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Frequent Feature (frequent + feature)
Selected AbstractsPeriodic spotted patterns in semi-arid vegetation explained by a propagation-inhibition modelJOURNAL OF ECOLOGY, Issue 4 2001P. Couteron Summary 1,Vegetation cover regularly punctuated by spots of bare soil is a frequent feature of certain semi-arid African landscapes, which are also characterized by banded vegetation patterns (i.e. tiger bush). 2,The propagation-inhibition (PI) model suggests that a periodic pattern characterized by a dominant wavelength can theoretically establish itself through a Turing-like spatial instability depending only on a trade-off between facilitative and competitive interactions among plants. Under strictly isotropic conditions, spotted and banded patterns are distinct outcomes of a unique process, whereas anisotropy leads to a banded structure. The model predicts that spotted patterns will have a lower dominant wavelength than bands. 3,We test some outcomes of the PI model against vegetation patterns observable in aerial photographs from West Africa. Two sites with rainfall of c. 500,600 mm year,1 were studied: a 525-ha plain in north-west Burkina Faso and a 300-ha plateau in southern Niger. Digitized photographs were subjected to spectral analysis by Fourier transform in order to quantify vegetation patterns in terms of dominant wavelengths and orientations. 4,Spotted vegetation proved highly periodic. The characteristic range of dominant wavelengths (30,50 m) was similar at two sites more than 500 km apart. The PI model suggests that spots may occur as a hexagonal lattice but there is little evidence of such patterning in the field. A dominant wavelength was far quicker to establish in simulations (c. 102,103 years for annual grasses) than a hexagonal symmetry (c. 105 years), and observed patterns are therefore likely to be far from the asymptotic structure. 5,Elongated and smudged spots that locally became flexuous bands have been observed in southern Niger. This pattern that had a dominant wavelength of 50 m but lacked any dominant orientation can be interpreted as a transition from spots to bands under fairly isotropic conditions. 6,The PI model provides a framework for further investigation of patterns in semi-arid vegetation and may be of a broader ecological application. [source] New insights into the nature of Warthin's tumourJOURNAL OF ORAL PATHOLOGY & MEDICINE, Issue 1 2009Iain David O'Neill Warthin's tumour is considered heterogeneous as to its pathogenesis with some data supporting a polyclonal origin for the epithelium, implying a non-neoplastic nature. After inconsistent reports, current information from molecular studies suggests that a recurrent t(11;19) and associated CRTC1-MAML2 fusion oncogene characterizes a subset of Warthin's tumours and supports a clonal origin in such cases. CRTC1-MAML2 is also a frequent feature of mucoepidermoid carcinoma. These findings, and the recent reports of Warthin's tumour and co-existent mucoepidermoid carcinoma with common CRTC1-MAML2 expression, provide a morphological and molecular framework for future studies as a basis for a fresh appraisal of the pathogenesis of Warthin's tumour. The underlying molecular basis and the pivotal studies defining such events are discussed. [source] CHARGE syndrome as unusual cause of hypogonadism: endocrine and molecular evaluationANDROLOGIA, Issue 5 2010L. Foppiani Summary Coloboma, heart defect, atresia choanae, retarded growth and development, genital hypoplasia, ear anomalies (CHARGE) syndrome is a genetic syndrome in which hypogonadism is a frequent feature. A causative mutation within the chromodomain helicase DNA-binding protein-7 gene, which plays an important role in the embryonic development, is present in 2/3 of affected patients. We describe the clinical, hormonal and molecular characteristics of a young man from Ecuador who was diagnosed as having CHARGE syndrome at an adult age. The patient showed several phenotypic features of the syndrome, associated with a prepubertal state and cryptorchidism; hypogonadotrophic hypogonadism with undetectable testosterone levels not responsive to hCG testing and severe osteoporosis were ascertained. Molecular evaluation of the CHD7 gene showed the novel frameshift truncating heterozygous mutation p.Tyr1046Glyfs*23 in exon 12. Magnetic resonance imaging revealed mild hypoplasia of the pituitary gland and hypoplasia of the posterior cranial fossa. Parenteral testosterone therapy led to sexual development over time and, in combination with diphophonate therapy and calcium,vitamin D supplementation, significantly improved bone mineralisation. Early proper hormonal treatment of hypogonadism in patients with complex genetic syndromes is important to achieve normal sexual maturation, improve quality of life and avoid significant comorbidities, such as osteoporosis. [source] Oblique rainfall and contemporary geomorphological dynamics (Serra da Estrela, Portugal)HYDROLOGICAL PROCESSES, Issue 4 2004Gonçalo Vieira Abstract Coarse sand accumulations are polygenic microforms that attain a width of several metres, a height up to 30,40 cm, a gradient of 8,12° and a slope length up to 1 m. These accumulations are frequent in the gruss-covered plateaus of the granite mountains of central and northern Portugal, but they have been described in other mountain areas (i.e. Cairngorms, Scotland). Though these microforms are frequent features, studies on them are rare. They have been attributed to complex genesis controlled primarily by aeolian processes, but also by wash and cryogenic dynamics. Results presented here add new insights into the origin of the sand accumulations and emphasize the importance of rainsplash-saltation induced by oblique rainfall as the main transportation mechanism. The study was conducted in the Serra da Estrela, a granite mountain in central Portugal (1993 m above sea level) and is supported by a detailed mapping of the orientation of the accumulations, monitoring of the surface material and analysis of meteorological data. The results are particularly significant since they indicate that the coarse sand accumulations are very active features that show a clear climatic and ecological signal. Copyright © 2004 John Wiley & Sons, Ltd. [source] Long-term trends of synoptic-scale breaking Rossby waves in the Northern Hemisphere between 1958 and 2001INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2008F. Isotta Abstract Breaking synoptic-scale Rossby waves are frequent features of the upper troposphere and lower stratosphere (UTLS) which affect both global- and regional-scale dynamics. Furthermore, they directly influence ozone distribution through meridional transport of ozone-rich air towards the south and ozone-poor air towards the north. Here, trends in the frequency of these breaking waves are assessed by analysing a 44-year climatology (1958,2002) of potential vorticity (PV) streamers on isentropic surfaces from 310 to 350 K. These streamers are viewed as breaking Rossby waves, whereby stratospheric (tropospheric) streamers indicate southward (northward) breaking waves. Two complementary techniques are used to analyse the trends. First, linear trends are computed using the least-squares regression technique. Statistically significant linear trends are found to vary in location and magnitude between isentropic levels and the four seasons. In winter significant trends are detected in the eastern Pacific between 340 and 350 K. A positive trend of stratospheric streamers in the Tropics is related to an increase of total column ozone, whereas the positive trend of tropospheric streamers in the mid-latitudes is associated with a decrease of total ozone. Secondly, a nonlinear trend analysis is performed using the seasonal-trend decomposition procedure based on Loess (STL). With this technique, the low-frequency variability of the time series is analysed during the 44-year period. For instance, over the eastern Atlantic on 350 K, a phase of decreasing PV streamer frequencies in the 1950s and 1960s is followed by a positive streamer tendency after the 1970s. Additionally, trends of the zonal wind are investigated. One prominent outcome of this analysis is the observation that equatorial easterlies over the Atlantic are weakening. A dynamically meaningful link exists between the trends observed in both wind velocity and PV streamers. Copyright © 2008 Royal Meteorological Society [source] | ||||||||||