Lack of Mitotic Delays at the Onset of Proliferation in Dormant Root Primordia Challenged by Ionizing Radiation

X-rays at doses between 2.5 and 20 Gy were applied to Allium cepa L. bulbs containing either dormant root primordia (before water imbibition) or actively proliferating meristems. Irradiation of the primordia that were enriched in G₀ cells neither delayed proliferation onset nor root sprouting. Under both protocols, irradiation decreased the final length of the roots to about 60 % (at 20 Gy) of that reached by the unirradiated controls. Irradiation of the proliferating meristems increased the mitotic index at some fixation times. This could not be due to a rise in the cell entry into mitosis, as the rate of root growth decreased simultaneously. The increased mitotic index should be the consequence of a delay in the relative time taken by mitosis in the whole cycle time. Lengthened mitosis probably allows the post-replicative repair of most DNA lesions, as the frequency of interphases with micronuclei was higher in the cells which were irradiated when still dormant than in those irradiated when cycling. Thus, the mitotic delays should be the consequence of a checkpoint pathway activated by the presence of DNA damage. This feedback mechanism seems only to develop after cell proliferation is restored. This revised version was published online in July 2006 with corrections to the Cover Date.     

具有变时滞竞争神经网络的指数稳定性

利用微分方程组的基解矩阵及推广的Halanay微分不等式等分析技巧,讨论了一类具有不同时间尺度的变时滞竞争神经网络的平衡点存在和唯一性,并给出指数稳定性判定的充分条件,最后通过数值仿真实例检验结果的正确性.     

变时延神经网络的全局指数稳定性及收敛性

利用Lyapunov泛函方法和线性矩阵不等式（LMI）技术,通过引入一系列参数,给出全局指数稳定的平衡点的判别条件和时延的最大上界和神经网络的收敛速度,所得结果较之一些文献中的结果简单、实用并且对于具体设计带时延神经网络有重要的指导意义.最后,通过实例表明给出的判定条件是有效、可行的.     

Global exponential stability of positive periodic solution of the n-species impulsive Gilpin–Ayala competition model with discrete and distributed time delays

In this paper, we study the n-species impulsive Gilpin–Ayala competition model with discrete and distributed time delays. The existence of positive periodic solution is proved by employing the fixed point theorem on cones. By constructing appropriate Lyapunov functional, we also obtain the global exponential stability of the positive periodic solution of this system. As an application, an interesting example is provided to illustrate the validity of our main results.     

A timesaving, accurate method for locating and re-locating plants in ecological field studies

We present a method for increasing the accuracy and acquisition rate of the initial location data of plants within fixed areas. Using a personal digital assistant (PDA) to create a link between various electronic measurement devices (pantograph, micrometers etc.) and a database of the study individual's location information, has significantly increased measurement reliability and speed in a demographic field study. The method also provides a means to quickly and accurately re-identify the same individuals during subsequent visits to the study plot. Application of the same method can be used in all ecological field studies with sessile organisms in permanent plots, increasing speed and accuracy of coordinate measurement.     

Stability in a class of cyclic epidemic models with delay

A detailed analysis of a general class of SIRS epidemic models is given. Sufficient conditions are derived which guarantee the global stability of the endemic equilibrium solution. Further conditions are found which ensure instability for the equilibrium. Finally, the dependence of the stability on the contact number and the ratio of the mean length of infection to the mean removed time is considered.     

Why wait? Three mechanisms selecting for environment‐dependent developmental delays

Many species delay development unless particular environments or rare disturbance events occur. How can such a strategy be favoured over continued development? Typically, it is assumed that continued development (e.g. germination) is not advantageous in environments that have low juvenile/seedling survival (mechanism 1), either due to abiotic or competitive effects. However, it has not previously been shown how low early survival must be in order to favour environment‐specific developmental delays for long‐lived species. Using seed dormancy as an example of developmental delays, we identify a threshold level of seedling survival in ‘bad’ environments below which selection can favour germination that is limited to ‘good’ environments. This can be used to evaluate whether observed differences in seedling survival are sufficient to favour conditional germination. We also present mathematical models that demonstrate two other, often overlooked, mechanisms that can favour conditional germination in the absence of differences in seedling survival. Specifically, physiological trade‐offs can make it difficult to have germination rates that are equally high in all environments (mechanism 2). We show that such trade‐offs can either favour conditional germination or intermediate (mixed) strategies, depending on the trade‐off shape. Finally, germination in every year increases the likelihood that some individuals are killed in population‐scale disturbances before reproducing; it can thus be favourable to only germinate immediately after a disturbance (mechanism 3). We demonstrate how demographic data can be used to evaluate these selection pressures. By presenting these three mechanisms and the conditions that favour conditional germination in each case, we provide three hypotheses that can be tested as explanations for the evolution of environment‐dependent developmental delays.     

The influence of experimental deformation on neurocranial Wormian bones in rats

Two hundred and twenty crania of Wistar rats were experimentally deformed. The growth of the anterior vault was restricted in one subgroup and the growth of the posterior vault was restricted in the second subgroup. Seventy-seven deformed animals survived up to the thirtieth day of age and were sacrificed. Both subgroups were compared with each other as well as with 37 surviving sham-operated animals and 51 controls, all samples being 30 days of age (group A). Additionally, 33 normal crania of animals sacrificed at 1, 10 and 20 days as well as 19 deformed crania of 10 and 20 days old were observed (group B). Chi-square and Z tests were employed. Wormian bones found in the skulls of normal growing rats apparently represent an epigenetic polymorphism. Higher frequencies of wormian bones were found in deformed crania than in sham-operated ones and controls. Experimental deformation may be an extra-genetic factor that affects the normal genetic expression of wormian bones. This concept is relevant to studies of human population differences based on discontinuous cranial traits.     

Species coexistence and chaotic behavior induced by multiple delays in a food chain system

In this paper, we present a food chain system composed of three species (resource, consumer and predator). Food digested periods corresponding to consumer-eat-resource and predator-eat-consumer are introduced for more realistic consideration, which are called resource digested delay (RDD) and consumer digested delay (CDD), respectively. In order to explore the combined influence of multiple delays on population dynamics, two different scenarios were explored, i.e. the intrinsic growth rate of resource is less/more than consumer and predator. In case 1, some types of species coexistence characterized by RDD and CDD independent are illustrated by the corresponding characteristic equation. Multiple delays can promote and suppress the recurrent bloom of species population, which is called the stability switching of species coexistence. In case 2, using RDD and CDD as the varying parameters, complex dynamical behaviors including multiple periodic motion and chaotic behavior are exhibited in detail by employing some numerical simulations, such as phase trajectory, power spectra, and bifurcation diagram. The population dynamics exhibit chaos behavior and then evolve into system collapse for species outbreak. Further, greater RDD and CDD make system population enter into system collapse easier.     

The complex hatching response of Aedes eggs to larval density

Abstract. 1. We investigated the effects of hatching medium, larval density and larval instar on egg hatching in Aedes triseriutus (Say) (Diptera: Cuficidae).
2. In a nutrient broth medium, hatching rates, responded positively to low densities of large larvae, but were suppressed by high densities.
3. The magnitude of suppression was moderated through the use of a yeast hatching medium, which provides a food source for the large larvae. Small instars generally exerted positive effects on hatching, with the exception of first in star larvae in the yeast medium, which inhibited hatching at low and moderate densities.
4. The results demonstrate an ability of eggs to assess indirectly the potential risks of hatching through their responses to changes in dissolved oxygen concentration.