共查询到20条相似文献,搜索用时 15 毫秒
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Carl A. Reese 《Grana》2013,52(4):257-260
Establishing a modern pollen analogue for remote, high‐deposition environments (e.g. ice caps) presents a unique problem that is difficult to address with current technology. Collecting modern pollen with traditional Tauber traps in these locations (at sub‐annual resolution) is nearly impossible due to the time and costs involved in frequent, long‐distance or remote travel. Presented in this paper is an automated, dry‐deposition Tauber trap with the ability to open and close on a timer, which allows for pollen collection at programmable intervals. This new trap (along with three traditional Tauber traps) was field tested in the summer of 2005 at the University of Colorado's Niwot Ridge Long‐Term Ecological Research Station (LTER) located in the Front Range of the Colorado Rockies. During the research, one of the traditional traps became damaged and was subsequently discarded from the study. Significant variability in pollen concentrations and percentages was found in the remaining traps. Statistical tests revealed that the pollen assemblage collected in the automated trap was statistically no different to the pollen collected in the traditional Tauber traps. Field testing in a cold, high‐wind environment also revealed the weaknesses of acrylic as a trap material, and identified several improvements that could be made to the overall design. Further testing aside, this modified, automated Tauber trap now permits modern pollen rain studies at higher resolutions in more remote locations. 相似文献
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The number of extant species of diatoms is estimated here to be at least 30,000 and probably ca. 100,000, by extrapolation from an eclectic sample of genera and species complexes. Available data, although few, indicate that the pseudocryptic species being discovered in many genera are not functionally equivalent. Molecular sequence data show that some diatom species are ubiquitously dispersed. A good case can be made that at least some diatom species and even a few genera are endemics, but many such claims are still weak. The combination of very large species numbers and relatively rapid dispersal in diatoms is inconsistent with some versions of the “ubiquity hypothesis” of protist biogeography, and appears paradoxical. However, population genetic data indicate geographical structure in all the (few) marine and freshwater species that have been examined in detail, sometimes over distances of a few tens of kilometres. The mode of speciation may often be parapatric, in the context of a constantly shifting mosaic of temporarily isolated (meta) populations, but if our “intermediate dispersal hypothesis” is true (that long‐distance dispersal is rare, but not extremely rare), allopatric speciation could also be maximized. 相似文献
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Hiroko Fujita Yaeko Igarashi Stefan Hotes Masayuki Takada Takashi Inoue Masami Kaneko 《Plant Ecology》2009,200(1):9-36
Hokkaido Island is located in the cool temperate zone, and its climate conditions facilitated the formation of a variety of
wetland types, the majority of them peat-forming mires. Most of these remained in a natural state until the early 20th century.
However, drainage and subsequent conversion mostly to agricultural land have since destroyed more than 70% of the original
wetland ecosystems. This paper (1) provides an overview of mire types, (2) reviews the development process of mires in Hokkaido
during the Holocene, (3) analyzes the causes of losses of wetland areas, and (4) gives a summary of the current conservation
and management status. Basic mire types that have been described in other parts of the northern hemisphere can also be recognized
in Hokkaido, although there are floristic differences, and the frequency and intensity of volcanic impact and tsunamis is
higher than in most other regions with abundant mire formation. Mire formation started at various points during the postglacial
period; a few mountain mires in southwest Hokkaido date back to the Lateglacial, but most mountain mires formed during the
mid to late Holocene. Most lowland mires developed at altitudes below 20 m and were influenced by the Jomon transgression
that peaked ca. 6,000 years BP. The largest lowland mires started forming after the sea retreated, and many are not older
than ca. 3,000 years. In 1996, the total number of wetlands (including peat-forming mires, freshwater marshes, and saltmarshes)
greater than 1 ha was 150, with a total area of 59,881 ha. In 1928, when many wetlands were yet undeveloped, their total area
was 200,642 ha. Most losses occurred between the 1950s and 1970s, when post-war development economics promoted agriculture
and large-scale reclamation projects. Currently, 90.7% of mountain wetlands are public land, while 81.3% of the lowland wetlands
are private or in mixed ownership. The ownership condition affects the possibilities for the protection of complete mire complexes
especially in the lowlands. For effective conservation of wetland ecosystems it is necessary to include the catchment area
in the planning of protected areas. 相似文献
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Background
Dehydrogenase enzymes belong to the oxidoreductase class and utilise the coenzymes NAD and NADP. Stereo-selectivity is focused on the C4 hydrogen atoms of the nicotinamide ring of NAD(P). Depending upon which hydrogen is transferred at the C4 location, the enzyme is designated as A or B stereospecific. 相似文献13.
Catherine Hervé du Penhoat Claude Gey Patrice Pellerin Serge Perez 《Journal of biomolecular NMR》1999,14(3):253-271
Rhamnogalacturonan II (RG-II) is a structurally complex pectic mega-oligosaccharide that is released enzymatically from the primary cell wall of higher plants. It contains roughly 30 monosaccharide units (MW 5 kDa) including very unusual residues such as Kdo, Dha, aceric acid and apiose. Previous studies have demonstrated that these monomers are arranged into four structurally well-defined oligosaccharide side chains (A–D), linked to a homogalacturonan mainchain, but the specific attachment sites of these branches on the pectic backbone have not yet been elucidated. In the present work, fairly complete assignments of the 750 MHz 1 H NMR spectra and partial assignments of the 13 C NMR spectra of the sodium-borohydride-reduced RG-II monomer were obtained for a 5 mM sample isolated from red wine. On the whole, these data corroborate the primary structures of the sidechains previously established by methylation analysis, partial hydrolysis and FAB-MS spectrometry but some heterogeneity has been demonstrated (partial substitution at B5, B6, and A5). The preferred orientations of the majority of the sidechain glycosidic linkages in the RG-II monomer have been determined from the sequential nOe data and the solution structure is generally in good agreement with the stable conformers previously obtained by molecular modeling (MM3) of the disaccharide and sidechain oligosaccharide building blocks. All of a two-residue, a three-residue, and a four-residue segment of the backbone have been tentatively identified from long range interactions between sidechain protons as well as in the mainchain. Taking into account the length of the 9-mer galacturonan mainchain described in prior work, these building blocks constitute almost the complete structure of RG-II (Scheme 2). 相似文献
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How is massive conformational change in integrins achieved on a rapid timescale? We report crystal structures of a metastable, putative transition state of integrin αXβ2. The αXβ2 ectodomain is bent; however, a lattice contact stabilizes its ligand-binding αI domain in a high affinity, open conformation. Much of the αI α7 helix unwinds, loses contact with the αI domain, and reshapes to form an internal ligand that binds to the interface between the β propeller and βI domains. Lift-off of the αI domain above this platform enables a range of extensional and rotational motions without precedent in allosteric machines. Movements of secondary structure elements in the β2 βI domain occur in an order different than in β3 integrins, showing that integrin β subunits can be specialized to assume different intermediate states between closed and open. Mutations demonstrate that the structure trapped here is metastable and can enable rapid equilibration between bent and extended-open integrin conformations and up-regulation of leukocyte adhesiveness. 相似文献
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《Biochemical Systematics and Ecology》1986,14(2):233-238
The first record of a natural intergeneric hybrid of Campostoma anomalum × Rhinichthys atratulus is described from the Youghiogheny River drainage. Morphometric and meristic characters of the parents and hybrid were used to calculate a mean hybrid index value of 49.8. Dentition of the hybrid was intermediate as was intestinal coiling. In addition to the hybrid index, C. anomalum, R. atratulus and the hybrid were examined electrophoretically. Of the 32 enzymatic loci resolved, six (ADH-A, EST-B, LDH-A, PGM-A, SDH-B and SOD-C) showed hybrid inheritance patterns identical to the enzyme pattern obtained by mixing tissue extracts composed of the two parental species. 相似文献
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Rusea Go Khor Hong Eng Muskhazli Mustafa Janna Ong Abdullah Ahmad Ainuddin Naruddin Nam Sook Lee Chang Shook Lee Sang Mi Eum Kwang-Woo Park Kyung Choi 《Biodiversity and Conservation》2011,20(10):2263-2272
A comprehensive study on the orchid diversity in Penang Hill, Penang, Malaysia was conducted from 2004 to 2008 with the objective
to evaluate the presence of orchid species listed by Curtis (J Strait Br R Asiat Soc 25:67–173, 1894) after more than 100 years. A total of 85 species were identified during this study, of which 52 are epiphytic or lithophytic
and 33 are terrestrial orchids. This study identified 57 species or 64.8% were the same as those recorded by Curtis (1894), and 78 species or 66.1% of Turner’s (Gardens’ Bull Singap 47(2):599–620, 1995) checklist of 118 species for the state of Penang including 18 species which were not recorded by Curtis (1894) and the current study but are actually collected from Penang Hill. A comparison table of the current findings against Curtis
(1894) and Turner (1995) is provided which shows only 56 species were the same in all three studies. The preferred account for comparison was Curtis’
(1894) list as his report was specifically for the areas around Penang Island especially Penang Hill, Georgetown and Ayer Itam
areas. Our study reveals that about 50% of Curtis’ collection localities have been converted to residential areas and agricultural
land, and this probably explains the decreasing numbers of species found in the current study especially for the terrestrial
species as epiphytic species have better adaptation capabilities towards environmental changes. Seven species were identified
as new records to Penang Hill as they were not recorded by Curtis (1894). None of the three species recorded as endemic to Penang by Turner (1995) was recollected during the current study, of which only Zeuxine rupestris was in Curtis’ (1894) list. Overall, we concluded that Penang hill harbours at least 136 species of orchids of which 78 species or 57.4% were
recollected in this study. This also indicates that this area is still suitable for orchid growth even though it is surrounded
by rapid development and mass conversion of forested land into fruit orchards and residential area. The designation of Penang
Hill as a Permanent Forest Reserve would better guarantee the survival of some orchid species unless human interventions and
climatic changes occur. 相似文献