Transient synchronization following invasion: revisiting Moran’s model and a case study

Synchrony in forest insect outbreaks is important because the resulting regionalized outbreak dilutes the regulating effects of natural enemies, reduces the landscape’s ability to buffer the disturbance, exacerbates the economic burden on individual stakeholders, and overwhelms the logistical abilities of managers to suppress populations and mitigate impacts. Understanding the process of synchronization of dynamics is therefore a crucial aspect of understanding outbreak dynamics. We studied the second-order log-linear (autoregressive) model to ask what patterns of synchronization across invasion fronts may be expected from Moran’s model. Generally, we show that the time to synchronization in the log-linear model is a complex function of a number of parameters of which the overall strength of regulation, the strength of delayed statistical density dependence, and the relaxation time seem to be of particular importance. Interestingly, while environmental correlation is the crucial determinant of the magnitude of asymptotic synchrony, it does not appear to influence the transient process of synchronization. However, synchronization proceeds much more quickly among weakly periodic populations than among populations that are strongly periodic. As a case study, we investigate synchronization following colonization by gypsy moth (Lymantria dispar) populations located along the species’s expanding invasion front in northeastern USA. Data consisted of more than 100 years of county quarantine records and 30 years of detailed defoliation maps. We found that the dynamics of new populations tended to be initially out of synch with the broadly synchronized outbreaks within the established range. However, the outbreak dynamics of these new populations lock on to the regional patterns very quickly—within 10–15 years of invasion. Focusing on parameters that produce periodicity comparable to that seen in real gypsy moth populations, we discuss how the observed synchronization compares to that predicted by the log-linear model. While our results are equivocal, the synchronization appears to be surprisingly rapid, so more mechanistic models may be needed to explain the synchronization observed in this case study.

Spatial mechanisms for coexistence of species sharing a common natural enemy

We examine the conditions under which spatial structure can mediate coexistence of apparent competitors. We use a spatially explicit, host-parasitoid metapopulation model incorporating local dynamics of Nicholson-Bailey type and global dispersal. Depending on the model parameters, the resulting system displays a plethora of asynchronous dynamical behaviors for which permanent or transient coexistence is observed. We identify a number of spatially mediated tradeoffs which apparent competitors can utilize and demonstrate that the dynamics of spatial coexistence can typically be understood from consideration of two and three patch systems. The phase relationships of species abundances are different for our model than for some other mechanisms of spatial coexistence. We discuss the implications of our findings relative to issues of community organization and biological conservation.     

天津八仙山国家级自然保护区野生勺鸡集群模式和时空活动规律

2017年6月至2021年8月,基于红外相机技术在天津八仙山国家级自然保护区对野生勺鸡(Pucrasia macrolopha)开展集群模式和时空活动规律研究,研究期间共筛选出野生勺鸡独立有效照片218张,从中辨认出野生勺鸡266只次。研究发现：野生勺鸡呈现7种集群模式,以一雄一雌模式为主(χ² = 29.722,df = 6,P < 0.001);野生勺鸡呈昼行性活动,冬季核密度曲线为单峰型,其余3个季节均为双峰型,其活动高峰时间为,春季07:00 ~ 09:00时和14:00 ~ 16:00时,夏季08:00 ~ 10:00时和12:00 ~ 14:00时,秋季07:00 ~ 09:00时和15:00 ~ 17:00时,冬季10:00 ~ 12:00时;勺鸡的日活动节律存在性别差异,其活动特点呈明显错峰现象,其中,秋季的日活动强度存在显著性别差异(日活动曲线的重叠面积比Δ = 0.665 5,P < 0.001);勺鸡在春、夏、秋3个季节的活动均偏向集中在10 ~ 20 ℃温度区段和600 ~ 799 m海拔区段,而冬季的活动偏向集中在﹣9 ~ 0 ℃温度区段和800 ~ 999 m海拔区段,不同温度区段的活动强度(Friedman：χ² = 0.231,df = 3,P > 0.05)和不同海拔区段的活动强度(Friedman：χ² = 5.615,df = 3,P > 0.05)均无显著季节差异。本研究为今后野生勺鸡的生态监测和保护管理提供了科学参考。     

视觉皮层复杂细胞时空编码特性

针对输入在视皮层的编码表达,在地空滤波窗口基础上构建了一个复杂细胞时空编码模型,对几种特殊的输入函数进行了编码仿真实验,结果说明了视皮层复杂细胞时空整合编码序列的精细时间结构进行视觉输入的神经表象。     

基于热消散探针法的植物水分运输的研究进展

液流是分析树木耗水特性、研究树木水分传输机制的重要途径之一,热消散探针法已广泛用于树干液流变化的监测.热消散探针法是目前研究不同时空尺度上植物蒸腾耗水特性较为灵活、可靠、经济的一种方法.但由于物种特性的差异,可能造成试验过程中出现测量值与实际值相比偏低的状况.此外,相当一部分植物依赖树干储存水进行蒸腾,因此木材含水量的...     

Fractal fluctuations in spatiotemporal variables when walking on a self-paced treadmill

This study investigated the fractal dynamic properties of stride time (ST), stride length (SL) and stride speed (SS) during walking on a self-paced treadmill (STM) in which the belt speed is automatically controlled by the walking speed. Twelve healthy young subjects participated in the study. The subjects walked at their preferred walking speed under four conditions: STM, STM with a metronome (STM+met), fixed-speed (conventional) treadmill (FTM), and FTM with a metronome (FTM+met). To compare the fractal dynamics between conditions, the mean, variability, and fractal dynamics of ST, SL, and SS were compared. Moreover, the relationship among the variables was examined under each walking condition using three types of surrogates. The mean values of all variables did not differ between the two treadmills, and the variability of all variables was generally larger for STM than for FTM. The use of a metronome resulted in a decrease in variability in ST and SS for all conditions. The fractal dynamic characteristics of SS were maintained with STM, in contrast to FTM, and only the fractal dynamic characteristics of ST disappeared when using a metronome. In addition, the fractal dynamic patterns of the cross-correlated surrogate results were identical to those of all variables for the two treadmills. In terms of the fractal dynamic properties, STM walking was generally closer to overground walking than FTM walking. Although further research is needed, the present results will be useful in research on gait fractal dynamics and rehabilitation.     

Spatial modeling of dimerization reaction dynamics in the plasma membrane: Monte Carlo vs. continuum differential equations

Bimolecular reactions in the plasma membrane, such as receptor dimerization, are a key signaling step for many signaling systems. For receptors to dimerize, they must first diffuse until a collision happens, upon which a dimerization reaction may occur. Therefore, study of the dynamics of cell signaling on the membrane may require the use of a spatial modeling framework. Despite the availability of spatial simulation methods, e.g., stochastic spatial Monte Carlo (MC) simulation and partial differential equation (PDE) based approaches, many biological models invoke well-mixed assumptions without completely evaluating the importance of spatial organization. Whether one is to utilize a spatial or non-spatial simulation framework is therefore an important decision. In order to evaluate the importance of spatial effects a priori, i.e., without performing simulations, we have assessed the applicability of a dimensionless number, known as second Damköhler number (Da), defined here as the ratio of time scales of collision and reaction, for 2-dimensional bimolecular reactions. Our study shows that dimerization reactions in the plasma membrane with Da ∼> 0.1 (tested in the receptor density range of 10²–10⁵/μm²) require spatial modeling. We also evaluated the effective reaction rate constants of MC and simple deterministic PDEs. Our simulations show that the effective reaction rate constant decreases with time due to time dependent changes in the spatial distribution of receptors. As a result, the effective reaction rate constant of simple PDEs can differ from that of MC by up to two orders of magnitude. Furthermore, we show that the fluctuations in the number of copies of signaling proteins (noise) may also depend on the diffusion properties of the system. Finally, we used the spatial MC model to explore the effect of plasma membrane heterogeneities, such as receptor localization and reduced diffusivity, on the dimerization rate. Interestingly, our simulations show that localization of epidermal growth factor receptor (EGFR) can cause the diffusion limited dimerization rate to be up to two orders of magnitude higher at higher average receptor densities reported for cancer cells, as compared to a normal cell.     

Transport and accumulation of PVP-Hypericin in cancer and normal cells characterized by image correlation spectroscopy techniques

PVP-Hypericin (PVP: polyvinylpyrrolidone) is a potent anti-cancer photosensitizer for photodynamic diagnosis (PDD) and therapy (PDT). However, cellular targets and mechanisms involved in the cancer-selectivity of the photosensitizer are not yet fully understood. This paper gives new insights into the differential transport and localization of PVP-Hypericin in cancer and normal cells which are essential to unravel the mechanisms of action and cancer-selectivity. Temporal (TICS) and spatiotemporal (STICS) image correlation spectroscopy are used for the assessment of PVP-Hypericin diffusion and/or velocity in the case of concerted flow in human cervical epithelial HeLa and human lung carcinoma A549 cells, as well as in human primary dendritic cells (DC) and human peripheral blood mononuclear cells (PBMC). Spatiotemporal image cross-correlation spectroscopy (STICCS) based on organelle specific fluorescent labeling is employed to study the accumulation of the photosensitizer in nucleus, mitochondria, early-endosomes and lysosomes of the cells and to assess the dynamics of co-migrating molecules. Whereas STICS and TICS did not show a remarkable difference between the dynamics of PVP-Hypericin in HeLa, A549 and DC cells, a significantly different diffusion rate of the photosensitizer was measured in PBMC. STICCS detected a stationary accumulation of PVP-Hypericin within the nucleus, mitochondria, early endosomes and lysosomes of HeLa and A549 cells. However, significant flow due to the directed motion of the organelles was detected. In contrast, no accumulation in the nucleus and mitochondria of DC and PBMC could be monitored.     

Systems-level Modeling with Molecular Resolution Elucidates the Rate-limiting Mechanisms of Cellulose Decomposition by Cellobiohydrolases

Interprotein and enzyme-substrate couplings in interfacial biocatalysis induce spatial correlations beyond the capabilities of classical mass-action principles in modeling reaction kinetics. To understand the impact of spatial constraints on enzyme kinetics, we developed a computational scheme to simulate the reaction network of enzymes with the structures of individual proteins and substrate molecules explicitly resolved in the three-dimensional space. This methodology was applied to elucidate the rate-limiting mechanisms of crystalline cellulose decomposition by cellobiohydrolases. We illustrate that the primary bottlenecks are slow complexation of glucan chains into the enzyme active site and excessive enzyme jamming along the crowded substrate. Jamming could be alleviated by increasing the decomplexation rate constant but at the expense of reduced processivity. We demonstrate that enhancing the apparent reaction rate required a subtle balance between accelerating the complexation driving force and simultaneously avoiding enzyme jamming. Via a spatiotemporal systems analysis, we developed a unified mechanistic framework that delineates the experimental conditions under which different sets of rate-limiting behaviors emerge. We found that optimization of the complexation-exchange kinetics is critical for overcoming the barriers imposed by interfacial confinement and accelerating the apparent rate of enzymatic cellulose decomposition.     

Conditional gene expression in invertebrate animal models

A mechanistic understanding of biology requires appreciating spatiotemporal aspects of gene expression and its functional implications.Conditional expression allows for (ir)reversible switching of genes on or off,with the potential of spatial and/or temporal control.This provides a valuable complement to the more often used constitutive gene (in)activation through mutagenesis,providing tools to answer a wider array of research questions across biological disciplines.Spatial and/or temporal control are granted primarily by(combinations of) specific promoters,temperature regimens,compound addition,or illumination.The use of such genetic tool kits is particularly widespread in invertebrate animal models because they can be applied to study biological processes in short time frames and on large scales,using organisms amenable to easy genetic manipulation.Recent years witnessed an exciting expansion and optimization of such tools,of which we provide a comprehensive overview and discussion regarding their use in invertebrates.The mechanism,applicability,benefits,and drawbacks of each of the systems,as well as further developments to be expected in the foreseeable future,are highlighted.