Fifty years ago, Lewis Wolpert introduced the concept of “positional information” to explain how patterns form in a multicellular embryonic field. Using morphogen gradients, whose continuous ...distributions of positional values are discretized via thresholds into distinct cellular states, he provided, at the theoretical level, an elegant solution to the “French Flag problem.” In the intervening years, many experimental studies have lent support to Wolpert's ideas. However, the embryonic patterning of highly repetitive morphological structures, as often occurring in nature, can reveal limitations in the strict implementation of his initial theory, given the number of distinct threshold values that would have to be specified. Here, we review how positional information is complemented to circumvent these inadequacies, to accommodate tissue growth and pattern periodicity. In particular, we focus on functional anatomical assemblies composed of such structures, like the vertebrate spine or tetrapod digits, where the resulting segmented architecture is intrinsically linked to periodic pattern formation and unidirectional growth. These systems integrate positional information and growth with additional patterning cues that, we suggest, increase robustness and evolvability. We discuss different experimental and theoretical models to study such patterning systems, and how the underlying processes are modulated over evolutionary timescales to enable morphological diversification.
Key Findings
Through a combination of positional information, directed growth and additional periodic patterning modules, repetitive morphological structures can be specified during development, in a robust, yet flexible manner.
Seeking condition indices characterizing the health state of a system is a key problem in condition-based maintenance. For this purpose, diagnostic and prognostic models have been unceasingly ...developed and improved over the past few decades; nevertheless none of them explains thoroughly the impacts of such indices on the effectiveness of maintenance operations. As a complement to these efforts, this paper analyzes the effectiveness of some well-known diagnostic and prognostic indices for maintenance decision-making. The study is based on a system subject to competing risks due to multiple crack paths. A periodic inspection scheme is used to monitor the system health state. Each inspection returns the perfect diagnostic information: the number of cracks, corresponding crack sizes, and the system failure/working state. Based on this information, two kinds of prognostic condition indices are predicted: the average value and probability law of the system residual useful life. The associated condition-based maintenance strategies and cost models are then developed and compared with the ones whose maintenance decisions are based on diagnostic condition indices. The comparison results allow us to conclude on the performance and on the robustness of these strategies, hence giving some suggestions on the choice of reliable condition indices for maintenance decision-making.
•Developing a new and generic degradation and failure model.•Synthesizing diagnostic and prognostic condition indices on the basis of the developed degradation and failure model.•Building diagnosis and prognosis-based maintenance strategies, and developing the associated cost models.•Assessing the performance and robustness of the considered strategies to find out reliable indices.
A few contemporary French novels mention christian and muslim religion, mixing a new realism (developed since the 1980’s in Western literature) with the need for transcendance. The liberal politics, ...forgeting their origins, that included the value of individualism in the society, leave the characters unsatisfied, but the novelists of our corpus, unlike some world literature, experience difficulties to imagine a constructive post secularism in their fictions.
Time series of environmental measurements are essential for detecting, measuring and understanding changes in the Earth system and its biological communities. Observational series have accumulated ...over the past 2–5 decades from measurements across the world's estuaries, bays, lagoons, inland seas and shelf waters influenced by runoff. We synthesize information contained in these time series to develop a global view of changes occurring in marine systems influenced by connectivity to land. Our review is organized around four themes: (i) human activities as drivers of change; (ii) variability of the climate system as a driver of change; (iii) successes, disappointments and challenges of managing change at the sea‐land interface; and (iv) discoveries made from observations over time. Multidecadal time series reveal that many of the world's estuarine–coastal ecosystems are in a continuing state of change, and the pace of change is faster than we could have imagined a decade ago. Some have been transformed into novel ecosystems with habitats, biogeochemistry and biological communities outside the natural range of variability. Change takes many forms including linear and nonlinear trends, abrupt state changes and oscillations. The challenge of managing change is daunting in the coastal zone where diverse human pressures are concentrated and intersect with different responses to climate variability over land and over ocean basins. The pace of change in estuarine–coastal ecosystems will likely accelerate as the human population and economies continue to grow and as global climate change accelerates. Wise stewardship of the resources upon which we depend is critically dependent upon a continuing flow of information from observations to measure, understand and anticipate future changes along the world's coastlines.
Summary
We present a synopsis and an identification key for the 13 species and one subspecies of the African genus
Whitfieldia
Hook. (Acanthaceae: Acanthoideae: Whitfieldieae). This work is based on ...the observation and analysis of morphological data from herbarium specimens and photographs of
Whitfieldia
species, and the study of all the relevant literature on the genus.
Whitfieldia latiflos
C.B.Clarke ex Stapf from Liberia is reinstated as an accepted species and a full description and illustration are provided. A new subspecies,
W. colorata
C.B.Clarke ex Stapf subsp.
tigrina
A.Grall & I.Darbysh., also from Liberia, is described. In addition,
W. arnoldiana
De Wild. & T.Durand and
W. letestui
Benoist are treated as synonyms of
W. laurentii
(Lindau) C.B.Clarke and
W. preussii
(Lindau) C.B.Clarke respectively for the first time. Eleven names in
Whitfieldia
are lectotypified. Reference specimens, relevant literature, habitat notes, distribution maps, taxonomic notes and a preliminary assessment of the conservation status and extinction risk are provided for each taxon. Four species are considered to be threatened of which two (
W. latiflos
and
W. rutilans
Heine) are assessed as Endangered (EN) while two others (
W. purpurata
(Benoist) Heine and
W. thollonii
(Baill.) Benoist) are placed in the Vulnerable (VU) category. In addition, two taxa are assessed as Near Threatened (NT) and one is currently considered to be Data Deficient (DD).
Using a call-by-value functional language as an example, this article illustrates the use of coinductive definitions and proofs in big-step operational semantics, enabling it to describe diverging ...evaluations in addition to terminating evaluations. We formalize the connections between the coinductive big-step semantics and the standard small-step semantics, proving that both semantics are equivalent. We then study the use of coinductive big-step semantics in proofs of type soundness and proofs of semantic preservation for compilers. A methodological originality of this paper is that all results have been proved using the Coq proof assistant. We explain the proof-theoretic presentation of coinductive definitions and proofs offered by Coq, and show that it facilitates the discovery and the presentation of the results.
Despite the growth in the number of studies on gambling disorders (GDs) and the potentially severe harm it may cause, problem gambling in older adults is rarely apparent in literature. Driven by the ...need to overcome this limitation, a broad systematic review is essential to cover the studies that have already assessed the determinants of GD in the elderly.
The aim of this systematic review is to understand the determinants related to GDs in elderly people.
A total of 51 studies met the inclusion criteria, and data were synthesized.
Three major types of determinants were identified in this review: individual, socio-financial and environmental.
This review explored the determinants influencing GDs in older people. The findings are relevant to academics, policymakers, patients, and practitioners interested in the identification and prevention of GD in older people.
► We define a model-based prognosis into mathematical terms, including specific observations on the system. ► We propose a two steps methodology to solve a RUL computation problem. ► We propose two ...new method to assess the reliability of a PDMP. ► We illustrate the methodology on two examples: one academic and one industrial.
This paper deals with the prognosis of complex systems using stochastic model-based techniques. Prognosis consists in this case in computing the distribution of the Remaining Useful Life (RUL) of the system conditionally to available information. In so doing, three main challenges arise from the industrial context. First, the model should unify the two classical approaches to describing complex systems: the bottom-up and the top-down approaches. The former uses elementary interacting components whilst the latter models the system’s physical behavior by means of a set of differential equations. Second, the prognosis must integrate online information to provide a specific result for each system depending on their life events. Online information can take different forms (e.g. inspections, component faults, non detection or false alarm, noisy signal) which must all be considered. Third, the prognosis must supply ready, meaningful numerical results, the error of which must also be under control. This paper proposes a method addressing those challenges. The method is illustrated with two different examples: a simplified spring-mass system and a pneumatic valve for aeronautical application.
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