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991.
992.
To study the mechanism of precise excision ofgypsy from genomic sites, the integrase domain ofgypsy pol was cloned and expressed inEscherichia coli. The endonuclease activity of recombinant integrase was assayed with synthetic substrates corresponding to 3′-U5 ofgypsy LTR and to the known genomic insertion sites ofgypsy. Integrase nicked the 5′-A ⇓ YR-3′ triplet in the (+) strand of the double-stranded substrates; cleavage of a single-stranded
substrate was nonspecific. Cleavage proved to be affected by the local conformation of the substrate: the (+) strand was cleaved
more efficiently when the (−) strand had an unpaired base in the triplet and was not cleaved when the (−) strand was interrupted
or branched. The triplet corresponded to the consensus region ofgypsy insertion (5′-YRYR ⇓ YR-3′), the site of cleavagein vitro coinciding with the site of insertionin vivo. The unique mechanism ofgypsy excision was assumed to depend to a great extent on the enzymic properties of its integrase. 相似文献
993.
The arrangement of resources in patchy landscapes: effects on distribution, survival, and resource acquisition of chironomids 总被引:3,自引:0,他引:3
The spatial arrangement of resources in patchy habitats influences the distribution of individuals and their ability to acquire
resources. We used Chironomus riparius, a ubiquitous aquatic insect that uses leaf particles as an important resource, to ask how the dispersion of resource patches
influences the distribution and resource acquisition of mobile individuals in patchy landscapes. Two experiments were conducted
in replicated laboratory landscapes (38×38 cm) created by arranging sand and leaf patches in a 5×5 grid so that the leaf patches
were either aggregated or uniformly dispersed in the grid. One-day-old C. riparius larvae were introduced into the landscapes in one of three densities (low, medium, high). In experiment 1, we sampled larvae
and pupae by coring each patch in each landscape 3, 6, 12, or 24 days after adding larvae. In experiment 2, emerging adults
were collected daily for 42 days from each patch in each landscape. In aggregated landscapes, individuals were aggregated
in one patch type or the other during a particular developmental stage, but the ”preferred” type changed depending on developmental
stage and initial density. Adult emergence was lower by about 30% in all aggregated landscapes. In dispersed landscapes, individuals
used both types of patch throughout their life cycles at all initial densities. Thus, patch arrangement influences the distribution
of mobile individuals in landscapes, and it influences resource acquisition even when average resource abundance is identical
among landscapes. Regardless of patch arrangement, high initial density caused accumulation of early instars in edge patches,
75% mortality of early instars, a 25% increase in development time, and a 60% reduction in adult emergence. Because mortality
was extremely high among early-instar larvae in high-density treatments, we do not have direct evidence that the mechanism
by which patch arrangement operates is density dependent. However, the results of our experiments strongly suggest that dispersion
of resource patches across a landscape reduces local densities by making non-resource patches available for use, thereby reducing
intraspecific competition.
Received: 20 July 1999 / Accepted: 28 January 2000 相似文献
994.
Habitat selection can be envisaged as a hierarchical spatial process, from choice of home range to choice of dietary item. The green woodpecker (Picus viridis) is described as being closely bound to cultivated land and deciduous forests, mainly due to its summer diet composed of ants (Hymenoptera: Formicidae) found on meadows and pastures. To explore possible responses of this woodpecker to recent changes in land use practice, we studied home ranges, feeding habitats and food selection of a marginal population (four radio-marked males and five females) in a 30,000-ha conifer-dominated landscape at the northern edge of its distribution range in south-central Scandinavia. We asked: (1) Is the green woodpecker confined to areas with cultivated land and deciduous forest? (2) If so, are important food items (ants) particularly abundant or exclusively found there? (3) Can clearcuts and young plantations substitute for cultivated land as feeding habitat? Home ranges (mean=100 ha) were invariably confined to the parts of the landscape that contained cultivated land (<1% of the total area). In summer, birds preferred to feed in cultivated land, presumably due to a higher overall biomass of ants compared to forest habitats. They avoided clearcuts, but preyed extensively upon soil-dwelling ants in young conifer stands (16–30 years old). We failed to find preferences for particular ant groups (Lasius niger and L. flavus) associated with cultivated land. The principal summer food was Serviformica, an ant group that was equally abundant in cultivated land and forest habitat. A positive correlation between ant body mass and a preference index suggests that the birds selected the larger ant species independent of habitat type. In winter, birds fed exclusively on mound-building Formica rufa-ants in closed-canopy, older forest stands. Our results indicate that the green woodpecker successfully utilizes young conifer plantations as feeding habitat. At a larger scale, we hypothesize that green woodpecker populations fail to establish in managed forest tracts, not because of food shortage, but because the landscapes lack cultivated land serving as a key stimulus encouraging individuals to settle. 相似文献
995.
大肠杆菌原核增强子样序列的克隆及其结构与功能的研究 总被引:3,自引:0,他引:3
利用氯霉素乙酰转移酶(cat)以及β-半乳糖苷酶(lacZ)基因作为报告基因,从大肠杆菌MC1061株染色体基因组中克隆到3个原核增强子样序列——MC2,MC8,MC9,这3个片段均具有正反向增强活性,对β-半乳糖苷酶基因的增强活性(正向)在2~5.5倍之间。采用体内转录,RNA Dot blot杂交的方法对MC8的功能进行了研究,结果表明,MC8片段对于基因表达的调控发生在转录水平上。用核酸外切酶III末端缺失的方法对MC8的功能区进行了定位。结果显示,MC8的功能区位于距其正向克隆5′端450~950bp长约500bp的区段内。在450~600bp以及840~950bp区段内至少分别含有一个功能位点。序列分析的结果表明,MC8功能区有3个AT丰富区,其中2个分别位于450~600bp以及840~950bp区段内。 相似文献
996.
福建种子植物区系地理研究 总被引:22,自引:0,他引:22
福建省地处中国东南沿海的亚热带 ,季风气候显著 ,水热条件良好 ,地貌以山地为主 ,致使种子植物成分丰富 ,自然分布的种类约有 1152属归 186科。区系地理成分分析表明 ,福建种子植物区系是典型的热带起源 ,热带成分占 56 .0 3% ,其中以泛热带成分最多 ,同时兼容了大量温带成分 ,说明温带成分并非源自本地 ,而是亚热带山区起源。区系成分中单型属和少型属以及特有属成分相当丰富 ,说明了残遗现象和个别特化现象比较明显 相似文献
997.
998.
999.
1000.
Abstract. In this paper we present an application of aerial remote sensing to the analysis of spatial information in a mountainous area of central Italy by applying texture measures and landscape indices. Land cover data acquired in different time periods are used to calculate measures of landscape pattern and structure at pixel level (tone and texture variables) and patch‐level (landscape indices). Images of the patches from the 1950s, 1980s and 1990s have been derived from the tone‐texture classification of scanned black‐and‐white photographs. Multitemporal analysis of landscape indices has been performed to detect changes of landscape elements and related effects on vegetation dynamics due to the reduction of human impact. 相似文献