Ecosystem-level patterns of primary productivity and herbivory in terrestrial habitats

Ecosystems are structurally organized as food webs within which energy is transmitted between trophic levels and dissipated into the environment. Energy flow between two trophic levels is given by the amount of production at the lower level and by the proportion of production that is consumed, assimilated and respired at the higher level. Considerable evidence indicates that food-web structure varies predictably in different habitats, but much less is known about quantitative relationships among food web fluxes. Many of the energetic properties of herbivores in African game parks are associated with rainfall and, by inference, with net primary productivity. Respiratory costs per unit production at the consumer trophic level are higher for homeotherms than for heterotherms. Plant secondary chemicals affect herbivore dietary choices and the allocation of plant resources to those chemicals varies with resource availability. How these phenomena are translated into ecosystem fluxes is unknown. We present evidence that herbivore biomass, consumption and productivity are closely correlated with plant productivity, suggesting that the latter is a principal integrator and indicator of functional processes in food webs.     

Myeloid leukaemia inhibitory factor maintains the developmental potential of embryonic stem cells

Embryonic stem (ES) cells, the totipotent outgrowths of blastocysts, can be cultured and manipulated in vitro and then returned to the embryonic environment where they develop normally and can contribute to all cell lineages. Maintenance of the stem-cell phenotype in vitro requires the presence of a feeder layer of fibroblasts or of a soluble factor, differentiation inhibitory activity (DIA) produced by a number of sources; in the absence of DIA the ES cells differentiate into a wide variety of cell types. We recently noted several similarities between partially purified DIA and a haemopoietic regulator, myeloid leukaemia inhibitory factor (LIF), a molecule which induces differentiation in M1 myeloid leukaemic cells and which we have recently purified, cloned and characterized. We demonstrate here that purified, recombinant LIF can substitute for DIA in the maintenance of totipotent ES cell lines that retain the potential to form chimaeric mice.     

拟南芥bHLH家族转录因子DYT1在花药发育过程中调控胼胝质降解(英文)

胼胝质的合成和降解是雄配子体减数分裂过程中的一个重要特征,对后期花粉成熟有重要作用.在此研究中,分离到了一个雄性不育突变体msl57,该突变体的绒毡层分化及胼胝质降解过程出现异常,导致花粉败育.图位克隆和遗传分析表明:MSl57基因与bHLI-I家族转录因子DYTI(At4g21330)是同一基因.因此,将ms157突变体改名为dyt1-2.反式激活作用实验揭示了DYTI的激活功能域位于基因的250 ～504bp之间.通过酵母双杂交实验发现DYT1蛋白在体内可以形成同源二聚体来执行其功能.RT-PCR及定量PCR分析表明胼胝质酶相关基因A6的表达在突变体背景下严重下调.因此,DVF1通过调控胼胝质的降解来影响花药发育过程.     

Fibronectin and VLA-4 in haematopoietic stem cell-microenvironment interactions

The self-renewal and differentiation of haematopoietic stem cells occurs in vivo and in vitro in direct contact with cells making up the haematopoietic microenvironment. In this study we used adhesive ligands and blocking antibodies to identify stromal cell-derived extracellular matrix proteins involved in promoting attachment of murine haematopoietic stem cells. Here we report that day-12 colony-forming-unit spleen (CFU-S12)5 cells and reconstituting haematopoietic stem cells attach to the C-terminal, heparin-binding fragment of fibronectin by recognizing the CS-1 peptide of the alternatively spliced non-type III connecting segment (IIICS) of human plasma fibronectin. Furthermore, CFU-S12 stem cells express the alpha 4 subunit of the VLA-4 integrin receptor, which is known to be a receptor for the CS-1 sequence, and monoclonal antibodies against the integrin alpha 4 subunit of VLA-4 block adhesion of CFU-S12 stem cells to plates coated with the C-terminal fibronectin fragment. Finally, polyclonal antibodies against the integrin beta 1 subunit of VLA-4 inhibit the formation of CFU-S12-derived spleen colonies and medullary haematopoiesis in vivo following intravenous infusion of antibody-treated bone marrow cells.     

Cadherin 23 is a component of the tip link in hair-cell stereocilia

Mechanoelectrical transduction, the conversion of mechanical force into electrochemical signals, underlies a range of sensory phenomena, including touch, hearing and balance. Hair cells of the vertebrate inner ear are specialized mechanosensors that transduce mechanical forces arising from sound waves and head movement to provide our senses of hearing and balance; however, the mechanotransduction channel of hair cells and the molecules that regulate channel activity have remained elusive. One molecule that might participate in mechanoelectrical transduction is cadherin 23 (CDH23), as mutations in its gene cause deafness and age-related hearing loss. Furthermore, CDH23 is large enough to be the tip link, the extracellular filament proposed to gate the mechanotransduction channel. Here we show that antibodies against CDH23 label the tip link, and that CDH23 has biochemical properties similar to those of the tip link. Moreover, CDH23 forms a complex with myosin-1c, the only known component of the mechanotransduction apparatus, suggesting that CDH23 and myosin-1c cooperate to regulate the activity of mechanically gated ion channels in hair cells.     

Cardano and the gambler’s habitus

Cardano’s Liber de ludo aleae is subjected to a reappraisal based largely on considerations of practice in the 16th century gambling arena. It is argued that Cardano’s purported failure to secure the foundations of a rigorous probability calculus can be explained as something that occurred precisely because of his gambling exposure, not in spite of it.     

隔板对燃烧室压力高频自激振荡的抑制作用

隔板在抑制液体火箭发动机高频燃烧不稳定性方面具有重要作用。该文研究隔板对液体火箭发动机中自激高频燃烧不稳定性的抑制作用机理。在初边值条件不施加任何激励的情况下,首先对无隔板的液氧/煤油模型发动机中的气液两相非稳态燃烧过程进行数值模拟,获得的燃烧室压力和流向平均速度与实验结果相符,并得到了燃烧室中的压力自激振荡。研究了隔板对这种压力自激振荡的抑制作用,并分析了压力波随时间的传递与演化过程。结果表明:无隔板燃烧室中发生了高频不稳定燃烧,频谱分析显示其具有1阶横向振型与2阶纵向振型;引入隔板后,燃烧室中的压力振荡减弱,且其横向振型消失,只有2阶纵向振型。结合燃烧室中压力波的传递和演化过程进行分析可知,压力自激振荡源主要出现在头部附近的区域,隔板的加入可以有效抑制压力波的横向传播,并提高激发不稳定燃烧所需的能量,从而抑制燃烧不稳定性的发生。