to forest edge does not affect crop production despite pollen limitation

A decline in pollination function has been linked to agriculture expansion and intensification. In northwest Argentina, pollinator visits to grapefruit, a self-compatible but pollinator-dependent crop, decline by approximately 50% at 1km from forest edges. We evaluated whether this decrease in visitation also reduces the pollination service in this crop. We analysed the quantity and quality of pollen deposited on stigmas, and associated limitation of fruit production at increasing distances (edge: 10, 100, 500 and 1000m) from the remnants of Yungas forest. We also examined the quantitative and qualitative efficiency of honeybees as pollen vectors. Pollen receipt and pollen tubes in styles decreased with increasing distance from forest edge; however, this decline did not affect fruit production. Supplementation of natural pollen with self- and cross-pollen revealed that both pollen quantity and quality limited fruit production. Despite pollen limitation, honeybees cannot raise fruit production because they often do not deposit sufficient high-quality pollen per visit to elicit fruit development. However, declines in visitation frequency well below seven visits during a flower's lifespan could decrease production beyond current yields. In this context, the preservation of forest remnants, which act as pollinator sources, could contribute to resilience in crop production. Like wild plants, pollen limitation of the yield among animal-pollinated crops may be common and indicative not only of pollinator scarcity, but also of poor pollination quality, whereby pollinator efficiency, rather than just abundance, can play a broader role than previously appreciated.     

Emergent material properties of developing epithelial tissues

Background

Force generation and the material properties of cells and tissues are central to morphogenesis but remain difficult to measure in vivo. Insight is often limited to the ratios of mechanical properties obtained through disruptive manipulation, and the appropriate models relating stress and strain are unknown. The Drosophila amnioserosa epithelium progressively contracts over 3 hours of dorsal closure, during which cell apices exhibit area fluctuations driven by medial myosin pulses with periods of 1.5–6 min. Linking these two timescales and understanding how pulsatile contractions drive morphogenetic movements is an urgent challenge.

Results

We present a novel framework to measure in a continuous manner the mechanical properties of epithelial cells in the natural context of a tissue undergoing morphogenesis. We show that the relationship between apicomedial myosin fluorescence intensity and strain during fluctuations is consistent with a linear behaviour, although with a lag. We thus used myosin fluorescence intensity as a proxy for active force generation and treated cells as natural experiments of mechanical response under cyclic loading, revealing unambiguous mechanical properties from the hysteresis loop relating stress to strain. Amnioserosa cells can be described as a contractile viscoelastic fluid. We show that their emergent mechanical behaviour can be described by a linear viscoelastic rheology at timescales relevant for tissue morphogenesis. For the first time, we establish relative changes in separate effective mechanical properties in vivo. Over the course of dorsal closure, the tissue solidifies and effective stiffness doubles as net contraction of the tissue commences. Combining our findings with those from previous laser ablation experiments, we show that both apicomedial and junctional stress also increase over time, with the relative increase in apicomedial stress approximately twice that of other obtained measures.

Conclusions

Our results show that in an epithelial tissue undergoing net contraction, stiffness and stress are coupled. Dorsal closure cell apical contraction is driven by the medial region where the relative increase in stress is greater than that of stiffness. At junctions, by contrast, the relative increase in the mechanical properties is the same, so the junctional contribution to tissue deformation is constant over time. An increase in myosin activity is likely to underlie, at least in part, the change in medioapical properties and we suggest that its greater effect on stress relative to stiffness is fundamental to actomyosin systems and confers on tissues the ability to regulate contraction rates in response to changes in external mechanics.

     

Seed characteristics and soil surface patch type interact to affect germination of semi-arid woodland species

Biological soil crusts are common in many arid and semi-arid regions and they can alter microenvironments which are likely to directly and indirectly influence vascular plant establishment. The effect of biological soil crusts on germination is also influenced by the biological characteristics of the seeds themselves but rarely have the effects of both crust type and seed morphology on germination been examined in the same study. In this study, seed of five semi-arid woodland species with contrasting seed morphology were sown on top of patch types that commonly occur in natural woodlands (foliose lichen, short-turf moss, tree leaf litter, disturbed crust) and their emergence was followed. Percent germination varied between patch types and, for the largest-seeded species (Maireana excavata), final germination was significantly lower on the biological soil crust and litter patch types because they strongly acted as a physical barrier to seed penetration into the soil. The remaining four species showed no significant differences in final percent germination with patch type because most seeds either completely or partially penetrated the surface layer. Germination time courses, however, showed that biological soil crusts delayed the timing of germination of these species. Hence, soil crusts might differentially affect the spatial patterning of species in semi-arid woodlands by their subtle influence on seedling emergence that is determined by differences in seed morphology and subsequent positioning within crusts.     

Emergent properties of conspecific attraction in fragmented landscapes

Attraction to conspecifics may have wide-ranging implications for habitat selection and metapopulation theory, yet little is known about the process of attraction and its effects relative to other habitat selection strategies. Using individual-based simulations, I investigated the emergent properties of conspecific attraction during habitat selection on survival, fecundity, short-term fitness (survival x fecundity), and distributions in fragmented landscapes. I simulated conspecific attraction during searching and settlement decisions and compared attraction with random, habitat-based (searching for the presence of habitat), and habitat quality sampling strategies (searching for and settling in high-quality habitat). Conspecific attraction during searching or settlement decisions had different consequences for animals: attraction while searching increased survival by decreasing time spent in nonsuitable habitat, whereas attraction during settlement increased fecundity by aggregating animals in high-quality habitats. Habitat-based sampling did not improve fitness over attraction, but directly sampling habitat quality resulted in the highest short-term fitness among strategies. These results suggest that attraction can improve fitness when animals cannot directly assess habitat quality. Interestingly, conspecific attraction influenced distributions by generating patch size effects and weak edge effects, highlighting that attraction is one potential, yet previously unappreciated, mechanism to explain the widespread patterns of animal sensitivity to habitat fragmentation.     

Emergent properties of electrically coupled smooth muscle cells

Asynchronous and synchronous calcium oscillations occur in a variety of cells. A well-established pathway for intercellular communication is provided by gap junctions which connect adjacent cells and can mediate electrical and chemical coupling. Several experimental studies report that cells presenting only a transient increase when freshly dispersed may oscillate when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. Here we illustrate the emergent properties of electrically coupled smooth muscle cells using a model that we recently proposed. A bifurcation analysis in the case of two cells reveals that synchronous and asynchronous calcium oscillations can be induced by electrical coupling. In a larger population of smooth muscle cells, electrical coupling may result in the creation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non-oscillating when uncoupled. All these observations remain true in the case of non-identical cells, except that the solution corresponding to synchronous calcium oscillations disappears and that the formation of groups is sensitive to the degree of heterogeneity. The first two authors contributed equally to this work.     

Emergent functional properties of neuronal networks with controlled topology

The interplay between anatomical connectivity and dynamics in neural networks plays a key role in the functional properties of the brain and in the associated connectivity changes induced by neural diseases. However, a detailed experimental investigation of this interplay at both cellular and population scales in the living brain is limited by accessibility. Alternatively, to investigate the basic operational principles with morphological, electrophysiological and computational methods, the activity emerging from large in vitro networks of primary neurons organized with imposed topologies can be studied. Here, we validated the use of a new bio-printing approach, which effectively maintains the topology of hippocampal cultures in vitro and investigated, by patch-clamp and MEA electrophysiology, the emerging functional properties of these grid-confined networks. In spite of differences in the organization of physical connectivity, our bio-patterned grid networks retained the key properties of synaptic transmission, short-term plasticity and overall network activity with respect to random networks. Interestingly, the imposed grid topology resulted in a reinforcement of functional connections along orthogonal directions, shorter connectivity links and a greatly increased spiking probability in response to focal stimulation. These results clearly demonstrate that reliable functional studies can nowadays be performed on large neuronal networks in the presence of sustained changes in the physical network connectivity.     

Unpreferred plants affect patch choice and spatial distribution of European brown hares

Many herbivore species prefer to forage on patches of intermediate biomass. Plant quality and forage efficiency are predicted to decrease with increasing plant standing crop which explains the lower preference of the herbivore. However, often is ignored that on the long-term, plant species composition is predicted to change with increasing plant standing crop. The amount of low-quality, unpreferred food plants increases with increasing plant standing crop. In the present study the effects of unpreferred plants on patch choice and distribution of European brown hare in a salt-marsh system were studied. In one experiment, unpreferred plants were removed from plots. In the second experiment, plots were planted with different densities of an unpreferred artificial plant. Removal of unpreferred plants increased hare-grazing pressure more than fivefold compared to unmanipulated plots. Planting of unpreferred plants reduced hare-grazing pressure, with a significant reduction of grazing already occurring at low unpreferred plant density. Spatial distribution of hares within this salt-marsh system was related to spatial arrangement of unpreferred plants. Hare-grazing intensity decreased strongly with increasing abundance of unpreferred plants despite a high abundance of principal food plants. The results of this study indicate that plant species replacement is an important factor determining patch choice and spatial distribution of hares next to changing plant quality. Increasing abundance of unpreferred plant species can strengthen the decreasing patch quality with increasing standing crop and can decrease grazing intensity when preferred food plants are still abundantly present.     

Emergent properties in the behaviour of a virtual spider robot

Using a virtual spider robot, we studied hypotheses about the weaving behaviour of orb spiders. Our model spiders built virtual webs that mimicked perfectly the visual architecture of real webs of the garden cross spider Araneus diadematus. The matching of capture spiral and auxiliary spiral pitch was an apparently emergent property in both types of web. This validated our interpretation of the garden spider''s web-building decision rules, which use strictly local interactions with previously placed threads to generate global architecture.     

How predator incursions affect critical patch size: the role of the functional response

Understanding the impact of habitat edges provides a key to deciphering how community dynamics change as functions of habitat structure and spatial scale. Motivated by studies of predation on bird nests in forest fragments and other cases of "cross-boundary subsidies," we present results from a partial differential equation model in which a patch-resident prey species suffers incidental mortality from a generalist predator species residing in the surrounding matrix habitat. We demonstrate that predator intrusions have the potential to induce critical patch size effects for the prey species, even when the prey's dynamics would otherwise preclude such effects. We also demonstrate that the existence of critical patch size effects depends on the functional response of the predator, with Lotka-Volterra and Type II functional responses generating the effect (but not Type III). We conclude by discussing how predator-induced critical patch size effects can influence opportunities for regionwide persistence of the prey by altering the fraction and spatial distribution of meaningful patches within a metapopulation.     

Emergent properties of proteostasis-COPII coupled systems in human health and disease

In eukaryotic membrane trafficking, emergent protein folding pathways dictated by the proteostasis network (the 'PN') in each cell type are linked to the coat protein complex II (COPII) system that initiates transport through the exocytic pathway. These coupled pathways direct the transit of protein cargo from the endoplasmic reticulum (ER) to diverse subcellular and extracellular destinations. Understanding how the COPII system selectively manages the trafficking of distinct folded states of nascent cargo (comprising one-third of the proteins synthesized by the eukaryotic genome) in close cooperation with the PN remains a formidable challenge to the field. Whereas the PN may contain a thousand component, the minimal COPII coat components that drive all vesicle budding from the ER include Sar1 (a GTPase), Sec12 (a guanine nucleotide exchange factor), Sec23-Sec24 complexes (protein cargo selectors) and the Sec13-Sec31 complex (that functions as a protein cargo collector and as a polymeric lattice generator to promote vesicle budding). A wealth of data suggests a hierarchical role of the PN and COPII components in coupling protein folding with recruitment and assembly of vesicle coats on the ER. In this minireview, we focus on insights recently gained from the study of inherited human disease states of the COPII machinery. We explore the relevance of the COPII system to human biology in the context of its inherent link with the remarkably flexible folding capacity of the PN in each cell type and in response to the environment. The pharmacological manipulation of this coupled system has important therapeutic implications for restoration of function in human disease.     

Transport properties and hydrodynamic centers of rigid macromolecules with arbitrary shapes

A general formalism, which includes translation–rotation coupling, is proposed for calculating translational and rotational transport properties, as well as intrinsic viscosities, of rigid macromolecules with an arbitrary shape. This formalism is based on Brenner's theory of translational–rotational dynamics and on methods for the calculation of hydrodynamic properties that have been already presented, and can be regarded as a generalization of the one proposed by Nakajima and Wada. The calculated transport properties depend on the origin as predicted by Brenner's theory, but in a disagreement with him, the center of resistance and the center of diffusion do not coincide. As one can define several hydrodynamic centers, which in practice turn out to be located at different points, the influence of the choice of the center on the calculated transport properties is discussed. An analysis of the translation–rotation coupling effects in translational diffusion reveals that they arise exclusively from hydrodynamic interactions and are rather small in some cases of interest. Finally, we present a study of the rotational diffusion of rigid bent rods with a fixed length-to-diameter ratio. The diffusion coefficients obtained can be useful to estimate changes with respect to a straight rod.     

Cutting edge: anti-inflammatory properties of low levels of IFN-gamma

Activation of naive T and B cells occurs only within the context of organized lymphoid tissue. Thus, the continuous recirculation of mature lymphocytes is crucial for the development of primary immune response to foreign Ags. We have previously shown that low levels of IFN-gamma inhibit homing of B cells to the secondary lymphoid organs. In this study, we demonstrate that similarly low doses of IFN-gamma down-regulate integrin-mediated adhesion and migration of naive T and Th2 cells, and have a profound effect on the in vivo homing of naive T cells to the lymph nodes. Moreover, we show that these low doses of IFN-gamma have anti-inflammatory effects in an in vivo asthma model. Thus, in contrast to the proinflammatory effects of IFN-gamma at relatively high concentrations, low dose IFN-gamma appears to exert global suppressory effects on T cell trafficking and may have clinical application as an anti-inflammatory agent.     

Emergent properties of coupled enzyme reaction systems 1. Switching and clustering behaviour

The dynamics of locally and globally coupled cells that convert a substrate to a product via an uncompetitive substrate-inhibition mechanism is studied. When the cell-cell coupling strength is below a threshold value, the coupled system exhibits a large number of steady states; however, all cells cluster to one state when coupling exceeds the threshold value. The coupled system also exhibits a buffering capacity that maintains low and almost constant intracellular and extracellular substrate levels; however, there exists a threshold value on the influx rate of extracellular substrate beyond which the system switches to higher substrate levels. This transition becomes sharper as the number of coupled cells increases. Propagation failure of concentration fronts between adjacent cells is also exhibited by the system.