Fingering instability of active nematic droplets

From the mitotic spindle up to tissues and biofilms, many biological systems behave as active droplets, which often break symmetry and change shape spontaneously. Here, I show that active nematic droplets can experience a fingering instability. I consider an active fluid that acquires nematic order through anchoring at the droplet interface, and I predict its morphological stability in terms of three dimensionless parameters: the anchoring angle, the penetration length of nematic order compared to droplet size, and an active capillary number. Droplets with extensile (contractile) stresses and planar (homeotropic) anchoring are unstable above a critical activity or droplet size. This instability is interfacial in nature: it arises through the coupling of active flows with interface motion, even when the bulk instability of active nematics cannot take place. In contrast to the dynamic states characteristic of active matter, the instability could produce static fingering patterns. The number of fingers increases with activity but varies non-monotonically with the nematic penetration length. Overall, these results pave the way towards understanding the self-organized shapes of biological systems, and towards designing patterns in active materials

How is rape a weapon of war?: feminist international relations, modes of critical explanation and the study of wartime sexual violence

Rape is a weapon of war. Establishing this now common claim has been an achievement of feminist scholarship and activism and reveals wartime sexual violence as a social act marked by gendered power. But the consensus that rape is a weapon of war obscures important, and frequently unacknowledged, differences in ways of understanding and explaining it. This article opens these differences to analysis. Drawing on recent debates regarding the philosophy of social science in IR and social theory, it interprets feminist accounts of wartime sexual violence in terms of modes of critical explanation – expansive styles of reasoning that foreground particular actors, mechanisms, reasons and stories in the formulation of research. The idea of a mode of critical explanation is expanded upon through a discussion of the role of three elements (analytical wagers, narrative scripts and normative orientations) which accomplish the theoretical work of modes. Substantive feminist accounts of wartime sexual violence are then differentiated in terms of three modes – of instrumentality, unreason and mythology – which implicitly structure different understandings of how rape might be a weapon of war. These modes shape political and ethical projects and so impact not only on questions of scholarly content but also on the ways in which we attempt to mitigate and abolish war rape. Thinking in terms of feminist modes of critical explanation consequently encourages further work in an unfolding research agenda. It clarifes the ways in which an apparently commonality of position can conceal meaningful disagreements about human action. Exposing these disagreements opens up new possibilities for the analysis of war rape

Cellular Sensing Governs the Stability of Chemotactic Fronts

In contexts ranging from embryonic development to bacterial ecology, cell populations migrate chemotactically along self-generated chemical gradients, often forming a propagating front. Here, we theoretically show that the stability of such chemotactic fronts to morphological perturbations is determined by limitations in the ability of individual cells to sense and thereby respond to the chemical gradient. Specifically, cells at bulging parts of a front are exposed to a smaller gradient, which slows them down and promotes stability, but they also respond more strongly to the gradient, which speeds them up and promotes instability. We predict that this competition leads to chemotactic fingering when sensing is limited at too low chemical concentrations. Guided by this finding and by experimental data on E. coli chemotaxis, we suggest that the cells' sensory machinery might have evolved to avoid these limitations and ensure stable front propagation. Finally, as sensing of any stimuli is necessarily limited in living and active matter in general, the principle of sensing-induced stability may operate in other types of directed migration such as durotaxis, electrotaxis, and phototaxis

Chemotactic smoothing of collective migration

Collective migration-the directed, coordinated motion of many self-propelled agents-is a fascinating emergent behavior exhibited by active matter with functional implications for biological systems. However, how migration can persist when a population is confronted with perturbations is poorly understood. Here, we address this gap in knowledge through studies of bacteria that migrate via directed motion, or chemotaxis, in response to a self-generated nutrient gradient. We find that bacterial populations autonomously smooth out large-scale perturbations in their overall morphology, enabling the cells to continue to migrate together. This smoothing process arises from spatial variations in the ability of cells to sense and respond to the local nutrient gradient-revealing a population-scale consequence of the manner in which individual cells transduce external signals. Altogether, our work provides insights to predict, and potentially control, the collective migration and morphology of cellular populations and diverse other forms of active matter. eLife digest Flocks of birds, schools of fish and herds of animals are all good examples of collective migration, where individuals co-ordinate their behavior to improve survival. This process also happens on a cellular level; for example, when bacteria consume a nutrient in their surroundings, they will collectively move to an area with a higher concentration of food via a process known as chemotaxis. Several studies have examined how disturbing collective migration can cause populations to fall apart. However, little is known about how groups withstand these interferences. To investigate, Bhattacharjee, Amchin, Alert et al. studied bacteria called Escherichia coli as they moved through a gel towards nutrients. The E. coli were injected into the gel using a three-dimensional printer, which deposited the bacteria into a wiggly shape that forces the cells apart, making it harder for them to move as a collective group. However, as the bacteria migrated through the gel, they smoothed out the line and gradually made it straighter so they could continue to travel together over longer distances. Computer simulations revealed that this smoothing process is achieved by differences in how the cells respond to local nutrient levels based on their position. Bacteria towards the front of the group are exposed to more nutrients, causing them to become oversaturated and respond less effectively to the nutrient gradient. As a result, they move more slowly, allowing the cells behind them to eventually catch-up. These findings reveal a general mechanism in which limitations in how individuals sense and respond to an external signal (in this case local nutrient concentrations) allows them to continue migrating together. This mechanism may apply to other systems that migrate via chemotaxis, as well as groups whose movement is directed by different external factors, such as temperature and light intensity

Frictiotaxis underlies focal adhesion-independent durotaxis

Cells move directionally along gradients of substrate stiffness — a process called durotaxis. In the situations studied so far, durotaxis relies on cell-substrate focal adhesions to sense stiffness and transmit forces that drive directed motion. However, whether and how durotaxis can take place in the absence of focal adhesions remains unclear. Here, we show that confined cells can perform durotaxis despite lacking focal adhesions. This durotactic migration depends on an asymmetric myosin distribution and actomyosin retrograde flow. We propose that the mechanism of this focal adhesion-independent durotaxis is that stiffer substrates offer higher friction. We put forward a physical model that predicts that non-adherent cells polarise and migrate towards regions of higher friction — a process that we call frictiotaxis. We demonstrate frictiotaxis in experiments by showing that cells migrate up a friction gradient even when stiffness is uniform. Our results broaden the potential of durotaxis to guide any cell that contacts a substrate, and they reveal a mode of directed migration based on friction. These findings have implications for cell migration during development, immune response and cancer progression, which usually takes place in confined environments that favour adhesion-independent amoeboid migration

Learning Form and Function by Dance-Dramatizing Cultural Legends to Drum Rhythms Wearing Student-Made Animal Masks

This study examined the self-efficacy in science, art, dance, and music; attitudes concerning contributions of people of various ethnic/cultural groups; and science learning of students involved in an after-school arts-integrated science enrichment project. Students dramatized three traditional animal legends from African, Native American, and Mexican cultures to drum beats while wearing student-made papier-mâché helmet crest masks of the animal characters. They learned the structure and functions of the featured animals through slide shows, embedded explanations in the play scripts, and hands-on form and function analogy materials that related the form and function of animal body parts to manufactured items. Although at least 40 students participated at times in the after-school program, matching pretest and posttest data were only available for 13 students. Results showed positive changes in students’ art self-efficacy with a medium effect size, improvements in knowledge of animal form and function with a large effect size, and a trend toward greater appreciation of the cultural contributions of different ethnic groups. Photographs of student-made masks and the animal legend scripts with added form-and-function content are provided

Emergence and spread of highly pathogenic avian influenza A(H5N8) in Europe in 2016‐2017

Circulation of highly pathogenic avian influenza (HPAI) viruses poses a continuousthreat to animal and public health. After the 2005–2006 H5N1 and the 2014–2015H5N8 epidemics, another H5N8 is currently affecting Europe. Up to August 2017,1,112 outbreaks in domestic and 955 in wild birds in 30 European countries havebeen reported, the largest epidemic by a HPAI virus in the continent. Here, the mainepidemiological findings are described. While some similarities with previous HPAIvirus epidemics were observed, for example in the pattern of emergence, significantdifferences were also patent, in particular the size and extent of the epidemic. Eventhough no human infections have been reported to date, the fact that A/H5N8 hasaffected so far 1,112 domestic holdings, increases the risk of exposure of humansand therefore represents a concern. Understanding the epidemiology of HPAIviruses is essential for the planning future surveillance and control activities.info:eu-repo/semantics/publishedVersio

Mosquito alert: leveraging citizen science to create a GBIF mosquito occurrence dataset

The Mosquito Alert dataset includes occurrence records of adult mosquitoes collected worldwide in 2014-2020 through Mosquito Alert, a citizen science system for investigating and managing disease-carrying mosquitoes. Records are linked to citizen science-submitted photographs and validated by entomologists to determine the presence of five targeted European mosquito vectors: Aedes albopictus, Ae. aegypti, Ae. japonicus, Ae. koreicus, and Culex pipiens. Most records are from Spain, reflecting Spanish national and regional funding, but since autumn 2020 substantial records from other European countries are included, thanks to volunteer entomologists coordinated by the AIM-COST Action, and to technological developments to increase scalability. Among other applications, the Mosquito Alert dataset will help develop citizen science-based early warning systems for mosquito-borne disease risk. It can also be reused for modelling vector exposure risk, or to train machine-learning detection and classification routines on the linked images, to assist with data validation and establishing automated alert systems

The role of innate immune responses against two strains of PEDV (S INDEL and non-S INDEL) in newborn and weaned piglets inoculated by combined orogastric and intranasal routes

IntroductionPorcine epidemic diarrhea (PED) is a severe gastrointestinal disease in swine caused by PED virus (PEDV), leading to significant economic losses worldwide. Newborn piglets are especially vulnerable, with nearly 100% mortality, unlike older pigs. Disease severity also varies depending on the PEDV strain, with non-S INDEL strains being more virulent than S INDEL ones.MethodsThis study examined early pathogenesis and innate immunity in 5-day-old suckling and 5-week-old weaned piglets (n=8 per age group, 4 per strain) inoculated with S INDEL or non-S INDEL PEDV strains via combined orogastric and intranasal route. Age matched negative controls (n=3 per age group) were included. Body weight, temperature, and clinical signs were monitored for 48 hours post-inoculation (hpi). PEDV RNA levels were assessed in rectal swabs (RS) at 0 and 48 hpi, while pathological analyses and viral RNA loads were measured in jejunal content and intestinal mucosa. Gene expression of 75 selected antiviral and inflammatory genes were determined in laser capture microdissection (LCM)–derived jejunal samples using microfluidic qPCR at 48 hpi.ResultsSuckling piglets showed severe clinical signs, while weaned piglets were mostly asymptomatic at 48 hpi. In general, clinical signs and lesions in suckling piglets were similar, regardless of the PEDV strain. Both viral strains produced comparable viral RNA loads in the small intestine and feces, as well as consistent villous atrophy and fusion across age groups. In LCM-derived jejunal samples, weaned piglets had higher expression of antiviral genes (type I/III interferons, ISGs) and Th1/Th17 pro-inflammatory genes, particularly with the non-S INDEL strain. Conversely, the anti-inflammatory cytokine IL-10 was overexpressed in suckling compared to weaned piglets for both strains.DiscussionOverall, PEDV-induced intestinal damage, viral replication, and excretion were similar in studied groups regardless of viral strain or piglet age. The reduced clinical severity in weaned piglets may result from their stronger intestinal antiviral and pro-inflammatory response

Previous SARS-CoV-2 Infection Increases B.1.1.7 Cross-Neutralization by Vaccinated Individuals

With the spread of new variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), there is a need to assess the protection conferred by both previous infections and current vaccination. Here we tested the neutralizing activity of infected and/or vaccinated individuals against pseudoviruses expressing the spike of the original SARS-CoV-2 isolate Wuhan-Hu-1 (WH1), the D614G mutant and the B.1.1.7 variant. Our data show that parameters of natural infection (time from infection and nature of the infecting variant) determined cross-neutralization. Uninfected vaccinees showed a small reduction in neutralization against the B.1.1.7 variant compared to both the WH1 strain and the D614G mutant. Interestingly, upon vaccination, previously infected individuals developed more robust neutralizing responses against B.1.1.7, suggesting that vaccines can boost the neutralization breadth conferred by natural infection