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521.
522.
Optimization of symbioses between plants and microbes has been suggested as method for enhancing nutrient uptake in low-input agroecosystems. In generall symbioses may be maximized through: (1) the selection of plant cultivars responsive to symbiontss; (2) ionoculation with highly efficient strains of symbionts; (3) habitat modification to encourage symbioses. These methods were tested under a traditional, slash mulch, ‘frijol tapado’ en agroecosystem and/or a high-input (‘espequeado’) system to increase nodulation and vesicular-arbuscular mycorrbizae (VAM) of beans with the following results: (1) Under the espequeado system (modified by the exclusion of fertilizer application), four cultivars of indeterminate, vining dry beans (Phaseolus vulgaris) nodulated more than two determinate, bush varieties, especially under low available soil phosphorus (P). VAM levels were not statistically different under these conditions. (2) Under the tapado system, pelletization with rock phosphate and Rhizobium inoculum did not significantly change nodule biomass, dinitrogen (N2)-fixation or yield. (3) Without P application, neither nodulation nor % VAM were significantly different between the two systems of bean production. However, as applied P increased, differences in %VAM of the two systems became significant, decreasing more sharply with increasing P in the tapado system.The tripartite symbiosis of beans can be influenced by various factors such as cultivar, nutrient application and production system. Although traditional varieties and systems may optimize certain microbial symbioses, the quantity of the symbiosis appears to depend on a number of complex factors. An intermediate agricultural technology, the mixing of traditional and modern, is exemplified here by the application of moderate levels of fertilizer to the tradional, tapado systems. This intermediate approach promises to decrease inputs in comparison to the espequedo system, as well as the need for shift cultivation, but long-term testing is needed. 相似文献
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526.
There are numerous reports of invertebrates that are visual mimics of ants, but no formal reports of mimicry of an ant, by
an ant. Two endemic Australian ants, Myrmecia fulvipes and Camponotus bendigensis are remarkably similar in colour and size; both are generally black but have red legs and golden gasters. The density and
hue of the pubescence of each ant's gaster are relatively uncommon in ants, but are very rare when combined with the black
forebody and red legs. The ants are similarly sized but are smaller than other species closely related to M. fulvipes. The range of C. bendigensis lies entirely within that of M. fulvipes, and both species excavate ground nests in open woodland. Finally, workers of both species are crepuscular and forage solitarily.
These data suggest that the relatively benign formicine C. bendigensis is a Batesian mimic of the formidable myrmeciine M. fulvipes.
Received: 9 August 1999 / Accepted in revised form: 22 December 1999 相似文献
527.
Chemical synapses are highly specialized cell–cell junctions designed for efficient signaling between nerve cells. Distinct
cytoskeletal matrices are assembled at either side of the synaptic junction. The presynaptic cytomatrix at the active zone
(CAZ) defines and organizes the site of neurotransmitter release from presynaptic nerve terminals. The postsynaptic density
(PSD) tethers neurotransmitter receptors and the postsynaptic signal transduction machinery. Recent progress in the identification
and characterization of novel CAZ and PSD components has revealed new insights into the molecular organization and assembly
mechanisms of the synaptic neurotransmission apparatus. On the presynaptic side, Bassoon and Piccolo, two related giant proteins,
are crucially involved in scaffolding the CAZ. On the postsynaptic side, two families of multi-domain adaptor proteins, the
MAGuKs (membrane-associated guanylate kinase homologs) and the ProSAP (proline-rich synapse-associated protein, also termed
Shank) family members are thought to be major organizing molecules of the PSD. 相似文献
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The ubiquity of circadian rhythms suggests that they have an intrinsic adaptive value (Ouyang et al. 1998; Ronneberg and
Foster 1997). Some experiments have shown that organisms have enhanced longevity, development time or growth rates when maintained
in environments whose periodicity closely matches their endogenous period (Aschoff et al. 1971; Highkin and Hanson 1954; Hillman
1956; Pittendrigh and Minis 1972; Went 1960). So far there has been no experimental evidence to show that circadian rhythms
per se (i.e. periodicity itself, as opposed to phasing properties of a rhythm) confer a fitness advantage. We show that the
circadian eclosion rhythm persists in a population of the fruitfly Drosophila melanogaster maintained in constant conditions of light, temperature, and humidity for over 600 generations. The results suggest that
even in the absence of any environmental cycle there exists some intrinsic fitness value of circadian rhythms.
Received: 2 November 1998 / Accepted in revised form: 22 April 1999 相似文献
530.