The recent breakup of an asteroid in the main-belt region

The present population of asteroids in the main belt is largely the result of many past collisions. Ideally, the asteroid fragments resulting from each impact event could help us understand the large-scale collisions that shaped the planets during early epochs. Most known asteroid fragment families, however, are very old and have therefore undergone significant collisional and dynamical evolution since their formation. This evolution has masked the properties of the original collisions. Here we report the discovery of a family of asteroids that formed in a disruption event only 5.8 +/- 0.2 million years ago, and which has subsequently undergone little dynamical and collisional evolution. We identified 39 fragments, two of which are large and comparable in size (diameters of approximately 19 and approximately 14 km), with the remainder exhibiting a continuum of sizes in the range 2-7 km. The low measured ejection velocities suggest that gravitational re-accumulation after a collision may be a common feature of asteroid evolution. Moreover, these data can be used to check numerical models of larger-scale collisions.     

Filamentous actin accumulates during plant cell penetration and cell wall plug formation in <Emphasis Type="Italic">Phytophthora infestans</Emphasis>

The oomycete Phytophthora infestans is the cause of late blight in potato and tomato. It is a devastating pathogen and there is an urgent need to design alternative strategies to control the disease. To find novel potential drug targets, we used Lifeact-eGFP expressing P. infestans for high resolution live cell imaging of the actin cytoskeleton in various developmental stages. Previously, we identified actin plaques as structures that are unique for oomycetes. Here we describe two additional novel actin configurations; one associated with plug deposition in germ tubes and the other with appressoria, infection structures formed prior to host cell penetration. Plugs are composed of cell wall material that is deposited in hyphae emerging from cysts to seal off the cytoplasm-depleted base after cytoplasm retraction towards the growing tip. Preceding plug formation there was a typical local actin accumulation and during plug deposition actin remained associated with the leading edge. In appressoria, formed either on an artificial surface or upon contact with plant cells, we observed a novel aster-like actin configuration that was localized at the contact point with the surface. Our findings strongly suggest a role for the actin cytoskeleton in plug formation and plant cell penetration.     

多笔画联机手写字符识别的动态时间规正算法

在单笔画符号(或字符)联机手写识别中,动态时间规正(DTW)算法遵循时间次序约束和边界约束,并具有较高的识别率.为了将此算法应用于多笔画符号识别,常用而简单的方法是按照人们的手写顺序连接多笔画符号为单笔画符号.但此方法存在一个问题:人们常使用不同的笔画顺序和笔画方向书写同一个符号,用朴素(Brute Force)方法寻找所有笔画可能性非常耗时.为了降低计算复杂度,文中提出了DTW A*算法.在部分笔画匹配时,此算法保留着次序约束,并用A*算法降低计算复杂度.文中还通过流程图数据库多笔画符号识别实验对比了DTW A*算法、DTW算法、改良Hausdorff距离3种算法的性能,结果表明DTW A*算法具有最高的识别率和最好的稳定性.     

Kirchhoff's theory of rods

Communicated by C. Truesdell     

Burrowing and denning habits of a captive colony of the Utah prairie dog

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Life strategies in the evolution of the Colorado squaqfish ( Ptychocheilus lucius )

The Colorado squawfish, a large predaceous cyprinid, is a generalist species adapted to the large seasonal water fluctuations, low food base, and changing riverine subsystems of the Colorado River. Extant at least as early as the Miocene epoch, Ptychocheilus has survived by incorporating life strategies to deal with changing climates varying from arid to pluvial. Migration and long-term movement patterns appear to have evolved as tactics to perpetuate a grand reproductive strategy for exploiting the changing habitats and general environmental conditions of the late Cenozoic era. Accordingly, high mobility of a large fish would aid in selection of optimum spawning, nursery, and adult habitats in the dynamic lacustrine/riverine system that existed at that time. A spatial separation of life stages thus produced would aid in the reduction of intraspecific competition. Large size, long life, and late spawning of Ptychocheilus indicate that mortality of young must be disproportionately high compared to that of the adult form. Growth to a large size should reduce predation by other fishes and, once attained, would facilitate long distance movement for reproduction, feeding, and other purposes. Such a strategy, formerly highly adaptive, may now be implicated in the decline of this species in controlled riverine systems.     

Shrew and heteromyid records from the Great Basin of Oregon and Utah

Oregon records from Harney County establish a minimum altitudinal occurrence for the water shrew ( Sorex palustris ), identify an isolated population of the montane shrew ( Sorex monticola obscures ), and provide an additional specimen of Merriam's shrew ( Sorex merriami ). Utah records extending the ranges in the southeastern part of the Bonneville Basin for the little pocket mouse ( Perognathus longimembris ), long-tailed pocket mouse ( Perognathus formosus ), and dark kangaroo mouse ( Microdipodops megacephalus ) are also given.