Mutations in a Saccharomyces cerevisme host showing increased holding stability of the heterologous plasmid pSRI

Summary We have isolated Saccharomyces cerevisiae mutants, smp, showing stable maintenance of plasmid pSRI, a Zygosaccharomyces rouxii plasmid. The smp mutants were recessive and were classified into at least three different complementation groups. The three mutants also showed increased stability of YRp plasmids and the mutations are additive for plasmid stability. One mutation, smp1, confers a respiration-deficient (rho ⁰) phenotype and several Rho^– mutants independently isolated by ethidium bromide treatment of the same yeast strain also showed increased stabilities of pSR1 and YRp plasmids. The wild-type S. cerevisiae cells showed a strongly biased distribution of pSR1 molecules as well as YRp plasmids to the mother cells at mitosis, while the smpf mutant did not show this bias. Another mutation, smp3, at a locus linked to ade2 on chromosome XV, confers temperature-sensitive growth. The SMP3 gene encodes a 59.9 kDa hydrophobic protein and disruption of the gene is lethal.     

Jack of one trade,master of none: host choice by Drosophila magnaquinaria

Summary Host selection by phytophagous insects is generally thought to be based on chemical or nutritional characteristics of the host. This is especially true for monophagous insects. However, many other factors may influence host choice. The present study examines host selection by Drosophila magnaquinaria, whose sole host is the yellow skunk cabbage, Lysichitum americanum. Utilization of skunk cabbage was tested relative to a set of alternative hosts. In the pre-alighting stage of host selection, skunk cabbage was found to be less attractive than tomato, cucumber, and commercial mushrooms. In pairwise oviposition tests, there were no differences among hosts. There were no differences in larval survivorship among skunk cabbage, tomato, cucumber, or Ramaria, and larvae developed into pupae earlier on tomato than on skunk cabbage. These results indicate that this monophagy is not based on characteristics of the host. We suggest that habitat selection is the more important factor in determining the association between D. magnaquinaria and skunk cabbage.     

On the opportunity cost of the photosynthate invested in stem elongation reactions mediated by phytochrome

Summary Seedlings of shade-intolerant species react to alterations of the light climate caused by their neighbors with morphological changes that may influence the pattern of resource acquisition and utilization at the whole-canopy level. One such change, the increased stem elongation rate that is triggered by low red (R, 660 nm) to far-red (FR, 730 nm) ratios (R:FR) in dense canopies, might reduce the amount of assimilates available for leaf area expansion or root growth, and in that way affect resource capture by the canopy. We have tested this hypothesis by comparing the growth of both isolated individuals and canopies of the weed Amaranthus quitensis under conditions differing only in the spectral distribution of the incident light. When canopies received the full spectrum of sunlight, the stems were a large proportion (40–57%) of total biomass. Filtering the FR waveband (and hence raising the R:FR ratio to eliminate the neighbors' proximity-signal) resulted in shorter canopies with lighter stems. However, the growth of leaves and roots was not promoted by this treatment, indicating that the opportunity cost of the assimilates invested in the stems was nil or very small. Filtering the FR had no effect on biomass accumulation when plants were grown as isolated individuals. The higher growth of the canopics under full spectrum could be due to a higher light interception or to a higher efficiency of light conversion into biomass. The first possibility is weakened by the observation that filtering the FR had no effect on the dynamics of soil covering by the crops. The second is indirectly strengthened by results of an experiment with isolated plants showing that stem elongation, stem growth, and total plant biomass can be increased by reducing the flux of R light received by the stems without affecting the light climate of the leaves. Further work is needed to distinguish between these two possibilities; whatever the cause, our results show that the elongation responses to decreased R:FR may lead to a net increase in canopy productivity, and do not necessarily have a negative impact on the growth of resource-harvesting organs.     

Influence of aphid size,age and behaviour on host choice by the parasitoid wasp Ephedrus californicus: a test of host-size models

Summary When host quality varies, parasitoid wasps are expected to oviposit selectively in high-quality hosts. We tested the assumption underlying host-size models that, for solitary species of wasps, quality is based on host size. Using Ephedrus californicus, a solitary endoparasitoid of the pea aphid, we evaluated the influence of aphid size (= mass), age and defensive behaviours on host selection. Experienced parasitoid females were given a choice among three classes of 5-day-old apterous nymphs: small aphids that had been starved daily for 4 h (S4) and 6 h (S6) respectively, and large aphids permitted to feed (F) normally. Wasps attacked more, and laid more eggs in, small than large aphids (S6>S4>F). This rank-order for attack did not change when females could choose among aphids of the same size that differed in age; however, wasps oviposited in all attacked aphids with equal probability. Host size did not influence parasitoid attack rates when aphids were anaesthetized so that they could not escape or defend themselves. As predicted by host-size models, wasp size increased with host size (F>S4; S6), but large wasps required longer to complete development than their smaller counterparts (S4host quality is not a linear function of current host size; it is influenced by the host's age and development stage which determine the host's potential to support parasitoid growth. Host choice in E. californicus reflects a trade-off between maximization of fitness gains per egg and the economics of search-time allocation. Because large aphids are more likely to escape parasitization, a wasp must balance her potential gain in fitness by ovipositinng in a high-quality (large) aphid against her potential cost in terms of lost opportunity time if the attack fails.     

Nutrient budgets in a dry heathland watershed in northeastern Spain

Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO₄-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO₄ ^2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H⁺ and Ca²⁺, had net losses of Na⁺ and Mg²⁺, and was close to steady state for K⁺, SO₄ ^2-, Cl^- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca²⁺ retention was higher than net exports of Na⁺ and Mg²⁺ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity.     

Host finding and discrimination in Diglyphus isaea,a parasitoid of the chrysanthemum leaf miner,Chromatomyia syngenesiae

Naive and experienced Diglyphus isaea were attracted by host plant odours of lettuce and chrysanthemum to search and probe on infested and uninfested leaves. A slight preference was shown for leaves infested with Chromatomyia syngenesiae. At close range, visual stimuli were unnecessary for oviposition and host‐feeding. Contact with uninfested lettuce and chrysanthemum elicited searching and probing behaviour whereas host frass did not. Host larval movement appeared to aid host detection at close range. The number of ovipositor probes increased with proximity to live hosts but not for stationary, dead hosts. Dead hosts were frequently walked over or missed when D. isaea passed within 0.5 cm of the stationary larvae. Dead hosts were also rejected for oviposition but not for host feeding. Both naive and experienced females discriminated between healthy hosts and those which had been attacked by conspecifics or encountered previously.     

用TOPSIS方法评价黄土丘陵区草场质量的研究

一、自然概况黄土高原位于我国西北部,西起青海日月山,东抵山西太行山,北至晋陕长城,南达甘陕秦岭,构成一个独具特色的、比较完整的自然区域,其地理范围大致是北纬34°—41°,东经103°—113°之间。高原到处有起伏的黄     

生态因子对中药甘草质量影响的初步研究

甘草是一种常用中药,由于具有“调和诸药”之功效而成为方剂中最常用的一味药。古有“十方九草”之说,可见对甘草的需求量之大;目前,对甘草的需求不仅仅限于医药界,在食品、轻工、畜牧等行业也有广泛的应用。然而,由于长期滥采、滥挖,只挖不种,使得甘草的蕴藏量急剧下降,有些地区的甘草资源濒临枯竭。例如,东北松嫩平原,原是我国“东草”商品的主产区之一,并大量出口国外,     

茶园生态环境对红茶芳香化学物质及品质影响

国内外研究均表明,芳香化学物质是构成红茶品质的主要化学物质。茶叶中芳香化学物质的形成,与茶树品种、加工工艺密切相关。至于茶园生态环境和气象因子对红茶     

青龙河底栖无脊椎动物群落结构及其水质评价

本文论述了青龙河底栖生物种类、数量、分布和结构等特点及其与环境因子间的关系。应用Beck、Gleason、Shannon、Simpson等生物指数对水质状况进行评价。结果表明青龙河除个别断面受污染外,大部分河段属尚清洁水。