POSITIVE VARIATIONS IN NITELLA

The reversible electrical variations hitherto described for plants and animals consist in a reversible loss of positive potential at a stimulated spot by which it becomes more negative. In this paper we describe changes which consist in a reversible loss of negative potential at a stimulated spot whereby it becomes more positive. We suggest that this be called a positive variation. The stimulation was produced in all cases by pinching or bending the cell. This produced a compression wave which traveled along the cell, producing a negative variation at a spot which was positive and a positive variation at a spot which was negative (due to application of 0.1 M KCl). The response produced by the compression wave differs in several respects from an ordinary propagated negative variation and may be termed a positive mechanical variation.     

Nano-mechanical Compliance of Müller Cells Investigated by Atomic Force Microscopy

It has been known that a single Müller cell displays a large variation in the cytoskeletal compositions along its cell body, suggesting different mechanical properties in different segments. Müller cells are thought to be involved in many retinal diseases such as retinoschisis, which can be facilitated by a mechanical stress. Thus, mapping of mechanical properties on localized nano-domains of Müller cells could provide essential information for understanding their structural functions in the retina and roles in their pathological progresses. Using Atomic Force Microscopy (AFM) - based bio-nano-mechanics, we have investigated the local variations of the mechanical properties of Müller cells in vitro. We have a particular interest in identifying elastic moduli in regions closer to three distinctive segments of the cells - process, endfoot, and soma. Using the modified spherical AFM probes, we were able to accurately determine mechanical properties, i.e., elastic moduli from the obtained force-distance curves. We found that the regions closer to soma were mechanically more compliant than regions closer to endfoot and process of Müller cells. We found that this lateral heterogeneity of the mechanical compliance within a single Müller cell is consistent with reports from other cell types. The local variation in mechanical compliances along a single Müller cell may support their diverse mechanical functions in the retina such as a soft mechanical embedding, mechanosensing, and neurotrophic functions for neurons.     

Interplay between traveling wave propagation and amplification at the apex of the mouse cochlea

Sounds entering the mammalian ear produce waves that travel from the base to the apex of the cochlea. An electromechanical active process amplifies traveling wave motions and enables sound processing over a broad range of frequencies and intensities. The cochlear amplifier requires combining the global traveling wave with the local cellular processes that change along the length of the cochlea given the gradual changes in hair cell and supporting cell anatomy and physiology. Thus, we measured basilar membrane (BM) traveling waves in vivo along the apical turn of the mouse cochlea using volumetric optical coherence tomography and vibrometry. We found that there was a gradual reduction in key features of the active process toward the apex. For example, the gain decreased from 23 to 19 dB and tuning sharpness decreased from 2.5 to 1.4. Furthermore, we measured the frequency and intensity dependence of traveling wave properties. The phase velocity was larger than the group velocity, and both quantities gradually decrease from the base to the apex denoting a strong dispersion characteristic near the helicotrema. Moreover, we found that the spatial wavelength along the BM was highly level dependent in vivo, such that increasing the sound intensity from 30 to 90 dB sound pressure level increased the wavelength from 504 to 874 μm, a factor of 1.73. We hypothesize that this wavelength variation with sound intensity gives rise to an increase of the fluid-loaded mass on the BM and tunes its local resonance frequency. Together, these data demonstrate a strong interplay between the traveling wave propagation and amplification along the length of the cochlea.     

PACEMAKERS IN NITELLA : III. ELECTRICAL ALTERNANS

An electrical impulse traveling along a Nitella cell may produce a complete or a partial response. The two kinds of response may occur in regular alternation. The partial response varies greatly and may be so far reduced as to appear as a local thickening in the upstroke of the action curve, usually accompanied by a more or less pronounced hump. In consequence a considerable variety of action curves is produced. The observations show that different regions of the cell may react differently.     

Testing genotypic variation of an invasive plant species in response to soil disturbance and herbivory

Herbivores, competitors, and predators can inhibit biological invasions (“biotic resistance” sensu Elton 1959), while disturbance typically promotes biological invasions. Although biotic resistance and disturbance are often considered separately in the invasion literature, these two forces may be linked. One mechanism by which disturbance may facilitate biological invasions is by decreasing the effectiveness of biotic resistance. The effects of both disturbance and biotic resistance may vary across invading genotypes, and genetic variation in the invasive propagule pool may increase the likelihood that some genotypes can overcome biotic resistance or take greater advantage of disturbance. We conducted an experimental field trial in which we manipulated soil disturbance (thatch removal and loosening soil) and the presence of insect herbivores and examined their effects on the invasion success of 44 Medicago polymorpha genotypes. As expected, insecticide reduced leaf damage and increased Medicago fecundity, suggesting that insect herbivores in this system provide some biotic resistance. Soil disturbance increased Medicago fecundity, but did not alter the effectiveness of biotic resistance by insect herbivores. We found significant genetic variation in Medicago in response to disturbance, but not in response to insect herbivores. These results suggest that the ability of Medicago to invade particular habitats depends on the amount of insect herbivory, the history of disturbance in the habitat, and how the specific genotypes in the invader pool respond to these factors.     

Fecundity and sexual size dimorphism of wolf spiders (Araneae: Lycosidae) along an elevational gradient in the Arctic

Fecundity and body size are central fitness-related traits, and their intra-specific responses to environmental variation are receiving increasing attention in the context of climate change. Recent results from Greenland indicate that temporal and spatial variation in body size differences between sexes (sexual size dimorphism) may be widespread among wolf spider species and could be related to climate. Here, we tested whether variation in elevation affected body size of three wolf spider (Araneae: Lycosidae) species in low-Arctic Canada, whether the sexes differed in their response to the cline, and whether changes in local density influenced this relationship. We also tested whether fecundity changed with elevation in two of the species, independent of body size variation. We found a significant sex–elevation interaction for Pardosa lapponica: female size decreased more in response to elevation than that of males. Males and females of Pardosa uintana decreased significantly in size with elevation at a similar rate. Alopecosa aculeata males increased in body size along the gradient while females did not. Pardosa lapponica females, but not P. uintana females, showed significant reduction in fecundity in response to elevation. P. uintana showed significant decreases in body size with increases in its population density. Changes in temperature and potential resource availability along the elevational gradient are probably causing these species- and sex-specific responses. Further summer warming of the region may alleviate current constraints on growth and reproduction of these species although sex-specific responses may affect their population dynamics.     

PACEMAKERS IN NITELLA : I. TEMPORARY LOCAL DIFFERENCES IN RHYTHM

A series of negative variations passing along the cell may reach a region where only every other variation registers. This condition may be temporary. It would seem to depend on a local change in the refractory period.     

Telomere shortening is associated with corticosterone stress response in adult barn swallows

When vertebrates face stressful events, the hypothalamic–pituitary–adrenal (HPA) axis is activated, generating a rapid increase in circulating glucocorticoid (GC) stress hormones followed by a return to baseline levels. However, repeated activation of HPA axis may lead to increase in oxidative stress. One target of oxidative stress is telomeres, nucleoprotein complexes at the end of chromosomes that shorten at each cell division. The susceptibility of telomeres to oxidizing molecules has led to the hypothesis that increased GC levels boost telomere shortening, but studies on this link are scanty. We studied if, in barn swallows Hirundo rustica, changes in adult erythrocyte telomere length between 2 consecutive breeding seasons are related to corticosterone (CORT) (the main avian GC) stress response induced by a standard capture-restraint protocol. Within-individual telomere length did not significantly change between consecutive breeding seasons. Second-year individuals showed the highest increase in circulating CORT concentrations following restraint. Moreover, we found a decline in female stress response along the breeding season. In addition, telomere shortening covaried with the stress response: a delayed activation of the negative feedback loop terminating the stress response was associated with greater telomere attrition. Hence, among-individual variation in stress response may affect telomere dynamics.     

The effect of human activities and their associated noise on ungulate behavior

Background

The effect of anthropogenic noise on terrestrial wildlife is a relatively new area of study with broad ranging management implications. Noise has been identified as a disturbance that has the potential to induce behavioral responses in animals similar to those associated with predation risk. This study investigated potential impacts of a variety of human activities and their associated noise on the behavior of elk (Cervus elaphus) and pronghorn (Antilocapra americana) along a transportation corridor in Grand Teton National Park.

Methodology/Principal Findings

We conducted roadside scan surveys and focal observations of ungulate behavior while concurrently recording human activity and anthropogenic noise. Although we expected ungulates to be more responsive with greater human activity and noise, as predicted by the risk disturbance hypothesis, they were actually less responsive (less likely to perform vigilant, flight, traveling and defensive behaviors) with increasing levels of vehicle traffic, the human activity most closely associated with noise. Noise levels themselves had relatively little effect on ungulate behavior, although there was a weak negative relationship between noise and responsiveness in our scan samples. In contrast, ungulates did increase their responsiveness with other forms of anthropogenic disturbance; they reacted to the presence of pedestrians (in our scan samples) and to passing motorcycles (in our focal observations).

Conclusions

These findings suggest that ungulates did not consistently associate noise and human activity with an increase in predation risk or that they could not afford to maintain responsiveness to the most frequent human stimuli. Although reduced responsiveness to certain disturbances may allow for greater investment in fitness-enhancing activities, it may also decrease detections of predators and other environmental cues and increase conflict with humans.     

Hydraulic development as a process of response

The relationship between political organization and hydraulic development is discussed, using illustrations from the Valley of Oaxaca, Mexico. It is suggested that change in political organization toward greater or lesser centralization of control over local water resource use may be part of a social response to environmental degradation (in this case, a receding water table) resulting from the effects of change in resource use (in this case, intensified water extraction). Economic stress due to environmental degradation varies in duration and intensity; such variations must be taken into account in understanding shifts in response patterns. Small-scale hydraulic development which places pressure on the availability of water for irrigation may elicit an initial response of centralization of control over its use to correct for the disturbance, water shortage. However, this response itself tends to exacerbate the disturbance through reducing previously existing homeostatic controls on the extraction of water and hence leads to greater economic stress, up to the point at which new responses must take place. Other kinds of environmental disturbance resulting from initial development attempts appear to have similar feedback effects which contribute to potential systemic instability.     

Analysis of inter-/intra-E-plate repeatability in the real-time cell analyzer

Over the past decade, the real-time cell analyzer (RTCA) has provided a good tool to the cell-based in vitro assay. Unlike the traditional systems that label the target cells with luminescence, fluorescence, or light absorption, RTCA monitors cell properties using noninvasive and label-free impedance measuring. However, realization of the maximum value of RTCA for applications will require assurance of within-experiment repeatability, day-to-day repeatability, and robustness to variations in conditions that might occur from different experiments. In this article, the performance and variability of RTCA is evaluated and a novel repeatability index (RI) is proposed to analyze the intra-/inter-E-plate repeatability of RTCA. The repeatability assay involves six cell lines and two media (water [H₂O] and dimethyl sulfoxide [DMSO]). First, six cell lines are exposed to the media individually, and time-dependent cellular response curves characterized as a cell index (CI) are recorded by RTCA. Then, the variations along sampling time and among repeated tests are calculated and RI values are obtained. Finally, a discriminating standard is set up to evaluate the degree of repeatability. As opposed to the standardized methodologies, it is shown that the presented index can give the quantitative evaluation for repeatability of RTCA within E-plate and variation on different days.     

Distant effects of toxic agents

Toxic solutions applied at one end of a Nitella cell 6 cm. long may produce little or no visible change in the structure of the protoplasm at the place of application but if the opposite end is covered with water its protoplasm soon disintegrates. If the middle of the cell is covered with mineral oil this region remains normal in appearance for half an hour or more. The result is due to the movement of substances in the cell. The loss of substances at the end where the toxic agent is applied results in loss at the opposite end if it is covered with water since water enters and travels along inside the cell carrying substances with it. This causes injury at the spot where the water enters. The conception developed here differs fundamentally from the usual view that the effects of injury spread gradually from the region where the toxic agent is applied to the immediately adjoining regions and thence to more remote places. The change produced by loss of substances produces an interesting pattern which deserves study.     

Invasive ants as back-seat drivers of native ant diversity decline in New Caledonia

Biological invasions are typically associated with disturbance, which often makes their impact on biodiversity unclear—biodiversity decline might be driven by disturbance, with the invader just being a ‘passenger’. Alternatively, an invader may act as a ‘back-seat driver’, being facilitated by disturbance that has already caused some biodiversity decline, but then causing further decline. Here we examine the interactive effects of anthropogenic fire and invasive ant species (Anoplolepis gracilipes or Wasmannia auropunctata) on native ant diversity in New Caledonia, a globally recognized biodiversity hotspot. We first examined native ant diversity at nine paired burnt and unburnt sites, with four pairs invaded by Anoplolepis, 5 years after an extensive fire. In the absence of invasion, native epigaeic ants were resilient to fire, but native ant richness and the abundance of Forest Opportunists were markedly lower in invaded burnt sites. Second, we examined native ant diversity along successional gradients from human-derived savanna to natural rainforest in the long-term absence of fire, where there was a disconnection between disturbance-mediated variation in microhabitat and the abundance of the disturbance specialist Wasmannia. All native ant diversity responses (total abundance, richness, species composition, functional group richness and the abundance of Forest Opportunists) declined independently of microhabitat variables but in direct association with high Wasmannia abundance. Our results indicate that invasive ants are acting as back-seat drivers of biodiversity decline in New Caledonia, with invasion facilitated by disturbance but then causing further biodiversity decline.     

Primate Abundance in an Unhunted Region of the Northern Peruvian Amazon and the Influence of Seismic Oil Exploration

The western Amazon, a relatively remote and unstudied region, is experiencing unprecedented levels of oil and gas exploration. Despite the widespread use of seismic reflection technology for oil and gas exploration, no studies have investigated the response of primate populations to this disturbance in the Amazon. We conducted distance sampling along transects in pristine, unhunted lowland rain forest inside a large oil concession (Block 39) in the northern Peruvian Amazon with ongoing 2D seismic exploration. We aimed to investigate seismic exploration effects on local primate abundance, with a particular focus on the region’s most endangered primates, lowland woolly monkeys (Lagothrix poeppigii) and white-bellied spider monkeys (Ateles belzebuth). We sampled transects before the arrival of exploration crews (control) and during the seismic operation (disturbance) and compared primate counts using mixed-effect models. We confirmed the presence of nine species of primates along sampled transects. Abundance of primate groups (species pooled) was not different between the control and disturbance period, yet abundance of primate individuals was significantly lower in the disturbance period. Although we encountered groups of Ateles belzebuth equally often during the control and disturbance periods, overall counts of individuals of this species were lower during the disturbance period owing to lower subgroup sizes. This suggests this species may have responded, at least temporarily, with localized spatial avoidance, and that it may be particularly sensitive to human disturbance regardless of hunting pressure. Our overall density estimate for Lagothrix poeppigii (30.6/km²) approaches the highest reported for the species. Although the relatively temporary nature of 2D seismic exploration may limit its effect on primate species, our data indicate some species may respond more negatively than others.     

Low sensitiveness of taxonomic distinctness indices to human impacts: Evidences across marine benthic organisms and habitat types

Ecological indicators likely constitute the mainstream tools in assessing the quality of aquatic ecosystems as they condense composite biological information into single measures, easier to handle for environmental managers and more understandable for non-scientific users. However, sampling settings can influence the performance of most indices, and their use is often constrained to specific habitat types. Average taxonomic distinctness (Δ⁺) and variation in taxonomic distinctness (Λ⁺) may represent promising tools in overcoming sampling bias, and potentially applicable to a wide range of environmental contexts. In marine systems, such metrics showed higher sensitivity than classical indices in discriminating among perturbed and unperturbed conditions, though a number of studies found Δ⁺ and Λ⁺ also varying along natural gradients, suggesting a lower ability to discern human-driven variations from natural variability than what expected. Here, analyzing existing data sets from previous impact assessment studies, we test the response of Δ⁺ and Λ⁺ in detecting the effects of different sources of anthropogenic disturbance on marine mollusks and polychaetes from Mediterranean soft sediments and hard substrates. Our results showed that neither classical univariate analyses on Δ⁺ and Λ⁺ values nor their associated statistical framework were able to discern among perturbed and unperturbed sites, highlighting a low sensitiveness of such metrics in detecting assemblage variations related to anthropogenic disturbance. Δ⁺ and Λ⁺ are based on presence/absence data, and assume impacts being likely to induce variations in taxonomic structures of assemblages. As a consequence, they could experience reduced discrimination power when impacts mostly affect relative abundances of organisms rather than assemblage composition, or drive species replacement within higher taxa. Our results also showed that habitat type could strongly affect taxonomic relatedness of species within assemblages, and that this effect can vary among different organisms, suggesting that the departures from expectation of Δ⁺ and Λ⁺ values might not be always univocally attributable to human perturbations, since possibly depending on habitat effects. Taxonomic distinctness indices can provide useful additional information on taxonomic diversity of assemblages, crucial to better address the wide concept of biological diversity. However, the effectiveness of such measures in disclosing the effects of human disturbance is still unclear, requiring further investigations especially on their potential application in defining the ecological status of coastal rocky systems.     

Patterns of Plant Biomass Allocation in Temperate Grasslands across a 2500-km Transect in Northern China

Plant biomass allocation between below- and above-ground parts can actively adapt to the ambient growth conditions and is a key parameter for estimating terrestrial ecosystem carbon (C) stocks. To investigate how climatic variations affect patterns of plant biomass allocation, we sampled 548 plants belonging to four dominant genera (Stipa spp., Cleistogenes spp., Agropyron spp., and Leymus spp.) along a large-scale (2500 km) climatic gradient across the temperate grasslands from west to east in northern China. Our results showed that Leymus spp. had the lowest root/shoot ratios among the each genus. Root/shoot ratios of each genera were positively correlated with mean annual temperature (MAT), and negatively correlated with mean annual precipitation (MAP) across the transect. Temperature contributed more to the variation of root/shoot ratios than precipitation for Cleistogenes spp. (C4 plants), whereas precipitation exerted a stronger influence than temperature on their variations for the other three genera (C3 plants). From east to west, investment of C into the belowground parts increased as precipitation decreased while temperature increased. Such changes in biomass allocation patterns in response to climatic factors may alter the competition regimes among co-existing plants, resulting in changes in community composition, structure and ecosystem functions. Our results suggested that future climate change would have great impact on C allocation and storage, as well as C turnover in the grassland ecosystems in northern China.     

MICROTUBULE ARMS AND CYTOPLASMIC STREAMING AND MICROTUBULE BENDING AND STRETCHING OF INTERTUBULE LINKS IN THE FEEDING TENTACLE OF THE SUCTORIAN CILIATE TOKOPHRYA

Microtubules attached to the pellicle at the tips of tentacles pivot through about 140° on these attachments, splay apart, and bend along their longitudinal axes when feeding occurs. The tubules could be bending in response to pellicular contractions; active bending, sliding, or contraction of the tubules may not be involved. Intertubule links apparently prevent tubules from splaying apart at certain levels. These links are probably under tension during feeding. They stretch; they sometimes become half as thick and eight times as long as they are before feeding. Often, tubules joined together by these links also change in shape; they become slightly flattened and elliptical in cross section. Cytoplasm from the ciliate Tetrahymena is drawn down a feeding tentacle inside an invagination of the Tokophrya cell membrane from the tentacle tip. The positions of arm-bearing microtubules around such invaginations indicate that arms are involved in moving invaginations along. The edges of the perforated Tetrahymena cell membrane are "sealed" to the cell membrane of Tokophrya around each feeding tentacle tip.     

Tensile properties and fiber alignment of human supraspinatus tendon in the transverse direction demonstrate inhomogeneity,nonlinearity, and regional isotropy

A recent study (Lake et al., 2009); reported the properties of human supraspinatus tendon (SST) tested along the predominant fiber direction. The SST was found to have a relatively disperse distribution of collagen fibers, which may represent an adaptation to multiaxial loads imposed by the complex loading environment of the rotator cuff. However, the multiaxial mechanical properties of human SST remain unknown. The objective of this study, therefore, was to evaluate the mechanical properties, fiber alignment, change in alignment with applied load, and structure–function relationships of SST in transverse testing. Samples from six SST locations were tested in uniaxial tension with samples oriented transverse to the tendon long-axis. Polarized light imaging was used to quantify collagen fiber alignment and change in alignment under applied load. The mechanical properties of samples taken near the tendon–bone insertion were much greater on the bursal surface compared to the joint surface (e.g., bursal moduli 15–30 times greater than joint; p<0.001). In fact, the transverse moduli values of the bursal samples were very similar to values obtained from samples tested along the tendon long-axis (Lake et al., 2009). This key and unexpected finding suggests planar mechanical isotropy for bursal surface samples near the insertion, which may be due to complex in vivo loading. Organizationally, fiber distributions became less aligned along the tendon long-axis in the toe-region of the stress–strain response. Alignment changes occurred to a slightly lesser degree in the linear-region, suggesting that movement of collagen fibers may play a role in mechanical nonlinearity. Transverse mechanical properties were significantly correlated with fiber alignment (e.g., for linear-region modulus r_s=0.74, p<0.0001), demonstrating strong structure–function relationships. These results greatly enhance current understanding of the properties of human SST and provide clinicians and scientists with vital information in attempting to treat or replace this complex tissue.     

Condition-dependent alteration of cellular immunity by secondary symbionts in the pea aphid,Acyrthosiphon pisum

Endosymbionts can fundamentally alter host physiology. Whether such changes are beneficial or detrimental to one or both partners may depend on the dynamics of the symbiotic relationship. Here we investigate the relationship between facultative symbionts and host immune responses. The pea aphid, Acyrthosiphon pisum, maintains an obligate primary symbiont, but may also harbour one or more facultative, secondary symbionts. Given their more transient nature and relatively recent adoption of a symbiotic lifestyle compared to primary symbionts, secondary symbionts may present a challenge for the host immune system. We assessed the response of several key components of the cellular immune system (phenoloxidase activity, encapsulation, immune cell counts) in the presence of alternative secondary symbionts, investigating the role of host and secondary symbiont genotype in specific responses. There was no effect of secondary symbiont presence on the phenoloxidase response, but we found variation in the encapsulation response and in immune cell counts based largely on the secondary symbiont. Host genotype was less influential in determining immunity outcomes. Our results highlight the importance of secondary symbionts in shaping host immunity. Understanding the complex physiological responses that can be propagated by host-symbiont associations has important consequences for host ecology, including symbiont and pathogen transmission dynamics.