Survey of Vegetated Areas and Muskox Populations in East-Central Ellesmere Island

The results of 1981-84 summer helicopter surveys and ground reconnaissance of east-central Ellesmere Island are presented. This was the first systematic ecological survey to be conducted in this region of the Canadian High Arctic. Central Ellesmere Island is dominated by two large ice fields separated by the deglaciated Sverdrup Pass (79 degrees N). Muskox migrate freely through the 70 km long corridor between the Fosheim Peninsula and some lowlands on the east coast, but large areas of suitable habitat were found unused on the central east coast. Muskox densities in Sverdrup Pass were comparable with those at other arctic sites, as were their reproduction rates (proportion of calves). Vegetated areas (>5 percent cover) constituted only 5 percent of the total surveyed land area and were largely restricted to coastal lowlands and the Sverdrup Pass valley.Key words: Ellesmere Island, Sverdup Pass, high arctic vegetation, polar oases, muskox, muskox habitatMots clés: île d'Ellesmere, Col Sverdrup, végétation dans le Grand Nord, oasis polaires, boeuf musqué, habitat du boeuf musqu&eacute

Entombed Plant Communities Released by a Retreating Glacier at Central Ellesmere Island, Canada

The release of a dead but well-preserved high arctic plant community, entombed for about 400 radiocarbon years (WAT-778 and 789) under glacial ice at Twin Glacier, central Ellesmere Island (78°53'N, 75°55'W) is reported. Remarkably intact plants have been emerging from under the ablating front of this polar glacier which has been retreating for several decades at an average rate of 4.1 m/y over the last 22 years. The vegetation can be readily recognized as a Cassiope tetragona-Dryas integrifolia-dominated community, similar in species composition and cover to an extant Cassiope-Dryas community 200 m below the ablation front. The excellent preservation of the plants supports the thesis that polar glaciers are frozen to their bases, and hence their movements are by internal deformation rather than by erosive basal sliding.Key words: Ellesmere Island, Twin Glacier, polar (cold) glaciers, retreating glacier, Little Ice Age, ice-buried plants, paleobotany, radiocarbon datingMots clés: île d'Ellesmere, glacier Twin, glaciers polaires (froids), glacier reculant, le petit âge glaciaire, plantes ensevelies dans la glace, paléobotanie, datation radiométriqu

Understanding Mechanochemical Coupling in Kinesins Using First-Passage Time Processes

Kinesins are processive motor proteins that move along microtubules in a stepwise manner, and their motion is powered by the hydrolysis of ATP. Recent experiments have investigated the coupling between the individual steps of single kinesin molecules and ATP hydrolysis, taking explicitly into account forward steps, backward steps and detachments. A theoretical study of mechanochemical coupling in kinesins, which extends the approach used successfully to describe the dynamics of conventional motor proteins, is presented. The possibility of irreversible detachments of kinesins from the microtubules is also explicitly taken into account. Using the method of first- passage times, experimental data on the mechanochemical coupling in kinesins are fully described using the simplest two-state model. It is shown that the dwell times for the kinesin to move one step forward or backward, or to dissociate irreversibly are the same, although the probabilities of these events are different. It is concluded that the current theoretical view, that only the forward motion of the motor protein molecule is coupled to ATP hydrolysis, is consistent with all available experimental observations for kinesins.Comment: Submitted to Biophysical Journa

Imaging high-resolution structure of GFP-expressing neurons in neocortex in vivo

To detect subtle changes in neuronal morphology in response to changes in experience, one must image neurons at high resolution in vivo over time scales of minutes to days. We accomplished this by infecting postmitotic neurons in rat and mouse barrel cortex with a Sindbis virus carrying the gene for enhanced green fluorescent protein. Visualized with 2-photon excitation laser scanning microscopy, infected neurons showed bright fluorescence that was distributed homogeneously throughout the cell, including axonal and dendritic arbors. Single dendritic spines could routinely be resolved and their morphological dynamics visualized. Viral infection and imaging were achieved throughout postnatal development up to early adulthood (P 8-30), although the viral efficiency of infection decreased with age. This relatively noninvasive method for fluorescent labeling and imaging of neurons allows the study of morphological dynamics of neocortical neurons and their circuits in vivo

Cooperative Cargo Transport by Several Molecular Motors

The transport of cargo particles which are pulled by several molecular motors in a cooperative manner is studied theoretically. The transport properties depend primarily on the maximal number, $N$, of motor molecules that may pull simultaneously on the cargo particle. Since each motor must unbind from the filament after a finite number of steps but can also rebind to it again, the actual number of pulling motors is not constant but varies with time between zero and $N$. An increase in the maximal number $N$ leads to a strong increase of the average walking distance (or run length) of the cargo particle. If the cargo is pulled by up to $N$ kinesin motors, e.g., the walking distance is estimated to be $5^{N-1}/N$ micrometers which implies that seven or eight kinesin molecules are sufficient to attain an average walking distance in the centimeter range. If the cargo particle is pulled against an external load force, this force is shared between the motors which provides a nontrivial motor-motor coupling and a generic mechanism for nonlinear force-velocity relationships. With increasing load force, the probability distribution of the instantenous velocity is shifted towards smaller values, becomes broader, and develops several peaks. Our theory is consistent with available experimental data and makes quantitative predictions that are accessible to systematic in vitro experiments.Comment: 24 pages, latex, 6 figures, includes Supporting Tex

Experimental Study on Thermomechanical Properties of New-Generation ODS Alloys

By using a combination of new technologies together with an unconventional use of different types of materials, specific mechanical properties and structures of the material can be achieved. Some possibilities are enabled by a combination of powder metallurgy in the preparation of a metal matrix with dispersed stable particles achieved by mechanical alloying and hot consolidation. This paper explains the thermomechanical properties of new generation of Oxide Dispersion Strengthened alloys (ODS) within three ranges of temperature with specified deformation profiles. The results show that the mechanical properties of new ODS alloys are significantly affected by the thermomechanical treatment