胞间连丝：共质体运输的动态通道

胞间连丝作为一种细胞质结构将相邻的细胞连系起来而形成植物的共质体。胞间连丝通过调控许多离子和分子的共质体运输而广泛地参与植物的生命活动。胞间连丝的主要构成部分是细胞质膜、连丝小管、以及位于二之间的环层细胞质。这三都很容易在电子显微镜下观察到。细胞骨架的成分（肌动蛋白和肌球蛋白）起到稳定胞间连丝的作用。同时,钙结合蛋白可能具有调节间连丝功能的作用。在胞间连丝里,环层细胞质为大多数溶质提供共质体运输的通道,而有些 共质体运输则可能是通过连丝小管的内腔、连丝小管的壳层、甚或是细胞质膜来实现的。共质体可以细分为数个区块,它们各自允许不同大小的分子（从低于1000到高于10000道尔顿）通过。从发生上看,胞间连丝可以是初生的,也可以是次生的。前是伴随着新细胞壁的形成则产生的,而后则是在已有的细胞壁上产生的。胞间连丝的动态性质还表现在它们的频率是处于变化之中,这是由于组织或植物整体的发育和生理状态决定的。虽然共质体运输的基本形式是扩散,但胞间连丝对于某些离子和分子却是选择性的。在病毒感染细胞时,病毒的移动蛋白作用于胞间连丝的受体蛋白,结果,胞间连丝被显地扩张（其机理尚不清楚）。于是,病毒的移动蛋白连同与之结合在一起的病毒基因组进入毗邻的健康细胞。一些植物源性的蛋白质也能够通过胞间连丝来运输;推测其方式类似于病毒的移动蛋白。有些植物蛋白质本身就是信号分子,它们调节分化和其他活动。与此相反,还有一些植物蛋白质的共质体运输并不是通过特异的方式来实现的。     

Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies ( Megarcys signata ) on fine sediment deposition and removal.
2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.
3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.
4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream-side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.
5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.
6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.     

Effects of Off-Highway Vehicle Use on the American Marten

Abstract Motorized recreation in North American wildlands is increasing, and technological developments in the power and range of vehicles has increased access to high-elevation habitats. The American marten (Martes americana) is vulnerable to this disturbance because martens, like other residents of high-elevation forests, are associated with remote wilderness conditions where the presence of motorized vehicles is a recent phenomenon. We evaluated the effects of vehicles at 2 study sites in California, USA, by comparing marten occupancy rates and probabilities of detection in areas where recreational vehicle use is legal and encouraged (use areas) with wilderness areas where vehicles are prohibited (non-use areas). We sampled vehicle occurrence in nearby use and non-use areas using sound level meters and determined marten occurrence using track and camera stations. We also included 2 secondary measures of potential effects of vehicles on martens: sex ratio and circadian pattern of activity. Martens were ubiquitous in use and non-use areas in both study sites, and there was no effect of vehicle use on marten occupancy or probability of detection. We predicted that females might be less common and martens more nocturnal in use than in non-use areas, but neither occurred. Martens were exposed to low levels of disturbance in our study sites. We estimated that a marten might be exposed to 0.5 vehicle passes/hour and that this exposure had the greatest effect on <20% of a typical home range area. Furthermore, vehicle use usually occurred when martens were inactive. We did not measure behavioral, physiological, or demographic responses, so it is possible that vehicles may have effects, alone or in concert with other threats (e.g., timber harvest), that we did not quantify. We encourage additional studies to determine whether other montane species that are year-round residents demonstrate the same response to motorized vehicles.     

Potential habitat dependence in deposition rate of presumptive annual lines in shells of the bivalve Protothaca staminea

Although repeating depositional features in shells of bivalve molluscs continue to be used in ecology and paleoecology to age individual specimens and thereby provide a potentially powerful tool for geochronological reconstruction of past events, few studies actually test the basic assumptions of growth line analysis: (1) that line deposition is truly periodic and (2) that the length of the period is one year. Recapture of marked individuals of the bivalve Protothaca staminea after 12 or 24 months in field plots in two habitats of Mugu Lagoon (California, U.S.A.) suggests a habitat-specific pattern of deposition of the major growth lines previously assumed to be annual. External (surface) and internal (cross-sectional) growth lines were in excellent agreement on any given specimen and showed that each specimen from a muddy-sand environment deposited a single annual line over 12 months, whereas all but one specimen from a clean-sand environment deposited several more major growth lines than predicted from the numbers of years of additional growth. Although more frequent handling or natural disturbance in the higher-energy, clean-sand habitat may explain these differences, the results demand caution in extrapolating tests of annual periodicity in growth line deposition across species or even across habitats for a single species.     

Unpredictability in the facultative association between larvae of Euphilotes enoptes (Lepidoptera: Lycaenidae) and ants

Larvae of the lycaenid butterfly, Euphilotes enoptes , are facultatively associated with ants. To determine whether variation in this association is predictable, I documented the proportion of larvae tended by ants and the identity of those ants over a three-year period in two study areas in central Washington state. Each study area contained numerous patches of Eriogonum compositum , the sole host plant of E. enoptes in this region. Using complete censuses and random subsampling, I estimated the dispersion and densities of E. enoptes larvae occupying each patch. The results of these observations indicated significant spatial and temporal variation in the identities of ants tending larvae and the frequency of ant attendance. In addition, early instar larvae were tended less frequently and by smaller ant species, than fourth instar larvae. Although larvae were aggregated at two spatial scales: individual plants within a patch and individual inflorescences on a plant, attendance rates did not vary consistently with aggregation size at either scale. Similarly, attendance rates were independent of the patch-wide density of larvae. I discuss the potential sources of such unpredictable variation and its implications for the evolution of further specialization in this facultative association.     

A new inexpensive suction apparatus for sampling arthropods in grassland

Abstract.

• 1 We describe a new technique for sampling arthropods from grassland and other vegetation, using a modified petrol-driven suction apparatus called a‘Blow & Vac’.
• 2 The new apparatus was tested by comparing its efficiency in extracting arthropods from vegetation with that of a conventional Dietrick, or‘D-Vac’, suction sampler.
• 3 Unimpeded air velocity inside the suction tube was approximately 4 times that of the D-Vac.
• 4 In field trials on semi-natural grasslands, the Blow & Vac caught more individuals of most spider and certain beetle species compared to the D-Vac and approximately equal numbers of Auchenorrhyncha.
• 5 We conclude that the‘Blow & Vac’has considerable potential for sampling arthropod populations. Its low cost and weight make it a suitable alternative to the conventional D-Vac apparatus.

     

Growth response of subalpine fir (Abies lasiocarpa) to climate in the Olympic Mountains, Washington, USA

Growth response of subalpine fir (Abies lasiocarpa) to climate was studied across its local geographical and elevation range in the Olympic Mountains, Washington. A dendroecological analysis of subalpine fir across a range of elevations (1350-1850 m) and annual precipitation (125-350 cm y^?1), was used to compare environmental factors affecting growth. Climate-growth relationships were explored using Pearson product-moment correlation coefficients; partial correlation analysis was used to assess relationships among site chronologies and climatic variables. Radial growth is negatively correlated with winter precipitation at high elevation and wet sites, but not at low and middle elevation dry sites. Growth is positively correlated with current growing season temperature at all sites; however, growth is negatively correlated with previous year August temperature, indicating that climate affects growth in subsequent years. Positive correlations between growth and summer precipitation during the growing season at low and middle elevation dry sites suggest that soil moisture is partially limiting to growth on these sites. If the climate of the Pacific Northwest becomes warmer and drier, then subalpine fir growth may increase at high elevation and wet sites, but may decrease at lower elevation dry sites in the Olympic Mountains. However, the growth response of subalpine fir to potentially rapid climate change will not be uniform because subalpine fir grows over a wide range of topographic features, habitats, and local climates at different geographical scales. A comparison of growth response to current growing season temperature suggests that the temperature-related growth response of subalpine fir is not adequately described by the parabolic curve used in JABOWA-based models.